Chapter IV. The Waters
Land artificially won from the Waters
Man, as we have seen, has done much to revolutionize the solid surface of the globe, and to change the distribution and proportions, if not the essential character, of the organisms which inhabit the land and even the waters. Besides the influence thus exerted upon the life which peoples the sea, his action upon the land has involved a certain amount of indirect encroachment upon the territorial jurisdiction of the ocean. So far as he has increased the erosion of running waters by the destruction of the forest or by other operations which lessen the cohesion of the soil, he has promoted the deposit of solid matter in the sea, thus reducing the depth of marine estuaries, advancing the coast-line, and diminishing the area covered by the waters. He has gone beyond this, and invaded the realm of the ocean by constructing within its borders wharves, piers, light-houses, breakwaters, fortresses, and other facilities for his commercial and military operations; and in some countries he has permanently rescued from tidal overflow, and even from the very bed of the deep, tracts of ground extensive enough to constitute valuable additions to his agricultural domain. The quantity of soil gained from the sea by these different modes of acquisition is, indeed, too inconsiderable to form an appreciable element in the comparison of the general proportion between the two great forms of terrestrial surface, land and water; but the results of such operations, considered in their physical and their moral bearings, are sufficiently important to entitle them to special notice in every comprehensive view of the relations between man and nature.
There are cases, as on the western shores of the Baltic, where, in consequence of the secular elevation of the coast, the sea appears to be retiring; others, where, from the slow sinking of the land, it seems to be advancing. These movements depend upon geological causes wholly out of our reach, and man can neither advance nor retard them.
There are also cases where similar apparent effects are produced by local oceanic currents, by river deposit or erosion, by tidal action, or by the influence of the wind upon the waves and the sands of the seabeach. A regular current may drift suspended earth and seaweed along a coast until they are caught by an eddy and finally deposited out of the reach of further disturbance, or it may scoop out the bed of the sea and undermine promontories and headlands; a powerful river, as the wind changes the direction of its flow at its outlet, may wash away shores and sandbanks at one point to deposit their material at another; the tide or waves, stirred to unusual depths by the wind, may gradually wear down the line of coast, or they may form shoals and coast-dunes by depositing the sand they have rolled up from the bottom of the ocean. These latter modes of action are slow in producing effects sufficiently important to be noticed in general geography, or even to be visible in the representations of coast-line laid down in ordinary maps; but they nevertheless form conspicuous features in local topography, and they are attended with consequences of great moment to the material and the moral interests of men. The forces which produce these limited results are all in a considerable degree subject to control, or rather to direction and resistance, by human power, and it is in guiding, combating, and compensating them that man has achieved some of his most remarkable and most honorable conquests over nature. The triumphs in question, or what we generally call harbor and coast improvements, whether we estimate their value by the money and labor expended upon them, or by their bearing upon the interests of commerce and the arts of civilization, must take a very high rank among the great works of man, and they are fast assuming a magnitude greatly exceeding their former relative importance.
The extension of commerce and of the military marine, and especially the introduction of vessels of increased burden and deeper draught of water, have imposed upon engineers tasks of a character which a century ago would have been pronounced, and, in fact, would have been, impracticable; but necessity has stimulated au ingenuity which has contrived means of executing them, and which gives promise of yet greater performance in time to come.
Indeed, although man, detached from the solid earth, is almost powerless to struggle against the sea, he is fast becoming invincible by it so long as his foot is planted on the shore, or even on the bottom of the rolling ocean; and though on some battle-fields between the waters and the land he is obliged slowly to yield his ground, yet he retreats still facing the foe, and will finally be able to say to the sea, "Thus far shalt thou come and no farther, and here shall thy proud waves be stayed!".
Great Works of Material Improvement
Men have ceased to admire the vain exercise of power which heaped up the great pyramid to gratify the pride of a despot with a giant sepulchre; for many great harbors, many important lines of internal communication, in the civilized world, now exhibit works which in volume and weight of material surpass the vastest remains of ancient architectural art, and demand the exercise of far greater constructive skill and involve a much heavier pecuniary expenditure than would now be required for the building of the tomb of Cheops. It is computed that the great pyramid, the solid contents of which when complete were about 3,000,000 cubic yards, could be erected for a million of pounds sterling. The breakwater at Cherbourg, founded in rough water sixty feet deep, at an average distance of more than two miles from the shore, contains double the mass of the pyramid, and many a comparatively unimportant canal has been constructed at twice the cost which would now build that stupendous monument.
The description of works of harbor and coast improvement which have only an economical value, not a true geographical importance, does not come within the plan of the present volume, and in treating this branch of my subject, I shall confine myself to such as are designed either to gain new soil by excluding the waters from grounds which they had permanently or occasionally covered, or to resist new encroachments of the sea upon the land.
Draining of Lincolnshire Fens
The draining of the Lincolnshire fens in England, which has converted about 400,000 acres of marsh, pool, and tide-washed flat into ploughland and pasturage, is a work, or rather series of works, of great magnitude, and it possesses much economical, and, indeed, no trifling geographical, importance. Its plans and methods were, at least in part, borrowed from the example of like improvements in Holland, and it is, in difficulty and extent, inferior to works executed for the same purpose on the opposite coast of the North Sea, by Dutch, Frisie, and Low German engineers. The space I can devote to such operations will be better employed in describing the latter, and I content myself with the simple statement I have already made of the quantity of worthless and even pestilential land which has been rendered both productive and salubrious in Lincolnshire, by diking out the sea, and the rivers which traverse the fens of that country.
The almost continued prevalence of west winds upon both coasts of the German Ocean occasions a constant set of the currents of that sea to the east, and both for this reason and on account of the greater violence of storms from the former quarter, the English shores of the North Sea are less exposed to invasion by the waves than those of the Netherlands and the provinces contiguous to them on the north. The old Netherlandish chronicles are filled with the most startling accounts of the damage done by the irruptions of the ocean, from west winds or extraordinarily high tides, at times long before any considerable extent of seacoast was diked. Several hundreds of those terrible inundations are recorded, and in many of them the loss of human lives is estimated as high as one hundred thousand. It is impossible to doubt that there must be enormous exaggeration in these numbers; for, with all the reckless hardihood shown by men in braving the dangers and privations attached by nature to their birthplace, it is inconceivable that so dense a population as such wholesale destruction of life supposes could find the means of subsistence, or content itself to dwell, on a territory liable, a dozen times in a century, to such fearful devastation. There can be no doubt, however, that the low continental shores of the German Ocean very frequently suffered immense injury from inundation by the sea, and it is natural, therefore, that the various arts of resistance to the encroachments of the ocean, and, finally, of aggressive warfare upon its domain, and of permanent conquest of its territory, should have been earlier studied and carried to higher perfection in the latter countries, than in England, which had less to lose or to gain by the incursions or the retreat of the waters.
Indeed, although the confinement of swelling rivers by artificial embankments is of great antiquity, I do not know that the defence or acquisition of land from the sea by diking was ever practised on a large scale until systematically undertaken by the Netherlanders, a few centuries after the commencement of the Christian era. The silence of the Roman historians affords a strong presumption that this art was unknown to the inhabitants of the Netherlands at the time of the Roman invasion, and the elder Pliny's description of the mode of life along the coast which has now been long diked in, applies precisely to the habits of the people who live on the low islands and mainland flats lying outside of the chain of dikes, and wholly unprotected by embankments of any sort.
Origin of Sea-dikes
It has been conjectured, and not without probability, that the causeways built by the Romans across the marshes of the Low Countries, in their campaigns against the Germanic tribes, gave the natives the first hint of the utility which might be derived from similar constructions applied to a different purpose. If this is so, it is one of the most interesting among the many instances in which the arts and enginery of war have been so modified as to be eminently promotive of the blessings of peace, thereby in some measure compensating the wrongs and sufferings they have inflicted on humanity. The Lowlanders are believed to have secured some coast and bay islands by ring-dikes and to have embanked some fresh-water channels, as early as the eighth or ninth century; but it does not appear that sea-dikes, important enough to be noticed in historical records, were constructed on the mainland before the thirteenth century. The practice of draining inland accumulations of water, whether fresh or salt, for the purpose of bringing under cultivation the ground they cover, is of later origin, and is said not to have been adopted until after the middle of the fifteenth century.
Gain and Loss of Land in the Netherlands
The total amount of surface gained to the agriculture of the Netherlands by diking out the sea and by draining shallow bays and lakes, is estimated by Staring at three hundred and fifty-five thousand bunder or hectares, equal to eight hundred and seventy-seven thousand two hundred and forty acres, which is one-tenth of the area of the kingdom. In very many instances the dikes have been partially, in some particularly exposed localities totally, destroyed by the violence of the sea, and the drained lands again flooded. In some cases the soil thus painfully won from the ocean has been entirely lost; in others it has been recovered by repairing or rebuilding the dikes and pumping out the water. Besides this, the weight of the dikes gradually sinks them into the soft soil beneath, and this loss of elevation must be compensated by raising the surface, while the increased burden thus added tends to sink them still lower. "Tetens declares," says Kohl, "that in some places the dikes have gradually sunk to the depth of sixty or even a hundred feet." For these reasons, the processes of dike-building have been almost everywhere again and again repeated, and thus the total expenditure of money and of labor upon the works in question is much greater than would appear from an estimate of the actual cost of diking-in a given extent of coast-land and draining a given area of water-surface.
Loss of Land by Incursions of Sea
On the other hand, by erosion of the coast-line, the drifting of sand-dunes into the interior, and the drowning of fens and morasses by incursions of the sea--all caused, or at least greatly aggravated, by human improvidence--the Netherlands have lost a far larger area of land since the commencement of the Christian era than they have gained by diking and draining. Staring despairs of the possibility of calculating the loss from the first-mentioned two causes of destruction, but he estimates that not less than six hundred and forty thousand bunder, or one million five hundred and eighty-one thousand acres, of fen and marsh have been washed away, or rather deprived of their vegetable surface and covered by water; and thirty-seven thousand bunder, or ninety-one thousand four hundred acres, of recovered land, have been lost by the destruction of the dikes which protected them. The average value of land gained from the sea is estimated at about nineteen pounds sterling, or ninety dollars, per acre; while the lost fen and morass was not worth more than one twenty-fifth part of the same price. The ground buried by the drifting of the dunes appears to have been almost entirely of this latter character, and, upon the whole, there is no doubt that the soil added by human industry to the territory of the Netherlands, within the historical period, greatly exceeds in pecuniary value that which has fallen a prey to the waves during the same era.
Upon most low and shelving coasts, like those of the Netherlands, the maritime currents are constantly changing, in consequence of the variability of the winds, and the shifting of the sand-banks, which the currents themselves now form and now displace. While, therefore, at one point the sea is advancing landward, and requiring great effort to prevent the undermining and washing away of the dikes, it is shoaling at another by its own deposits, and exposing, at low water, a gradually widening belt of sands and ooze. The coast-lands selected for diking-in are always at points where the sea is depositing productive soil. The Eider, the Elbe, the Weser, the Ems, the Rhine, the Maas, and the Schelde bring down large quantities of fine earth. The prevalence of west winds prevents the waters from carrying this material far out from the coast, and it is at last deposited northward or southward from the mouth of the rivers which contribute it, according to the varying drift of the currents.
The process of natural deposit which prepares the coast for diking-in is thus described by Staring: "All sea-deposited soil is composed of the same constituents. First comes a stratum of sand, with marine shells, or the shells of mollusks living in brackish water. If there be tides, and, of course, flowing and ebbing currents, mud is let fall upon the sand only after the latter has been raised above low-water mark; for then only, at the change from flood to ebb, is the water still enough to form a deposit of so light a material. Where mud is found at great depths, as, for example, in a large proportion of the Ij, it is a proof that at this point there was never any considerable tidal flow or other current. ... The powerful tidal currents, flowing and ebbing twice a day, drift sand with them. They scoop out the bottom at one point, raise it at another, and the sand-banks in the current are continually shifting. As soon as a bank raises itself above low-water mark, flags and reeds establish themselves upon it. The mechanical resistance of these plants checks the retreat of the high water and favors the deposit of the earth suspended in it, and the formation of land goes on with surprising rapidity. When it has risen to high-water level, it is soon covered with grasses, and becomes what is called schor in Zeeland, kwelder in Friesland. Such grounds are the foundation or starting-point of the process of diking. When they are once elevated to the flood-tide level, no more mud is deposited upon them except by extraordinary high tides. Their further rise is, accordingly, very slow, and it is seldom advantageous to delay longer the operation of diking."
Sea-dikes of the Netherlands
The formation of new banks by the sea is constantly going on at points favorable for the deposit of sand and earth, and hence opportunity is continually afforded for enclosure of new land outside of that already diked in, the coast is fast advancing seaward, and every new embankment increases the security of former enclosures. The province of Zeeland consists of islands washed by the sea on their western coasts, and separated by the many channels through which the Schelde and some other rivers find their way to the ocean. In the twelfth century these islands were much smaller and more numerous than at present. They have been gradually enlarged, and, in several instances, at last connected by the extension of their system of dikes. Walcheren is formed of ten islets united into one about the end of the fourteenth century. At the middle of the fifteenth century, Goeree and Overflakkee consisted of separate islands, containing altogether about ten thousand acres; by means of above sixty successive advances of the dikes, they have been brought to compose a single island, whose area is not less than sixty thousand acres.
In the Netherlands--which the first Napoleon characterized as a deposit of the Rhine, and as, therefore, by natural law, rightfully the property of him who controlled the sources of that great river--and on the adjacent Frisie, Low German, and Danish shores and islands, sea and river dikes have been constructed on a grander and more imposing scale than in any other country. The whole economy of the art has been there most thoroughly studied, and the literature of the subject is very extensive. For my present aim, which is concerned with results rather than with processes, it is not worth while to refer to professional treatises, and I shall content myself with presenting such information as can be gathered from works of a more popular character.
The superior strata of the lowlands upon and near the coast are, as we have seen, principally composed of soil brought down by the great rivers I have mentioned, and either directly deposited by them upon the sands of the bottom, or carried out to sea by their currents, and then, after a shorter or longer exposure to the chemical and mechanical action of salt-water and marine currents, restored again to the land by tidal overflow and subsidence from the waters in which it was suspended. At a very remote period the coast-flats were, at many points, raised so high by successive alluvious or tidal deposits as to be above ordinary high-water level, but they were still liable to occasional inundation from river-floods, and from the seawater also, when heavy or long-continued west winds drove it landwards. The extraordinary fertility of this soil and its security as a retreat from hostile violence attracted to it a considerable population, while its want of protection against inundation exposed it to the devastations of which the chroniclers of the Middle Ages have left such highly colored pictures. The first permanent dwellings on the coast-flats were erected upon artificial mounds, and many similar precarious habitations still exist on the unwalled islands and shores beyond the chain of dikes. River embankments, which, as is familiarly known, have from the earliest antiquity been employed in many countries where sea-dikes are unknown, were probably the first works of this character constructed in the Low Countries, and when two neighboring streams of fresh water had been embanked, the next step in the process would naturally be to connect the river-walls together by a transverse dike or raised causeway, which would serve as a means of communication between different hamlets and at the same time secure the intermediate ground both against the backwater of river-floods and against overflow by the sea. The oldest true sea-dikes described in historical records, however, are those enclosing islands in the estuaries of the great rivers, and it is not impossible that the double character they possess as a security against maritime floods and as a military rampart, led to their adoption upon those islands before similar constructions had been attempted upon the mainland.
At some points of the coast, various contrivances, such as piers, piles, and, in fact, obstructions of all sorts to the ebb of the current, are employed to facilitate the deposit of slime, before a regular enclosure is commenced. Usually, however, the first step is to build low and cheap embankments, extending from an older dike, or from high ground, around the parcel of flat intended to be secured. These are called summer dikes. They are erected when a sufficient extent of ground to repay the cost has been elevated enough to be covered with coarse vegetation fit for pasturage. They serve both to secure the ground from overflow by the ordinary flood-tides of mild weather, and to retain the slime deposited by very high water, which would otherwise be partly carried off by the retreating ebb. The elevation of the soil goes on slowly after this; but when it has at last been sufficiently enriched, and raised high enough to justify the necessary outlay, permanent dikes are constructed by which the water is excluded at all seasons. These embankments are constructed of sand from the coast-dunes or from sand-banks, and of earth from the mainland or from flats outside the dikes, bound and strengthened by fascines, and provided with sluices, which are generally founded on piles and of very expensive construction, for drainage at low water. The outward slope of the sea-dikes is gentle, experience having shown that this form is least exposed to injury both from the waves and from floating ice, and the most modern dikes are even more moderate in the inclination of the seaward scarp than the older ones. The crown of the dike, however, for the last three or four feet of its height, is much steeper, being intended rather as a protection against the spray than against the waves, and the inner slope is always comparatively abrupt.
The height and thickness of dikes varies according to the elevation of the ground they enclose, the rise of the tides, the direction of the prevailing winds, and other special causes of exposure, but it may be said that they are, in general, raised from fifteen to twenty feet above ordinary high-water mark. The water-slopes of river-dikes are protected by plantations of willows or strong semi-aquatic shrubs or grasses, but as these will not grow upon banks exposed to salt-water, sea-dikes must be faced with stone, fascines, or some other revetement. Upon the coast of Schleswig and Holstein, where the people have less capital at their command, they defend their embankments against ice and the waves by a coating of twisted straw or reeds, which must be renewed as often as once, sometimes twice a year. The inhabitants of these coasts call the chain of dikes "the golden border," a name it well deserves, whether we suppose it to refer to its enormous cost, or, as is more probable, to its immense value as a protection to their fields and their firesides.
When outlying flats are enclosed by building new embankments the old interior dikes are suffered to remain, both as an additional security against the waves, and because the removal of them would be expensive. They serve, also, as roads or causeways, a purpose for which the embankments nearest the sea are seldom employed, because the whole structure might be endangered from the breaking of the turf by wheels and the hoofs of horses. Where successive rows of dikes have been thus constructed, it is observed that the ground defended by the more ancient embankments is lower than that embraced within the newer enclosures, and this depression of level has been ascribed to a general subsidence of the coast from geological causes; but the better opinion seems to be that it is, in most cases, due merely to the consolidation and settling of the earth from being more effectually dried, from the weight of the dikes, from the tread of men and cattle, and from the movement of the heavy wagons which carry off the crops.
Notwithstanding this slow sinking, most of the land enclosed by dikes is still above low-water mark, and can, therefore, be wholly or partially freed from rain-water, and from that received by infiltration from higher ground, by sluices opened at the ebb of the tide. For this purpose the land is carefully ditched, and advantage is taken of every favorable occasion for discharging the water through the sluices. But the ground cannot be effectually drained by this means, unless it is elevated four or five feet, at least, above the level of the ebb-tide because the ditches would not otherwise have a sufficient descent to carry the water off in the short interval between ebb and flow, and because the moisture of the saturated sub-soil is always rising by capillary attraction. Whenever, therefore, the soil has sunk below the level I have mentioned, and in cases where its surface has never been raised above it, pumps, worked by wind or some other mechanical power, must be very frequently employed to keep the land dry enough for pasturage and cultivation.
Draining of the Lake of Haarlem
The substitution of steam-engines for the feeble and uncertain action of windmills, in driving pumps, has much facilitated the removal of water from the polders as well as the draining of lakes, marshes, and shallow bays, and thus given such an impulse to these enterprises, that not less than one hundred and ten thousand acres wore reclaimed from the waters, and added to the agricultural domain of the Netherlands, between 1815 and 1855. The most important of these undertaking was the draining of the Lake of Haarlem, and for this purpose some of the most powerful hydraulic engines over constructed were designed and executed. The origin of this lake is unknown. It is supposed by some geographers to be a part of an ancient bed of the Rhine, the channel of which, as there is good reason to believe, has undergone great changes since the Roman invasion of the Netherlands; by others it is thought to have once formed an inland marine channel, separated from the sea by a chain of low islands, which the sand washed up by the tides has since connected with the mainland and converted into a continuous line of coast. The best authorities, however, find geological evidence that the surface occupied by the lake was originally a marshy tract containing within its limits little solid ground, but many ponds and inlets, and much floating as well as fixed fen.
In consequence of the cutting of turf for fuel, and the destruction of the few trees and shrubs which held the loose soil together with their roots, the ponds are supposed to have gradually extended themselves, until the action of the wind upon their enlarged surface gave their waves sufficient force to overcome the resistance of the feeble barriers which separated them, and to unite them all into a single lake. Popular tradition, it is true, ascribes the formation of the Lake of Haarlem to a single irruption of the sea, at a remote period, and connects it with one or another of the destructive inundations of which the Netherland chronicles describe so many; but on a map of the year 1531, a chain of four smaller waters occupies nearly the ground afterwards covered by the Lake of Haarlem, and they have most probably been united by gradual encroachments resulting from the improvident practices above referred to, though no doubt the consummation may have been hastened by floods, and by the neglect to maintain dikes, or the intentional destruction of them, in the long wars of the sixteenth century.
The Lake of Haarlem was a body of water not far from fifteen miles in length, by seven in greatest width, lying between the cities of Amsterdam and Leyden, running parallel with the coast of Holland at the distance of about five miles from the sea, and covering an area of about 45,000 acres. By means of the Ij, it communicated with the Zuiderzee, the Mediterranean of the Netherlands, and its surface was little above the mean elevation of that of the sea. Whenever, therefore, the waters of the Zuiderzee were acted upon by strong north-west winds, those of the Lake of Haarlem were raised proportionally and driven southwards, while winds from the south tended to create a flow in the opposite direction. The shores of the lake were everywhere low, and though between the years 1767 and 1848 more than $1,700,000 had been expended in checking its encroachments, it often burst its barriers, and produced destructive inundations. In November, 1836, a south wind brought its waters to the very gates of Amsterdam, and in December of the same year, in a north-west gale, they overflowed twenty thousand acres of land at the southern extremity of the lake, and flooded a part of the city of Leyden. The depth of water in the lake did not, in general, exceed fourteen feet, but the bottom was a semi-fluid ooze or slime, which partook of the agitation of the waves, and added considerably to their mechanical force. Serious fears were entertained that the lake would form a junction with the inland waters of the Legmeer and Mijdrecht, swallow up a vast extent of valuable soil, and finally endanger the security of a large proportion of the land which the industry of Holland had gained in the course of centuries from the ocean.
For this reason, and for the sake of the large addition the bottom of the lake would make to the cultivable soil of the state, it was resolved to drain it, and the preliminary steps for that purpose were commenced in the year 1840. The first operation was to surround the entire lake with a ring-canal and dike, in order to cut off the communication with the Ij, and to exclude the water of the streams and morasses which discharged themselves into it from the land side. The dike was composed of different materials, according to the means of supply at different points, such as sand from the coast-dunes, earth and turf excavated from the line of the ring-canal, and floating turf, fascines being everywhere used to bind and compact the mass together. This operation was completed in 1848, and three steam-pumps were then employed for five years in discharging the water. The whole enterprise was conducted at the expense of the state, and in 1853 the recovered lands were offered for sale for its benefit. Up to 1858, forty-two thousand acres had been sold at not far from sixteen pounds sterling or seventy-seven dollars an acre, amounting altogether to L661,000 sterling or $3,200,000. The unsold lands were valued at more than L6,000 or nearly $30,000, and as the total cost was L764,500 or about $3,700,000, the direct loss to the state, exclusive of interest on the capital expended, may be stated at L100,000 or something less than $500,000.
The success of this operation has encouraged others of like nature in Holland. The Zuid Plas, which covered 11,500 acres and was two feet deeper than the Lake of Haarlem, has been drained, and a similar work now in course of execution on an arm of the Scheld, will recover about 35,000 acres.
In a country like the United States, of almost boundless extent of sparsely inhabited territory, such an expenditure for such an object would be poor economy. But Holland has a narrow domain, great pecuniary resources, an excessively crowded population, and a consequent need of enlarged room and opportunity for the exercise of industry. Under such circumstances, and especially with an exposure to dangers so formidable, there is no question of the wisdom of the measure. It has already provided homes and occupation for more than five thousand citizens, and furnished a profitable investment for a private capital of not less than L400,000 sterling or $2,000,000, which has been expended in improvements over and above the purchase money of the soil; and the greater part of this sum, as well as of the cost of drainage, has been paid as a compensation for labor. The excess of governmental expenditure over the receipts, if employed in constructing ships of war or fortifications, would have added little to the military strength of the kingdom; but the increase of territory, the multiplication of homes and firesides which the people have an interest in defending, and the augmentation of agricultural resources, constitute a stronger bulwark against foreign invasion than a ship of the line or a fortress armed with a hundred cannon.
Draining of the Zuiderzee
I have referred to the draining of the Lake of Haarlem as an operation of great geographical as well as economical and mechanical interest. A much more gigantic project, of a similar character, is now engaging the attention of the Netherlandish engineers. It is proposed to drain the great salt-water basin called the Zuiderzee. This inland sea covers an area of not less than two thousand square miles, or about one million three hundred thousand acres. The seaward half, or that portion lying north-west of a line drawn from Enkhuizen to Stavoren, is believed to have been converted from a marsh to an open bay since the fifth century after Christ, and this change is ascribed, partly if not wholly, to the interference of man with the order of nature. The Zuiderzee communicates with the sea by at least six considerable channels, separated from each other by low islands, and the tide rises within the basin to the height of three feet. To drain the Zuiderzee, these channels must first be closed and the passage of the tidal flood through them cut off. If this be done, the coast currents will be restored approximately to the lines they followed fourteen or fifteen centuries ago, and thero can be little doubt that an appreciable effect will thus be produced upon all the tidal phenomena of that coast, and, of course, upon the maritime geography of Holland.
A ring-dike and canal must then be constructed around the landward side of the basin, to exclude and carry off the freshwater streams which now empty into it. One of these, the Ijssel, a considerable river, has a course of eighty miles, and is, in fact, one of the outlets of the Rhine, though augmented by the waters of several independent tributaries. These preparations being made, and perhaps transverse dikes erected at convenient points for dividing the gulf into smaller portions, the water must be pumped out by machinery, in substantially the same way as in the case of the Lake of Haarlem. No safe calculations can be made as to the expenditure of time and money required for the execution of this stupendous enterprise, but I believe its practicability is not denied by competent judges, though doubts are entertained as to its financial expediency. The geographical results of this improvement would be analogous to those of the draining of the Lake of Haarlem, but many times multiplied in extent, and its meteorological effects, though perhaps not perceptible on the coast, could hardly fail to be appreciable in the interior of Holland.
The bearing of the works I have noticed, and of others similar in character, upon the social and moral, as well as the purely economical, interests of the people of the Netherlands, has induced me to describe them more in detail than the general purpose of this volume may be thought to justify; but if we consider them simply from a geographical point of view, we shall find that they are possessed of no small importance as modifications of the natural condition of terrestrial surface. There is good reason to believe that before the establishment of a partially civilized race upon the territory now occupied by Dutch, Frisic, and Low German communities, the grounds not exposed to inundation were overgrown with dense woods; that the lowlands between these forests and the sea-coasts were marshes, covered and partially solidified by a thick matting of peat-plants and shrubs interspersed with trees; and that even the sand-dunes of the shore were protected by a vegetable growth which, in a great measure, prevented the drifting and translocation of them.
The present causes of river and coast erosion existed, indeed, at the period in question; but some of them must have acted with less intensity, there were strong natural safeguards against the influence of marine and fresh-water currents, and the conflicting tendencies had arrived at a condition of approximate equilibrium, which permitted but slow and gradual changes in the face of nature. The destruction of the forests around the sources and along the valleys of the rivers by man gave them a more torrential character. The felling of the trees, and the extirpation of the shrubbery upon the fens by domestic cattle, deprived the surface of its cohesion and consistence, and the cutting of peat for fuel opened cavities in it, which, filling at once with water, rapidly extended themselves by abrasion of their borders, and finally enlarged to pools, lakes, and gulfs, like the Lake of Haarlem and the northern part of the Zuiderzee. The cutting of the wood and the depasturing of the grasses upon the sand-dunes converted them from solid bulwarks against the ocean to loose accumulations of dust, which every sea-breeze drove farther landward, burying, perhaps, fertile soil and choking up water-courses on one side, and exposing the coast to erosion by the sea upon the other.
Geographical Effect of Physical Improvements in the Netherlands
The changes which human action has produced within twenty centuries in the Netherlands and the neighboring provinces, are, certainly of no small geographical importance, considered simply as a direct question of loss and gain of territory. They have also, as we shall see hereafter, undoubtedly been attended with some climatic consequences, they have exercised a great influence on the spontaneous animal and vegetable life of this region, and they cannot have failed to produce effects upon tidal and other oceanic currents, the range of which may be very extensive. The force of the tidal wave, the height to which it rises, the direction of its currents, and, in fact, all the phenomena which characterize it, as well as all the effects it produces, depend as much upon the configuration of the coast it washes, and the depth of water, and form of bottom near the shore, as upon the attraction which occasions it. Every one of the terrestrial conditions which affect the character of tidal and other marine currents has been very sensibly modified by the operations I have described, and on this coast, at least, man has acted almost as powerfully on the physical geography of the sea as on that of the land.
Ancient Hydraulic Works
The hydraulic works of the Netherlands and of the neighboring states are of such magnitude that--with the exception of the dikes of the Mississippi--they quite throw into the shade all other known artificial arrangements for defending the land against the encroachments of the rivers and the sea, and for reclaiming to the domain of agriculture and civilization soil long covered by the waters. But although the recovery and protection of lands flooded by the sea seems to be an art wholly of Netherlandish origin, we have abundant evidence that, in ancient as well as in comparatively modern times, great enterprises more or less analogous in character have been successfully undertaken, both in inland Europe and in the less familiar countries of the East.
In many cases no historical record remains to inform us when or by whom such works were constructed. The Greeks and Romans, the latter especially, were more inclined to undertake and carry out stupendous material enterprises than to boast of them; and many of the grandest and most important constructions of those nations are absolutely unnoticed by contemporary annalists, and are first mentioned by writers living after all knowledge of the epochs of the projectors of these works had perished. Thus the aqueduct known as the Pont du Gard, near Nimes, which, though not surpassing in volume or in probable cost other analogous constructions of ancient and of modern ages, is yet among the most majestic and imposing remains of ancient civil architecture, is not so much as spoken of by any Roman author, and we are in absolute ignorance of the age or the construction of the remarkable tunnel cut to drain Lake Copais in Boeotia. This lake, now reduced by sedimentary deposit and the growth of aquatic and semi-aquatic vegetation to the condition of a marsh, was originally partially drained by natural subterranean outlets in the underlying limestone rock, many of which still exist. But these emissaries, or katavothra, as they are called in both ancient and modern Greek, were insufficient for the discharge of the water, and besides, they were constantly liable to be choked by earth and vegetables, and in such cases the lake rose to a height which produced much injury. To remedy this evil and secure a great accession of fertile soil, at some period anterior to the existence of a written literature in Greece and ages before the time of any prose author whose works have come down to us, two tunnels, one of them four miles long, and of course not inferior to the Torlonian emissary in length, were cut through the solid rock, and may still be followed throughout their whole extent. They were repaired in the time of Alexander the Great, in the fourth century before Christ, and their date was at that time traditionally referred to the reign of rulers who lived as early as the period of the Trojan war.
One of the best known hydraulic works of the Romans is the tunnel which serves to discharge the surplus waters of the Lake of Albano, about fourteen miles from [[[Rome]]. This lake, about six miles in circuit, occupies one of the craters of an extinct volcanic range, and the surface of its waters is about nine hundred feet above the sea. It is fed by rivulets and subterranean springs originating in the Alban Mount, or Monte Cavo, the most elevated peak of the volcanic group just mentioned, which rises to the height of about three thousand feet. At present the lake has no discoverable natural outlet, and it is not known that the water ever stood at such a height as to flow regularly over the lip of the crater. It seems that at the earliest period of which we have any authentic memorials, its level was usually kept by evaporation, or by discharge through subterranean channels, considerably below the rim of the basin which encompassed it, but in the year 397 B.C., the water, either from the obstruction of such channels, or in consequence of increased supplies from unknown sources, rose to such a height as to flow over the edge of the crater, and threaten inundation to the country below by bursting through its walls. To obviate this danger, a tunnel for carrying off the water was pierced at a level much below the height to which it had risen. This gallery, cut entirely with the chisel through the rock for a distance of six thousand feet, or nearly a mile and one-seventh, is still in so good condition as to serve its original purpose. The fact that this work was contemporaneous with the siege of Veii, has given to ancient annalists occasion to connect the two events, but modern critics are inclined to reject Livy's account of the matter, as one of the many improbable fables which disfigure the pages of that historian. It is, however, repeated by Cicero and by Dionysius of Halicarnassus, and it is by no means impossible that, in an age when priests and soothsayers monopolized both the arts of natural magic and the little which yet existed of physical science, the Government of Rome, by their aid, availed itself at once of the superstition and of the military ardor of its citizens to obtain their sanction to an enterprise which sounder arguments might not have induced them to approve.
Still more remarkable is the tunnel cut by the Emperor Claudius to drain the Lake Fucinus, now Lago di Celano, in the former Neapolitan territory, about fifty miles eastward of Rome. This lake, as far as its history is known, has varied very considerably in its dimensions at different periods, according to the character of the seasons. It lies 2,200 feet above the sea, and has no visible outlet, but was originally either drained by natural subterranean conduits, or kept within certain extreme limits by evaporation. In years of uncommon moisture it spread over the adjacent soil and destroyed the crops; in dry seasons it retreated, and produced epidemic disease by poisonous exhalations from the decay of vegetable and animal matter upon its exposed bed. Julius Caesar had proposed the construction of a tunnel to lower the bed of the lake and provide a regular discharge for its waters, but the enterprise was not actually undertaken until the reign of Claudius, when--after a temporary failure, from errors in levelling by the engineers, as was pretended at the time, or, as now appears certain, in consequence of frauds by the contractors in the execution of the work--it was at least partially completed. From this imperfect construction, it soon got out of repair, but was restored by Hadrian, and is said to have answered its design for some centuries. In the barbarism which followed the downfall of the empire, it again fell into decay, and though numerous attempts were made to repair it during the Middle Ages, no tolerable success seems to have attended any of these efforts until the present generation.
Draining of Lake Celano by Prince Torlonia
Works have been some years in progress and are now substantially completed, at a cost of about six millions of dollars, for restoring, or rather enlarging and rebuilding, this ancient tunnel, upon a scale of grandeur which does infinite honor to the liberality and public spirit of the projectors, and with an ingenuity of design and a constructive skill which reflect the highest credit upon the professional ability of the engineers who have planned the works and directed their execution. The length of the Roman tunnel was 18,634 feet, or rather more than three miles and a half, but as the new emissary is designed to drain the lake to the bottom, it must be continued to the lowest part of the basin. It will consequently have a length of not less than 21,000 feet, and, of course, is among the longest subterranean galleries in Europe. Many curious particulars in the design and execution of the original work have been observed in the course of the restoration, but these cannot here be noticed. The difference between the lowest and highest known levels of the surface of the lake is rather more than forty feet and the difference between the areas covered by water at these levels is not less than nine thousand acres. The complete drainage of the lake, including the ground occasionally flooded, will recover, for agricultural occupation, and permanently secure from inundation, about forty-two thousand acres of as fertile soil as any in Italy. The ground already dry enough for cultivation furnishes occupation and a livelihood for a population of 16,000 persons, and it is thought that this number will be augmented to 40,000 when the drainage shall be completely effected.
The new tunnel follows the line of the Claudian emissary--which though badly executed was admirably engineered--but its axis is at a somewhat lower level than that of the old gallery, and its cross-section is about two hundred and fifteen square feet, allowing a discharge of about 2,400 cubic feet to the second, while the Roman work had a cross-section of only one hundred and two square feet, with a possible delivery of 424 cubic feet to the second.
In consequence of the nature of the rock and of the soil, which had been loosened and shattered by the falling in of much of the crown and walls of the old tunnel--every stone of which it was necessary to remove in the progress of the work--and the great head of water in the lake from unusually wet seasons, the technical difficulties to be surmounted were most baffling and discouraging in character, and of such extreme gravity that it may well be doubted whether the art of engineering has anywhere triumphed over more serious obstacles. This great "victory of peace"--probably the grandest work of physical improvement ever effected by the means, the energy, and the munificence of a single individual--is of no small geographical and economical, as well as sanitary, importance, but it has a still higher moral value as an almost unique example of the exercise of public spirit, courage, and perseverance in the accomplishment of a noble and beneficent enterprise by a private citizen.
The crater-lake of Nemi, in the same volcanic region as that of Albano, is also drained by a subterranean tunnel probably of very ancient construction, and the Valle-Riccia appears to have once been the basin of a lake long since laid dry, but whether by the bursting of its banks or by human art we are unable to say.
The success of the Lake Celano tunnel has suggested other like improvements in Italy. A gallery has been cut, under circumstances of great difficulty, to drain Lake Agnano near Naples, and a project for the execution of a similar operation on the Lake of Perugia, the ancient Trasimenus, which covers more than 40,000 acres, is under discussion.
Many similar enterprises have been conceived and executed in modern times, both for the purpose of reclaiming land covered by water and for sanitary reasons. They are sometimes attended with wholly unexpected evils, as, for example, in the case of Barton Pond, in Vermont, and in that of a lake near Ragunda in Sweden, already mentioned on a former page. Another still less obvious consequence of the withdrawal of the waters has occasionally been observed in these operations. The hydrostatic force with which the water, in virtue of its specific gravity, presses against the banks that confine it, has a tendency to sustain them whenever their composition and texture are not such as to expose them to softening and dissolution by the infiltration of the water. If, then, the slope of the banks is considerable, or if the earth of which they are composed rests on a smooth and slippery stratum inclining towards the bed of the lake, they are liable to fall or slide forward when the mechanical support of the water is removed, and this sometimes happens on a considerable scale. A few years ago the surface of the Lake of Lungern, in the Canton of Unterwalden, in Switzerland, was lowered by driving a tunnel about a quarter of a mile long through the narrow ridge, called the Kaiserstuhl, which forms a barrier at the north end of the basin. When the water was drawn off, the banks, which are steep, cracked and burst, several acres of ground slid down as low as the water receded, and even the whole village of Lungern was thought to be in no small danger.
Other inconveniences of a very serious character have often resulted from the natural wearing down, or, much more frequently, the imprudent destruction, of the barriers which confine mountain lakes. In their natural condition, such basins serve both to receive and retain the rocks and other detritus brought down by the torrents which empty into them, and to check the impetus of the rushing waters by bringing them to a temporary pause; but if the outlets are lowered so as to drain the reservoirs, the torrents continue their rapid flow through the ancient bed of the basins, and carry down with them the sand and gravel with which they are charged, instead of depositing their burden as before in the still waters of the lakes.
It is a common opinion in America that the river meadows, bottoms, or intervales, as they are popularly called, are generally the beds of ancient lakes which have burst their barriers and left running currents in their place. It was shown by Dr. Dwight, many years ago, that this is very far from being universally true; but there is no doubt that mountain lakes were of much more frequent occurrence in primitive than in modern geography, and there are many chains of such still existing in regions where man has yet little disturbed the original features of the earth. In the long valleys of the Adirondack range in Northern New York, and in the mountainous parts of Maine, eight, ten, and even more lakes and lakelets are sometimes found in succession, each emptying into the next lower pool, and so all at last into some considerable river. When the mountain slopes which supply these basins shall be stripped of their woods, the augmented swelling of the lakes will break down their barriers, their waters will run off, and the valleys will present successions of flats with rivers running through them, instead of chains of lakes connected by natural canals.
A similar state of things seems to have existed in the ancient geography of France. "Nature," says Lavergne, "has not excavated on the flanks of our Alps reservoirs as magnificent as those of Lombardy; she had, however, constructed smaller but more numerous lakes, which the improvidence of man has permitted to disappear. Auguste de Gasparin demonstrated more than thirty years ago that many natural dikes formerly existed in the mountain valleys, which have been swept away by the waters.".
Many Alpine valleys in Switzerland and Italy present unquestionable evidence of the former existence of chains of lakes in their basins, and this may be regarded as a general fact in regard to the primitive topography of mountainous regions. Where the forests have not been destroyed, the lakes remain as characteristic features of the geographical surface. But when the woods are felled, these reservoirs are sooner or later filled up by wash from the shores, and of course disappear. Geologists have calculated the period when the bottom of the Lake of Geneva will be levelled up and its outlet worn down. The Rhone will then flow, in an unbroken current, from its source in the great Rhone glacier to the Mediterranean Sea.
Draining of Swamps
The reclamation of bogs and swamps by draining off the surface-water is doubtless much more ancient than the draining of lakes. The beneficial results of the former mode of improvement are more unequivocal, and balanced by fewer disadvantages, and, at the same time, the processes by which it is effected are much simpler and more obvious. It has accordingly been practised through the whole historical period, and in recent times operations for this purpose have assumed a magnitude, and been attended with economical as well as sanitary and geographical effects, which entitle them to a high place in the efforts of man to ameliorate the natural conditions of the soil he occupies.
The methods by which the draining of marshes is ordinarily accomplished are too familiar, and examples of their successful employment too frequent, to require description, and I shall content myself, for the moment, with a brief notice of some recent operations of this sort which are less generally known than their importance merits.
Within the present century more than half a million acres of swamp-land have been drained and brought under cultivation in Hungary, and works are in progress which will ultimately recover a still larger area for human use. The most remarkable feature of these operations, and at the same time the process which has been most immediately successful and remunerative, is what is called in Europe the regulation of water-courses, and especially of the River Theiss, on the lower course of which stream alone not less than 250,000 acres of pestilential and wholly unproductive marsh have been converted into a healthful region of the most exuberant fertility.
The regulation of a river consists in straightening its channel by cutting off bends, securing its banks from erosion by floods, and, where necessary, by constructing embankments to confine the waters and prevent them from overflowing and stagnating upon the low grounds which skirt their current. In the course of the Theiss about sixty bends, including some of considerable length, have been cut off, and dikes sufficient for securing the land along its banks against inundation have been constructed.
Many thousand acres of land have been recently permanently improved in Italy by the draining of swamps, and extensive operations have been projected and commenced on the lower Rhone, and elsewhere in France, with the same object. But there is probably no country where greater improvements of this sort have either been lately effected, or are now in course of accomplishment, than in our own. Not to speak of well-known works on the New Jersey seacoast and the shores of Lake Michigan, the people of the new State of California are engaging in this mode of subduing nature with as much enterprise and energy as they have shown in the search for gold. The Report of the Agricultural Department of the United States for January, 1872, notices, with more or less detail, several highly successful experiments in California in the way of swamp-drainage and securing land from overflow, and it appears that not far from 200,000 acres have either very recently undergone or will soon be subjected to this method of improvement.
I have commenced this chapter with a description of the dikes and other hydraulic works of the Netherland engineers, because both the immediate and the remote results of such operations are more obvious and more easily measured, though certainly not more important, than those of much older and more widely diffused modes of resisting or directing the flow of waters, which have been practised from remote antiquity in the interior of all civilized countries. Draining and irrigation are habitually regarded as purely agricultural processes, having little or no relation to technical geography; but we shall find that they exert a powerful influence on soil, climate, and animal and vegetable life, and may, therefore, justly claim to be regarded as geographical elements.
Superficial draining is a necessity in all lands newly reclaimed from the forest. The face of the ground in the woods is never so regularly inclined as to permit water to flow freely over it. There are, even on the hillsides, small ridges depressions, partly belonging to the original distribution of the soil, and partly occasioned by irregularities in the growth and deposit of vegetable matter. These, in the husbandry of nature, serve as dams and reservoirs to collect a larger supply of moisture than the spongy earth can at once imbibe. Besides this, the vegetable mould is, even under the most favorable circumstances, slow in parting with the humidity it has accumulated under the protection of the woods, and the infiltration from neighboring forests contributes to keep the soil of small clearings too wet for the advantageous cultivation of artificial crops. For these reasons, surface draining must have commenced with agriculture itself, and there is probably no cultivated district, one may almost say no single field, which is not provided with artificial arrangements for facilitating the escape of superficial water, and thus carrying off moisture which, in the natural condition of the earth, would have been imbibed by the soil.
All these processes belong to the incipient civilization of the ante-historical periods, but the construction of subterranean channels for the removal of infiltrated water marks ages and countries distinguished by a great advance in agricultural theory and practice, a great accumulation of pecuniary capital and a density of population which creates a ready demand and a high price for all products of rural industry. Under draining, too, would be most advantageous in damp and cool climates, where evaporation is slow, and upon soils where the natural inclination of surface does not promote a very rapid flow of the surface-waters. All the conditions required to make this mode of rural improvement, if not absolutely necessary, at least profitable, exist in Great Britain, and it is, therefore, very natural that the wealthy and intelligent farmers of England should have carried this practice farther, and reaped a more abundant pecuniary return from it, than those of any other country.
Besides superficial and subsoil drains, there is another method of disposing of superfluous surface-water, which, however, can rarely be practised, because the necessary conditions for its employment are not of frequent occurrence. Whenever a tenacious water-holding stratum rests on a loose, gravelly bed so situated as to admit of a free discharge of water from or through it by means of the outcropping of the bed at a lower level, or of deep-lying conduits leading to distant points of discharge, superficial waters may be carried off by opening a passage for them through the impervious into the permeable stratum. Thus, according to Bischof, as early as the time of King Rene, in the first half of the fifteenth century, when subsoil drainage was scarcely known, the plain of Paluns, near Marseilles, was laid dry by boring, and Wittwer informs us that drainage is effected at Munich by conducting the superfluous water into large excavations, from which it filters through into a lower stratum of pebble and gravel lying a little above the level of the river Isar. So at Washington, in the western part of the city, which lies high above the rivers Potomac and Rock Creek, many houses are provided with dry wells for draining their cellars and foundations. These extend through hard, tenacious earth to the depth of thirty or forty feet, when they strike a stratum of gravel, through which the water readily passes off. This practice has been extensively employed at Paris, not merely for carrying off ordinary surface-water, but for the discharge of offensive and deleterious fluids from chemical and manufacturing establishments. A well of this sort received, in the winter of 1832-'33, twenty thousand gallons per day of the foul water from a starch factory, and the same process was largely used in other factories. The apprehension of injury to common and artesian wells and springs led to an investigation on this subject by Girard and Parent Duchatelet, in the latter year. The report of these gentlemen, published in the Annales des Ponts et Chaussees for 1833, second half-year, is full of curious and instructive facts respecting the position and distribution of the subterranean waters under and near Paris; but it must suffice to say that the report came to the conclusion that, in consequence of the absolute immobility of these waters, and the relatively small quantity of noxious fluid to be conveyed to them, there was no danger of the diffusion of such fluid if discharged into them. This result will not surprise those who know that, in another work, Duchatelet maintains analogous opinions as to the effect of the discharge of the city sewers into the Seine or the waters of that river. The quantity of matter delivered by them he holds to be so nearly infinitesimal, as compared with the volume of water of the river, that it cannot possibly affect it to a sensible degree, and therefore cannot render the Seine water unfit for drinking.
Meteorological Effects of Draining
The draining of lakes diminishes the water-surface of the soil, and consequently, in many cases, the evaporation from it, as well as the refrigeration which attends all evaporation. On the other hand, if the volume of water abstracted is great, its removal deprives its basin of an equalizing and moderating influence; for large bodies of water take very slowly the temperature of the air in contact with their surface, and are almost constantly either sending off heat into the atmosphere or absorbing heat from it. Besides, as we have seen, lakes in elevated positions discharge more or less water by infiltration, and contribute it by the same process to other lakes, to springs, and to rivulets, at lower levels. Hence the draining of lakes, on a considerable scale, must modify both the humidity and the temperature of the atmosphere of the neighboring regions, and the permanent supply of ground-water for the lands lying below them.
Meteorological Action of Marshes
The shallow water of marshes, indeed, performs this latter function, but, under ordinary circumstances, marshes exercise in but a very small degree the compensating meteorological action which I have ascribed to large expansions of deeper water. The direct rays of the sun and the warmth of the atmosphere penetrate to the soil beneath, and raise the temperature of the water which covers it; and there is usually a much greater evaporation from marshes than from lakes in the same region during the warmer half of the year. This evaporation implies refrigeration, and consequently the diminution of evaporation by the drainage of swamps tends to prevent the lowering of the atmospheric temperature, and to lessen the frequency and severity of frosts. Accordingly it is a fact of experience that, other things being equal, dry soils, and the air in contact with them, are perceptibly warmer during the season of vegetation, when evaporation is most rapid, than moist lands and the atmospheric stratum resting upon them. Instrumental observation on this special point has not yet been undertaken on a large scale, but still we have thermometric data sufficient to warrant the general conclusion, and the influence of drainage in diminishing the frequency of frost appears to be even better established than a direct increase of atmospheric temperature. The steep and dry uplands of the Green Mountain range in New England often escape frosts when the Indian-corn harvest on moister grounds, five hundred or even a thousand feet lower, is destroyed or greatly injured by them. The neighborhood of a marsh is sure to be exposed to late spring and early autumnal frosts, but they cease to be feared after it is drained, and this is particularly observable in very cold climates, as, for example, in Lapland.
In England, under-drains are not generally laid below the reach of daily variations of temperature, or below a point from which moisture, if not carried off by the drains, might be brought to the surface by capillary attraction, and evaporated by the heat of the sun. They, therefore, like surface-drains, withdraw from local solar action much moisture which would otherwise be vaporized by it, and, at the same time, by drying the soil above them, they increase its effective hygroscopicity, and it consequently absorbs from the atmosphere a greater quantity of water than it did when, for want of under-drainage, the subsoil was always humid, if not saturated. Under-drains, then, contribute to the dryness as well as to the warmth of the atmosphere, and, as dry ground is more readily heated by the rays of the sun than wet, they tend also to raise the mean, and especially the summer, temperature of the soil.
Effects of Draining Lake of Haarlem
The meteorological influence of the draining of lakes and of humid soils has not, so far as I know, received much attention from experimental physicists; but we are not altogether without direct proof in support of theoretical and a priori conclusions. Thermometrical observations have been regularly made at Zwanenburg, near the northern extremity of the Lake of Haarlem, for more than a century; and since 1845 a similiar registry has been kept at the Helder, forty or fifty miles more to the north. In comparing these two series of observations, it is found that towards the end of 1852, when the draining of the lake was finished, and the following summer had completely dried the newly exposed soil--and, of course, greatly diminished the water-surface--a change took place in the relative temperature of those two stations. Taking the mean of each successive period of five days, from 1845 to 1852, both inclusive, the temperature of Zwanenburg was thirty-three hundredths of a degree centigrade LOWER than at the Helder. From the end of 1852 the thermometer at Zwanenburg has stood, from the 11th of April to the 20th of September, twenty-two hundredths of a degree HIGHER than that at Helder; but from the 14th of October to the 17th of March, it has marked one-tenth of a degree LOWER than its mean between the same dates before 1853.
There is no reason to doubt that these differences are due to the draining of the lake. In summer, solar irradiation has acted more powerfully on the now exposed earth and of course on the air in contact with it; and there is no longer a large expanse of water still retaining and of course imparting something of the winter temperature; in winter, the earth has lost more heat by radiation than when covered by water and the influence of the lake, as a reservoir of warmth accumulated in summer and gradually given out in winter, was of course lost by its drainage. Doubtless the quantity of moisture contained in the atmosphere has been modified by the same cause, but it does not appear that observations have been made upon this point. Facts lately observed by Glaisher tend to prove an elevation of not far from two degrees in the mean temperature of England during the course of the last hundred years. For reasons which I have explained elsewhere, the early observations upon which these conclusions are founded do not deserve entire confidence; but admitting the fact of the alleged elevation, its most probable explanation would be found in the more thorough draining of the soil by superficial and by subterranean conduits.
So far as respects the immediate improvement of soil and climate, and the increased abundance of the harvests, the English system of surface and subsoil drainage has fully justified the eulogiums of its advocates; but its extensive adoption appears to have been attended with some altogether unforeseen and undesirable consequences, very analogous to those which I have described as resulting from the clearing of the forests. The under-drains carry off very rapidly the water imbibed by the soil from precipitation, and through infiltration from neighboring springs or other sources of supply. Consequently, in wet seasons, or after heavy rains, a river bordered by artificially drained lands receives in a few hours, from superficial and from subterranean conduits, an accession of water which, in the natural state of the earth, would have reached it only by small instalments after percolating through hidden paths for weeks or even months, and would have furnished perennial and comparatively regular contributions, instead of swelling deluges, to its channel. Thus, when human impatience rashly substitutes swiftly acting artificial contrivances for the slow methods by which nature drains the surface and superficial strata of a river-basin, the original equilibrium is disturbed, the waters of the heavens are no longer stored up in the earth to be gradually given out again, but are hurried out of man's domain with wasteful haste; and while the inundations of the river are sudden and disastrous, its current, when the drains have run dry, is reduced to a rivulet, it ceases to supply the power to drive the machinery for which it was once amply sufficient, and scarcely even waters the herds that pasture upon its margin.
The water of subterranean currents and reservoirs, as well as that of springs and common wells, is doubtless principally furnished by infiltration, and hence its quantity must vary with every change of natural surface which tends to accelerate or to retard the drainage of the surface-soil. The drainage of marshes, therefore, and all other methods of drying the superficial strata, whether by open ditches or by underground tubes or drains, has the same effect as clearing off the forest in depriving the subterranean waters of accessions which they would otherwise receive by infiltration, and in proportion as the sphere of such operation is extended, their influence will make itself felt in the diminished supply of water in springs and wells.
Geographical and Meteorological Effects of Aqueducts, Reservoirs, and Canals
Many of the great processes of internal improvement, such as aqueducts for the supply of great cities, railroad cuts and embankments, and the like, divert water from its natural channels and affect its distribution and ultimate discharge. The collecting of the waters of a considerable district into reservoirs, to be thence carried off by means of aqueducts, as, for example, in the forest of Belgrade, near Constantinople, deprives the grounds originally watered by the springs and rivulets of the necessary moisture, and reduces them to barrenness. Similar effects must have followed from the construction of the numerous aqueducts which supplied ancient Rome with such a profuse abundance of water. On the other hand, the filtration of water through the banks or walls of an aqueduct carried upon a high level across low ground, often injures the adjacent soil, and is prejudicial to the health of the neighboring population; and it has been observed in Switzerland and elsewhere, that fevers have been produced by the stagnation of the water in excavations from which earth had been taken to form embankments for railways.
If we consider only the influence of physical improvements on civilized life, we shall perhaps ascribe to navigable canals a higher importance, or at least a more diversified influence, than to aqueducts or to any other works of man designed to control the waters of the earth, and to affect their distribution. They bind distant regions together by social ties, through the agency of the commerce they promote; they facilitate the transportation of military stores and engines, and of other heavy material connected with the discharge of the functions of government; they encourage industry by giving marketable value to raw material and to objects of artificial elaboration which would otherwise be worthless on account of the cost of conveyance; they supply from their surplus waters means of irrigation and of mechanical power; and, in many other ways, they contribute much to advance the prosperity and civilization of nations. Nor are they wholly without geographical importance. They sometimes drain lands by conveying off water which would otherwise stagnate on the surface, and, on the other hand, like aqueducts, they render the neighboring soil cold and moist by the percolation of water through their embankments; they dam up, check, and divert the course of natural currents, and deliver them at points opposite to, or distant from, their original outlets; they often require extensive reservoirs to feed them thus retaining through the year accumulations of water--which would otherwise run off, or evaporate in the dry season--and thereby enlarging the evaporable surface of the country; and we have already seen that they interchange the flora and the fauna of provinces widely separated by nature. All these modes of action certainly influence climate and the character of terrestrial surface, though our means of observation are not yet perfected enough to enable us to appreciate and measure their effects.
Antiquity of Irrigation
We know little of the history of the extinct civilizations which preceded the culture of the classic ages, and no nation has, in modern times, spontaneously emerged from barbarian and created for itself the arts of social life. The improvements of the savage races whose history we can distinctly trace are borrowed and imitative, and our theories as to the origin and natural development of industrial art are conjectural. Of course, the relative antiquity of particular branches of human industry depends much upon the natural character of soil, climate, and spontaneous vegetable and animal life in different countries; and while the geographical influence of man would, under given circumstances, be exerted in one direction, it would, under different conditions, act in an opposite or a diverging line. I have given some reasons for thinking that in the climates to which our attention has been chiefly directed, man's first interference with the natural arrangement and disposal of the waters was in the way of drainage of surface. But if we are to judge from existing remains alone, we should probably conclude that irrigation is older than drainage; for, in the regions regarded by general tradition as the cradle of the human race, we find traces of canals evidently constructed for the former purpose at a period long preceding the ages of which we have any written memorials. There are, in ancient Armenia, extensive districts which were already abandoned to desolation at the earliest historical epoch, but which, in a yet remoter antiquity, had been irrigated by a complicated and highly artificial system of canals, the lines of which can still be followed; and there are, in all the highlands where the sources of the Euphrates rise, in Persia, in Egypt, in India, and in China, works of this sort which must have been in existence before man had begun to record his own annals.
In warm countries, such as most of those just mentioned, the effects I have described as usually resulting from the clearing of the forests would very soon follow. In such climates, the rains are inclined to be periodical; they are also violent, and for these reasons the soil would be parched in summer and liable to wash in winter. In these countries, therefore, the necessity for irrigation must soon have been felt, and its introduction into mountainous regions like Armenia must have been immediately followed by a system of terracing, or at least scarping the hillsides. Pasture and meadow, indeed, may be irrigated even when the surface is both steep and irregular, as may be observed abundantly on the Swiss as well as on the Piedmontese slope of the Alps; but in dry climates, ploughland and gardens on hilly grounds require terracing, both for supporting the soil and for administering water by irrigation, and it should be remembered that terracing, of itself, even without special arrangements for controlling the distribution of water, prevents or at least checks the flow of rain-water, and gives it time to sink into the ground instead of running off over the surface.
The summers in Egypt, in Syria, and in Asia Minor and even Rumelia, are almost rainless. In such climates, the neccssity of irrigation is obvious, and the loss of the ancient means of furnishing it helps to explain the diminished fertility of most of the countries in question. The surface of Palestine, for example, is composed, in a great measure, of rounded limestone hills, once, no doubt, covered with forests. These were partially removed before the Jewish conquest. When the soil began to suffer from drought, reservoirs to retain the waters of winter were hewn in the rock near the tops of the hills, and the declivities were terraced. So long as the cisterns were in good order, and the terraces kept up, the fertility of Palestine was unsurpassed, but when misgovernment and foreign and intestine war occasioned the neglect or destruction of these works--traces of which still meet the traveller's eye at every step,--when the reservoirs were broken and the terrace walls had fallen down, there was no longer water for irrigation in summer, the rains of winter soon washed away most of the thin layer of earth upon the rocks, and Palestine was reduced almost to the condition of a desert.
The course of events has been the same in Idumaea. The observing traveller discovers everywhere about Petra, particularly if he enters the city by the route of Wadi Ksheibeh, very extensive traces of ancient cultivation, and upon the neighboring ridges are the ruins of numerous cisterns evidently constructed to furnish a supply of water for irrigation. In primitive ages, the precipitation of winter in these hilly countries was, in great part, retained for a time in the superficial soil, first by the vegetable mould of the forests, and then by the artificial arrangements I have described. The water imbibed by the earth was partly taken up by direct evaporation, partly absorbed by vegetation, and partly carried down by infiltration to subjacent strata which gave it out in springs at lower levels, and thus a fertility of soil and a condition of the atmosphere were maintained sufficient to admit of the dense population that once inhabited those now arid wastes. At present, the rain-water runs immediately off from the surface and is carried down to the sea, or is drunk up by the sands of the wadis, and the hillsides which once teemed with plenty are bare of vegetation, and seared by the scorching winds of the desert.
In fact, climatic conditions render irrigation a necessity in all the oriental countries which have any importance in ancient or in modern history, and there can be no doubt that this diffusion of water over large surfaces has a certain reaction on climate. Some idea of the extent of artificially watered soil in India may be formed from the fact that in fourteen districts of the Presidency of Madras, not less than 43,000 reservoirs, constructed by the ancient native rulers for the purpose of irrigation, are now in use, and that there are in those districts at least 10,000 more which are in ruins and useless. These reservoirs are generally formed by damming the outlets of natural valleys; and the dams average half a mile in length, though some of them are thirty miles long and form ponds covering from 37,000 to 50,000 acres. The areas of these reservoirs alone considerably increase the water-surface, and each one of them irrigates an extent of cultivated ground much larger than itself. Hence there is a great augmentation of humid surface from those constructions.
The cultivable area of Egypt, or the space between desert and desert where cultivation would be possible, is now estimated at ten thousand square statute miles. Much of the surface, though not out of the reach of irrigation, lies too high to be economically watered, and irrigation and cultivation are therefore at present confined to an area of seven thousand square miles, nearly the whole of which is regularly and constantly watered when not covered by the inundation, except in the short interval between the harvest and the rise of the waters. For nearly half of the year, then, irrigation adds seven thousand square miles to the humid surface of the Nile valley, or, in other words, more than decuples the area from which an appreciable quantity of moisture would otherwise be evaporated; for after the Nile has retired within its banks, its waters by no means cover one-tenth of the space just mentioned.
The Nile receives not a single tributary in its course below Khartoum; there is not so much as one living spring in the whole land, and, with the exception of a narrow strip of coast, where the annual precipitation is said to amount to six inches, the fall of rain in the territory of the Pharaohs is not two inches in the year. The subsoil of the whole valley is pervaded with moisture by infiltration from the Nile, and water can everywhere be found at the depth of a few feet. Were irrigation suspended, and Egypt abandoned, as in that case it must be, to the operations of nature, there is no doubt that trees, the roots of which penetrate deeply, would in time establish themselves on the deserted soil, fill the valley with verdure, and perhaps at last temper the climate, and even call down abundant rain from the heavens. But the immediate effect of discontinuing irrigation would be, first, an immense reduction of the evaporation from the valley in the dry season, and then a greatly augmented dryness and heat of the atmosphere. Even the almost constant north wind--the strength of which would be increased in consequence of these changes--would little reduce the temperature of the narrow cleft between the burning mountains which hem in the channel of the Nile, so that a single year would transform the most fertile of soils to the most barren of deserts, and render uninhabitable a territory that irrigation makes capable of sustaining as dense a population as has ever existed in any part of the world. Whether man found the valley of the Nile a forest, or such a waste as I have just described, we do not historically know. In either case, he has not simply converted a wilderness into a garden, but has unquestionably produced extensive climatic change.
The fields of Egypt are more regularly watered than those of any other country bordering on the Mediterranean, except the rice-grounds in Italy, and perhaps the marcite or winter meadows of Lombardy; but irrigation is more or less employed throughout almost the entire basin of that sea, and is everywhere attended with effects which, if less in degree, are analogous in character, to those resulting from it in Egypt.
There are few things in European husbandry which surprise English or American observers so much as the extent to which irrigation is employed in agriculture, and that, too, on soils, and with a temperature, where their own experience would have led them to suppose it would be injurious to vegetation rather than beneficial to it. In Switzerland, for example, grass-grounds on the very borders of glaciers are freely irrigated, and on the Italian slope of the Alps water is applied to meadows at heights exceeding 6,000 feet. The summers in Northern Italy, though longer, are very often not warmer than in the Northern United States; and in ordinary years, the summer rains are as frequent and as abundant in the former country as in the latter. Yet in Piedmont and Lombardy irrigation is bestowed upon almost every crop, while in our Northern States it is never employed at all in farming husbandry, or indeed for any purpose except in kitchen-gardens, and possibly, in rare cases, in some other small branch of agricultural industry.
In general, it may be said that irrigation is employed only in the seasons when the evaporating power of the sun and the capacity of the air for absorbing humidity are greatest, or, in other words, that the soil is nowhere artificially watered except when it is so dry that little moisture would be evaporated from it, and, consequently, every acre of irrigated ground is so much added to the evaporable surface of the country. When the supply of water is unlimited, it is allowed, after serving its purpose on one field, to run into drains, canals, or rivers. But in most regions where irrigation is regularly employed, it is necessary to economize the water; after passing over or through one parcel of ground, it is conducted to another; no more is usually withdrawn from the canals at anyone point than is absorbed by the soil it irrigates, or evaporated from it, and, consequently, it is not restored to liquid circulation, except by infiltration or precipitation. We are safe, then, in saying that the humidity evaporated from any artificially watered soil is increased by a quantity bearing a large proportion to the whole amount distributed over it, for most even of that which is absorbed by the earth is immediately given out again either by vegetables or by evaporation; and the hygrometrical and thermometrical condition of the atmosphere in irrigated countries is modified proportionally to the extent of the practice.
It is not easy to ascertain precisely either the extent of surface thus watered, or the amount of water supplied, in any given country, because these quantities vary with the character of the season; but there are not many districts in Southern Europe where the management of the arrangements for irrigation is not one of the most important branches of agricultural labor. The eminent engineer Lombardini describes the system of irrigation in Lombardy as, "every day in summer, diffusing over 550,000 hectares [1,375,000 acres] of land 45,000,000 cubic metres [nearly 600,000,000 cubic yards] of water, which is equal to the entire volume of the Seine, at an ordinary flood, or a rise of three metres above the hydrometer at the bridge of La Tournelle at Paris."
Niel states the quantity of land irrigated in the former kingdom of Sardinia, including Savoy, in 1856, at 240,000 hectares, or not much Ices than 600,000 acres. This is about four-thirteenths of the cultivable soil of the kingdom. According to the same author, the irrigated lands in Franco did not exceed 100,000 hectares, or 247,000 acres, while those in Lombardy amounted to 450,000 hectares, more than 1,100,000 acres.
In these three states alone, then, there were more than three thousand square miles of artificially watered land, and if we add the irrigated soils of the rest of Italy, of the Mediterranean islands, of the Spanish peninsula, of Turkey in Europe and in Asia Minor, of Syria, of Egypt and the remainder of Northern Africa, we shall see that irrigation increases the evaporable surface of the Mediterranean basin by a quantity bearing no inconsiderable proportion to the area naturally covered by water within it.
Arrangements are concluded, and new plans proposed, for an immense increase of the lands fertilized by irrigation in France and in Belgium, as well as in Spain and Italy, and there is every reason to believe that the artificially watered soil of the latter country will be doubled, that of France quadrupled, before the end of this century. There can be no doubt that by these operations man is exercising a powerful influence on the soil, on vegetable and animal life, and on climate, and hence that in this, as in many other fields of industry, he is truly a geographical agency.
The rice-grounds and the marcite of Lombardy are not included in these estimates of the amount of water applied. The meteorological effect of irrigation on a large scale, which would seem prima facie most probable, would be an increase of precipitation in the region watered. Hitherto scientific observation has recorded no such increase, but in a question of so purely local a character, we must ascribe very great importance to a consideration which I have noticed elsewhere, but which, has been frequently overlooked by meteorologists, namely, that vapors exhaled in one district may very probably be condensed and precipitated in another very distant from their source. If then it were proved that an extension of irrigated soil was not followed by an increase of rain-fall in the same territory, the probability that the precipitation was augmented SOMEWHERE would not be in the least diminished.
But though we cannot show that in the irrigated portions of Italy the summer rain is more abundant than it was before irrigation was practised--for we know nothing of the meteorological conditions of that country at so remote a period--the fact that there is a very considerable precipitation in the summer months in Lombardy is a strong argument in favor of such increase. In the otherwise similar climate of Rumelia and of much of Asia Minor, irrigation is indeed practiced, but in a relatively small proportion. In those provinces there is little or no summer rain. Is it not highly probable that the difference between Italy and Turkey in this respect is to be ascribed, in part at least, to extensive irrigation in the former country, and the want of it in the latter It is true that, in its accessible strata, the atmosphere of Lombardy is extremely dry during the period of irrigation, but it receives an immense quantity of moisture by the evaporation from the watered soil, and the rapidity with which the aqueous vapor is carried up to higher regions--where, if not driven elsewhere by the wind, it would be condensed by the cold into drops of rain or at least visible clouds--is the reason why it is so little perceptible in the air near the ground.
But the question of an influence on temperature rests on a different ground; for though the condensation of vapor may not take place within days of time and degrees of distance from the hour and the place where it was exhaled from the surface, a local refrigeration must necessarily accompany a local evaporation. Hence, though the summer temperature of Lombardy is high, we are warranted in affirming that it must have been still higher before the introduction of irrigation, and would again become so if that practice were discontinued.
The quantity of water artificially withdrawn from running streams for the purpose of irrigation is such as very sensibly to affect their volume, and it is, therefore, an important element in the geography of rivers. Brooks of no trifling current are often wholly diverted from their natural channels to supply the canals, and their entire mass of water is completely absorbed or evaporated, so that only such proportion as is transmitted by infiltration reaches the river they originally fed. Irrigation, therefore, diminishes great rivers in warm countries by cutting off their sources of supply as well as by direct abstraction of water from their main channels. We have just seen that the system of irrigation in Lombardy deprives the Po of a quantity of water equal to the total delivery of the Seine at ordinary flood, or, in other words, of the equivalent of a tributary navigable for hundreds of miles by vessels of considerable burden.
The new canals executed and projected will greatly increase the loss. The water required for irrigation in Egypt is less than would be supposed from the exceeding rapidity of evaporation in that arid climate; for the soil is thoroughly saturated during the inundation, and infiltration from the Nile continues to supply a considerable amount of humidity in the dryest season. Linant Bey computed that, in the Delta, fifteen and one-third cubic yards per day sufficed to irrigate an acre. If we suppose water to be applied for one hundred and fifty days during the season of growth, this would be equivalent to a total precipitation of about seventeen inches and one-third. Taking the area of actually cultivated soil in Egypt at the estimate of 4,500,000 acres, and the average amount of water daily applied in both Upper and Lower Egypt at twelve hundredths of an inch in depth, we have an abstraction of about 74,000,000 cubic yards, which--the mean daily delivery of the Nile being in round numbers 320,000,000 cubic yards--is twenty-three per cent of the average quantity of water contributed to the Mediterranean by that river.
In estimating the effect of this abstraction of water upon the volume of great rivers, especially in temperate climates and in countries with a hilly surface, we must remember that all the water thus withdrawn--except that which is absorbed by vegetation, that which enters into new inorganic compounds, and that which is carried off by evaporation--is finally restored to the original current by superficial flow or by infiltration. It is generally estimated that from one-third to one-half of the water applied to the fields is absorbed by the earth, and this, with the deductions just given, is returned to the river by direct infiltration, or descends through invisible channels to moisten lower grounds, and thence in part escapes again into the bed of the river, by similar conduits, or in the form of springs and rivulets. Interesting observations have lately been made on this subject in France and important practical results arrived at. It was maintained that mountain irrigation is not ultimately injurious to that of the plains below, because lands liberally watered in the spring, when the supply is abundant, act as reservoirs, storing up by absorption water which afterwards filters down to lower grounds or escapes into the channel of the river and keeps up its current in the dry summer months, so as to compensate for what, during those months, is withdrawn from it for upland irrigation. Careful investigation showed that though this proposition is not universally true, it is so in many cases, and there can be no doubt that the loss in the volume of rivers by the abstraction of water for irrigation is very considerably less than the measure of the quantity withdrawn. Irrigation, as employed for certain special purposes in Europe and America, is productive of very prejudicial climatic effects. I refer particularly to the cultivation of rice in the Southern States of the American Union and in Italy. The climate of the Southern States is in general not necessarily unhealthy for the white man, but he can scarcely sleep a single night in the vicinity of the rice-grounds without being attacked by a dangerous fever. The neighborhood of the rice-fields is possibly less pestilential in Lombardy and Piedmont than in South Carolina and Georgia, but still very insalubrious to both man and beast. "Not only does the population decrease where rice is grown," says Escourron-Milliago, "but even the flocks are attacked by typhus. In the rice-grounds the soil is divided into compartments rising in gradual succession to the level of the irrigating canal, in order that the water, after having flowed one field, may be drawn off to another, and thus a single current serve for several compartments, the lowest field, of course, still being higher than the ditch which at last drains both it and the adjacent soil. This arrangement gives a certain force of hydrostatic pressure to the water with which the rice is irrigated, and the infiltration from these fields is said to extend through neighboring grounds, sometimes to the distance of not less than a myriametre, or six English miles, and to be destructive to crops and even trees reached by it. Land thus affected can no longer be employed for any purpose but growing rice, and when prepared for that crop, it propagates still further the evils under which it had itself suffered, and, of course, the mischief is a growing one."
Salts deposited by Water of Irrigation
The attentive traveller in Egypt and Nubia cannot fail to notice many localities, generally of small extent, where the soil is rendered infertile by an excess of saline matter in its composition. In many cases, perhaps in all, these barren spots lie rather above the level usually flooded by the inundations of the Nile, and yet they exhibit traces of former cultivation. Observations in India suggest a possible explanation of this fact. A saline efflorescence called "Reh" and "Kuller" is gradually invading many of the most fertile districts of Northern and Western India, and changing them into sterile deserts. It consists principally of sulphate of soda (Glauber's salts), with varying proportions of common salt. These salts (which in small quantities are favorable to fertility of soil) are said to be the gradual result of concentration by evaporation of river and canal waters, which contain them in very minute quantities, and with which the lands are either irrigated or occasionally overflowed. The river inundations in hot countries usually take place but once in a year, and, though the banks remain submerged for days or even weeks, the water at that period, being derived principally from rains and snows, must be less highly charged with mineral matter than at lower stages, and besides, it is always in motion. The water of irrigation, on the other hand, is applied for many months in succession, it is drawn from rivers and canals at the seasons when the proportion of salts is greatest, and it either sinks into the superficial soil, carrying with it the saline substances it holds in solution, or is evaporated from the surface, leaving them upon it. Hence irrigation must impart to the soil more salts than natural inundation. The sterilized grounds in Egypt and Nubia lying above the reach of the floods, as I have said, we may suppose them to have been first cultivated in that remote antiquity when the Nile valley received its earliest inhabitants, and when its lower grounds were in the condition of morasses. They must have been artificially irrigated from the beginning; they may have been under cultivation many centuries before the soil at a lower level was invaded by man, and hence it is natural that they should be more strongly impregnated with saline matter than fields which are exposed every year, for some weeks, to the action of running water so nearly pure that it would be more likely to dissolve salts than to deposit them.
I have frequently alluded to a branch of physical geography, the importance of which is but recently adequately recognized--the subterranean waters of the earth considered as stationary reservoirs, as flowing currents, and as filtrating fluids. The earth drinks in moisture by direct absorption from the atmosphere, by the deposition of dew, by rain and snow, by percolation from rivers and other superficial bodies of water, and sometimes by currents flowing into caves or smaller visible apertures. Some of this humidity is exhaled again by the soil, some is taken up by organic growths and by inorganic compounds, some poured out upon the surface by springs and either immediately evaporated or carried down to larger streams and to the sea, some flows by subterranean courses into the bed of fresh-water rivers or of the ocean, and some remains, though even here not in forever motionless repose, to fill deep cavities and underground channels. In every case the aqueous vapors of the air are the ultimate source of supply and all these hidden stores are again returned to the atmosphere by evaporation.
The proportion of the water of precipitation taken up by direct evaporation from the surface of the ground seems to have been generally exaggerated, sufficient allowance not being made for moisture carried downwards or in a lateral direction by infiltration or by crevices in the superior rocky or earthy strata. According to Wittwer, Mariotte found that but one-sixth of the precipitation in the basin of the Seine was delivered into that sea by the river, "so that five-sixths remained for evaporation and consumption by the organic world." Maury estimates the annual amount of precipitation in the valley of the Mississippi at 620 cubic miles, the discharge of that river into the sea at 107 cubic miles, and concludes that "this would leave 513 cubic miles of water to be evaporated from this river-basin annually." In these and other like computations, the water carried down into the earth by capillary and larger conduits is wholly lost sight of, and no thought is bestowed upon the supply for springs, for common and artesian wells, and for underground rivers, like those in the great caves of Kentucky, which may gush up in fresh-water currents at the bottom of the Caribbean Sea, or rise to the light of day in the far-off peninsula of Florida.
The progress of the emphatically modern science of geology has corrected these erroneous views, because the observations on which it depends have demonstrated not only the existence, but the movement, of water in nearly all geological formations, have collected evidence of the presence of large reservoirs at greater or less depths beneath surfaces of almost every character, and have investigated the rationale of the attendant phenomena. The distribution of these waters has been minutely studied with reference to a great number of localities, and though the actual mode and rate of their vertical and horizontal transmission is still involved in much obscurity, the laws which determine their aggregation are so well understood, that, when the geology of a given district is known, it is not difficult to determine at what depth water will be reached by the borer, and to what height it will rise. The same principles have been successfully applied to the discovery of small subterranean collections or currents of water, and some persons have acquired, by a moderate knowledge of the superficial structure of the earth combined with long practice, a skill in the selection of favorable places for digging wells which seems to common observers little less than miraculous. The Abbe Paramelle--a French ecclesiastic who devoted himself for some years to this subject and was extensively employed as a well-finder--states, in his work on Fountains, that in the course of thirty-four years he had pointed out more than ten thousand subterranean springs; and though his geological speculations were often erroneous, high scientific authorities have testified to the great practical value of his methods, and the general accuracy of his predictions. Hydrographical researches have demonstrated the existence of subterranean currents and reservoirs in many regions where superficial geology had not indicated their probable presence. Thus, a much larger proportion of the precipitation in the valley of the Tiber suddenly disappears than can be accounted for by evaporation and visible flow into the channel of the river. Castelli suspected that the excess was received by underground caverns, and slowly conducted by percolation to the bed of the Tiber. Lombardini--than whom there is no higher authority--concludes that the quantity of water gradually discharged into the river by subterranean conduits, is not less than three-quarters of the total delivery of its basin. What is true of the hydrology of the Tiber is doubtless more or less true of that of other rivers, and the immense value of natural arrangements which diminish the danger of sudden floods by retaining a large proportion of the precipitation, and of an excessive reduction of river currents in the droughts of summer, by slowly conducting into their beds water accumulated and stored up in subterranean reservoirs in rainy seasons, is too obvious to require to be dwelt upon. The readiness with which water not obstructed by impermeable strata diffuses itself through the earth in all directions--and consequently, the importance of keeping up the supply of subterranaean reservoirs--find a familiar illustration in the effect of paving the ground about the stems of vines and trees. The surface-earth around the trunk of a tree may be made almost impervious to water, by flagstones and cement, for a distance as great as the spread of the roots; and yet the tree will not suffer for want of moisture, except in droughts severe enough sensibly to affect the supply in deep wells and springs. Both forest and fruit trees attain a considerable age and size in cities where the streets and courts are closely paved, and where even the lateral access of water to the roots is more or less obstructed by deep cellars and foundation walls. The deep-lying veins and sheets of water, supplied by infiltration from often comparatively distant sources, send up moisture by capillary attraction, and the pavement prevents the soil beneath it from losing its humidity by evaporation. Hence, city-grown trees find moisture enough for their roots, and though plagued with smoke and dust, often retain their freshness, while those planted in the open fields, where sun and wind dry up the soil faster than the subterranean fountains can water it, are withering from drought. Without the help of artificial conduit or of water-carrier, the Thames and the Seine refresh the ornamental trees that shade the thoroughfares of London and of Paris, and beneath the hot and reeking mould of Egypt, the Nile sends currents to the extremest border of its valley.
The existence of artesian wells depends upon that of subterranean reservoirs and rivers, and the supply yielded by borings is regulated by the abundance of such sources. The waters of the earth are, in many cases, derived from superficial currents which are seen to pour into chasms opened, as it were, expressly for their reception; and in others, where no apertures in the crust of the earth have been detected, their existence is proved by the fact that artesian wells sometimes bring up from great depths seeds, leaves, and even livin fish, which must have been carried down through channels large enough to admit a considerable stream. But in general, the sheet and currents of water reached by deep boring appear to be primarily due to infiltration from highlands where the water is first collected in superficial or subterranean reservoirs. By means of channels conforming to the dip of the strata, these reservoirs communicate with the lower basins, and exert upon them a fluid pressure sufficient to raise a column to the surface, whenever an orifice is opened. The water delivered by an artesian well is, therefore, often derived from distant sources, and may be wholly unaffected by geographical or meteorological changes in its immediate neighborhood, while the same changes may quite dry up common wells and springs which are fed only by the local infiltration of their own narrow basins.
In most cases, artesian wells have been bored for purely economical or industrial purposes, such as to obtain good water for domestic use or for driving light machinery, to reach saline or other mineral springs, and recently, in America, to open fountains of petroleum or rock-oil. The geographical and geological effects of such abstraction of fluids from the bowels of the earth are too remote and uncertain to be here noticed; but artesian wells have lately been employed in Algeria for a purpose which has even now a substantial, and may hereafter acquire a very great geographical importance. It was observed by many earlier as well as recent travellers in the East, among whom Shaw deserves special mention, that the Libyan desert, bordering upon the cultivated shores of the Mediterranean, appeared in many places to rest upon a subterranean lake at an accessible distance below the surface. The Moors are vaguely said to bore artesian wells down to this reservoir, to obtain water for domestic use and irrigation, and there is evidence that this art was practised in Northern Africa in the Middle Ages. But it had been lost by the modern Moors, and the universal astonishment and incredulity with which the native tribes viewed the operations of the French engineers sent into the desert for that purpose, are a sufficient proof that this mode of reaching the subterranean waters was new to them. They were, however, aware of the existence of water below the sands, and were dexterous in digging wells--square shafts lined with a framework of palm-tree stems--to the level of the sheet. The wells so constructed, though not technically artesian wells, answer the same purpose; for the water rises to the surface and flows over it as from a spring.
These wells, however, are too few and too scanty in supply to serve any other purposes than the domestic wells of other countries, and it is but recently that the transformation of desert into cultivable land by this means has been seriously attempted. The French Government has bored a large number of artesian wells in the Algerian desert within a few years, and the native sheikhs are beginning to avail themselves of the process. Every well becomes the nucleus of a settlement proportioned to the supply of water, and before the end of the year 1860, several nomade tribes had abandoned their wandering life, established themselves around the wells, and planted more than 30,000 palm trees, besides other perennial vegetables. The water is found at a small depth, generally from 100 to 200 feet, and though containing too large a proportion of mineral matter to be acceptable to a European palate, it answers well for irrigation, and does not prove unwholesome to the natives.
The most obvious use of artesian wells in the desert at present is that of creating stations for the establishment of military posts and halting-places for the desert traveller; but if the supply of water shall prove adequate for the indefinite extension of the system, it is probably destined to produce a greater geographical transformation than has ever been effected by any scheme of human improvement.
The most striking contrast of landscape scenery that nature brings near together in time or place, is that between the greenery of the tropics, or of a northern summer, and the snowy pall of leafless winter. Next to this in startling novelty of effect, we must rank the sudden transition from the shady and verdant oasis of the desert to the bare and burning party-colored ocean of sand and rock which surrounds it. The most sanguine believer in indefinite human progress hardly expects that man's cunning will accomplish the universal fulfilment of the prophecy, "the desert shall blossom as the rose," in its literal sense; but sober geographers have thought the future conversion of the sand plains of Northern Africa into fruitful gardens, by means of artesian wells, not an improbable expectation. They have gone farther, and argued that, if the soil were covered with fields and forests, vegetation would call down moisture from the Libyan sky, and that the showers which are now wasted on the sea, or so often deluge Southern Europe with destructive inundation, would in part be condensed over the arid wastes of Africa, and thus, without further aid from man, bestow abundance on regions which nature seems to have condemned to perpetual desolation.
An equally bold speculation, founded on the well-known fact that the temperature of the earth and of its internal waters increases as we descend beneath the surface, has suggested that artesian wells might supply heat for industrial and domestic purposes, for hot-house cultivation, and even for the local amelioration of climate. The success with which Count Lardarel has employed natural hot springs for the evaporation of water charged with boracic acid, and other fortunate applications of the heat of thermal sources, lend some countenance to the latter project; but both must, for the present, be ranked among the vague possibilities of science, not regarded as probable future triumphs of man over nature.
A more plausible and inviting scheme is that of the creation of perennial springs by husbanding rain and snow water, storing it up in artificial reservoirs of earth, and filtering it through purifying strata, in analogy with the operations of nature. The sagacious Palissy--starting from the theory that all springs are primarily derived from precipitation, and reasoning justly on the accumulation and movement of water in the earth--proposed to reduce theory to practice, and to imitate the natural processes by which rain is absorbed by the earth and given out again in running fountains. "When I had long and diligently considered the cause of the springing of natural fountains and the places where they be wont to issue," says he, "I did plainly perceive, at last, that they do proceed and are engendered of nought but the rains. And it is this, look you, which hath moved me to enterprise the gathering together of rain-water after the manner of nature, and the most closely according to her fashion that I am able; and I am well assured that by following the formulary of the Supreme Contriver of fountains, I can make springs, the water whereof shall be as good and pure and clear as of such which be natural." Palissy discusses the subject of the origin of springs at length and with much ability, dwelling specially on infiltration, and, among other things, thus explains the frequency of springs in mountainous regions: "Having well considered the which, thou mayest plainly see the reason why there be more springs and rivulets proceeding from the mountains than from the rest of the earth; which is for no other cause but that the rocks and mountains do retain the water of the rains like vessels of brass. And the said waters falling upon the said mountains descend continually through the earth, and through crevices, and stop not till they find some place that is bottomed with stone or close and thick rocks; and they rest upon such bottom until they find some channel or other manner of issue, and then they flow out in springs or brooks or rivers, according to the greatness of the reservoirs and of the outlets thereof."
After a full exposition of his theory, Palissy proceeds to describe his method of creating springs, which is substantially the same as that lately proposed by Babinet in the following terms: "Choose a piece of ground containing four or five acres, with a sandy soil, and with a gentle slope to determine the flow of the water. Along its upper line, dig a trench five or six feet deep and six feet wide. Level the bottom of the trench, and make it impermeable by paving, by macadamizing, by bitumen, or, more simply and cheaply, by a layer of clay. By the side of this trench dig another, and throw the earth from it into the first, and so on until you have rendered the subsoil of the whole parcel impermeable to rain-water. Build a wall along the lower line with an aperture in the middle for the water, and plant fruit or other low trees upon the whole, to shade the ground and check the currents of air which promote evaporation. This will infallibly give you a good spring which will flow without intermission, and supply the wants of a whole hamlet or a large chateau." Babinet states that the whole amount of precipitation on a reservoir of the proposed area, in the climate of Paris, would be about 13,000 cubic yards, not above one half of which, he thinks, would be lost, and, of course, the other half would remain available to supply the spring. I much doubt whether this expectation would be realized in practice, in its whole extent; for if Babinet is right in supposing that the summer rain is wholly evaporated, the winter rains, being much less in quantity, would hardly suffice to keep the earth saturated and give off so large a surplus. The method of Palissy, though, as I have said, similar in principle to that of Babinet, would be cheaper of execution, and, at the same time, more efficient. He proposes the construction of relatively small filtering receptacles, into which he would conduct the rain falling upon a large area of rocky hillside, or other sloping ground not readily absorbing water. This process would, in all probability, be a very successful, as well as an inexpensive, mode of economizing atmospheric precipitation, and compelling the rain and snow to form perennial fountains at will.
The methods suggested by Palissy and by Babinet are of limited application, and designed only to supply a sufficient quantity of water for the domestic use of small villages or large private establishments. Dumas has proposed a much more extensive system for collecting and retaining the whole precipitation in considerable valleys, and storing it in reservoirs, whence it is to be drawn for household and mechanical purposes, for irrigation, and, in short, for all the uses to which the water of natural springs and brooks is applicable. His plan consists in draining both surface and subsoil, by means of conduits differing in construction according to local circumstances, but in the main not unlike those employed in improved agriculture, collecting the water in a central channel, securing its proper filterage, checking its too rapid flow by barriers at convenient points, and finally receiving the whole in spacious, covered reservoirs, from which it may he discharged in a constant flow or at intervals as convenience may dictate.
There is no reasonable doubt that a very wide employment of these various contrivances for economizing and supplying water is practicable, and the expediency of resorting to them is almost purely an economical question. There appears to be no serious reason to apprehend collateral evils from them, and in fact all of them, except artesian wells, are simply indirect methods of returning to the original arrangements of nature, or, in other words, of restoring the fluid circulation of the globe; for when the earth was covered with the forest, perennial springs gushed from the foot of every hill, brooks flowed down the bed of every valley. The partial recovery of the fountains and rivulets which once abundantly watered the face of the agricultural world seems practicable by such means, even without any general replanting of the forests; and the cost of one year's warfare--or in some countries of that armed peace which has been called "Platonic war"--if judiciously expended in a combination of both methods of improvement, would secure, to almost every country that man has exhausted, an amelioration of climate, a renovated fertility of soil, and a general physical improvement, which might almost be characterized as a new creation.
Inundations and Torrents
In pointing out in a former chapter the evils which have resulted from the too extensive destruction of the forests, I dwelt at some length on the increased violence of river inundations, and especially on the devastations of torrents, in countries improvidently deprived of their woods, and I spoke of the replanting of the forests as probably the most effectual method of preventing the frequent recurrence of disastrous floods. There are many regions where, from the loss of the superficial soil, from financial considerations, and from other special causes, the general restoration of the woods is not, under present circumstances, either possible or desirable. In all inhabited countries, the necessities of agriculture and other considerations of human convenience will always require the occupation of much the largest proportion of the surface for purposes inconsistent with the growth of extensive forests. Even where large plantations are possible and in actual process of execution, many years must elapse before the action of the destructive causes in question can be arrested or perhaps even sensibly mitigated by their influence; and besides, floods will always occur in years of excessive precipitation, whether the surface of the soil be generally cleared or generally wooded. Physical improvement in this respect, then, cannot be confined to merely preventive measures, but, in countries subject to damage by inundation, means must be contrived to obviate dangers and diminish injuries to which human life and all the works of human industry will occasionally be exposed, in spite of every effort to lessen the frequency of their recurrence by acting directly on the causes that produce them. As every civilized country is, in some degree, subject to inundation by the overflow of rivers, the evil is a familiar one, and needs no general description. In discussing this branch of the subject, therefore, I may confine myself chiefly to the means that have been or may be employed to resist the force and limit the ravages of floods, which, left wholly unrestrained, would not only inflict immense injury upon the material interests of man, but produce geographical revolutions of no little magnitude.
Inundations of 1856 in France
The month of May, 1856, was remarkable for violent and almost uninterrupted rains, and most of the river-basins of France were inundated to an extraordinary height. In the val-leys of the Loire and its aflluents, about a million of acres, including many towns and villages, were laid under water, and the amount of pecuniary damage was almost incalculable. The flood was not less destructive in the valley of the Rhone, and in fact an invasion by a hostile army could hardly have been more disastrous to the inhabitants of the plains than was this terrible deluge. There had been a flood of this latter river in the year 1840, which, for height and quantity of water, was almost as remarkable as that of 1856, but it took place in the month of November, when the crops had all been harvested, and the injury inflicted by it upon agriculturists was, therefore, of a character to be less severely and less immediately felt than the consequences of the inundation of 1856.
In the fifteen years between these two great floods, the population and the rural improvements of the river valleys had much increased, common roads, bridges, and railways had been multiplied and extended, telegraph lines had been constructed, all of which shared in the general ruin, and hence greater and more diversified interests were affected by the catastrophe of 1856 than by any former like calamity. The great flood of 1840 had excited the attention and roused the sympathies of the French people, and the subject was invested with new interest by the still more formidable character of the inundations of 1856. It was felt that these scourges had ceased to be a matter of merely local concern, for, although they bore most heavily on those whose homes and fields were situated within the immediate reach of the swelling waters, yet they frequently destroyed harvests valuable enough to be a matter of national interest, endangered the personal security of the population of important political centres, interrupted communication for days and even weeks together on great lines of traffic and travel--thus severing, as it were, all South-western France from the rest of the empire--and finally threatened to produce great and permanent geographical changes. The well-being of the whole commonwealth was seen to be involved in preventing the recurrence and in limiting the range of such devastations. The Government encouraged scientific investigation of the phenomena and their laws. Their causes, their history, their immediate and remote consequences, and the possible safeguards to be employed against them, have been carefully studied by the most eminent physicists, as well as by the ablest theoretical and practical engineers of France. Many hitherto unobserved facts have been collected, many new hypotheses suggested, and many plans, more or less original in character, have been devised for combating the evil; but thus far, the most competent judges are not well agreed as to the mode, or even the possibility, of applying an effectual remedy. I have noticed in the next preceding chapter the recent legislation of France upon the preservation and restoration of the forests, with reference to their utility in subduing torrents and lessening the frequency and diminishing the violence of river inundations. The provisions of those laws are preventive rather than remedial, but most beneficial effects have already been experienced from the measures adopted in pursuance of them, though sufficient time has not yet elapsed for the complete execution of the greater operations of the system.
Basins of Reception
Destructive inundations of large rivers are seldom, if ever, produced by precipitation within the limits of the principal valley, but almost uniformly by sudden thaws or excessive rains on the mountain ranges where the tributaries take their rise. It is therefore plain that any measures which shall check the flow of surface-waters into the channels of the affluents, or which shall retard the delivery of such waters into the principal stream by its tributaries, will diminish in the same proportion the dangers and the evils of inundation by great rivers. The retention of the surface-waters upon or in the soil can hardly be accomplished except by the methods already mentioned, replanting of forests, and furrowing or terracing. The current of mountain streams can be checked by various methods, among which the most familiar and obvious is the erection of barriers or dams across their channels, at points convenient for forming reservoirs large enough to retain the superfluous waters of great rains and thaws.
Besides the utility of such basins in preventing floods, the construction of them is recommended by very strong considerations, such as the furnishing of a constant supply of water for agricultural and mechanical purposes, and, also, their value as ponds for breeding and rearing fish, and, perhaps, for cultivating aquatic vegetables.
The objections to the general adoption of the system of reservoirs are these: the expense of their construction and maintenance; the reduction of cultivable area by the amount of surface they must cover; the interruption they would occasion to free communication; the probability that they would soon be filled up with sediment, and the obvious fact that when full of earth, or even water, they would no longer serve their principal purpose; the great danger to which they would expose the country below them in case of the bursting of their barriers; the evil consequences they would occasion by prolonging the flow of inundations in proportion as they diminished their height; the injurious effects it is supposed they would produce upon the salubrity of the neighbouring districts; and, lastly, the alleged impossibility of constructing artificial basins sufficient in capacity to prevent, or in any considerable measure to mitigate, the evils they are intended to guard against.
The last argument is more easily reduced to a numerical question than the others. The mean and extreme annual precipitation of all the basins where the construction of such works would be seriously proposed is already approximately known by meteorological tables, and the quantity of water, delivered by the greatest floods which have occurred within the memory of man, may be roughly estimated from their visible traces. From these elements, or from meteorological records, the capacity of the necessary reservoirs can be calculated. Let us take the case of the Ardeche. In the inundation of 1857, that river poured into the Rhone 1,305,000,000 cubic yards of water in three days. If we suppose that half this quantity might have been suffered to flow down its channel without inconvenience, we shall have about 650,000,000 cubic yards to provide for by reservoirs. The Ardeche and its principal affluent, the Chassezae, have, together, about twelve considerable tributaries rising near the crest of the mountains which bound the basin. If reservoirs of equal capacity were constructed upon all of them, each reservoir must be able to contain 54,000,000 cubic yards, or, in other words, must be equal to a lake 3,000 yards long, 1,000 yards wide, and 18 yards deep, and besides, in order to render any effectual service, the reservoirs must all have been empty at the commencement of the rains which produced the inundation.
Thus far I have supposed the swelling of the waters to be uniform throughout the whole basin; but such was by no means the fact in the inundation of 1857, for the rise of the Chassezae, which is as large as the Ardeche proper, did not exceed the limits of ordinary floods, and the dangerous excess came solely from the headwaters of the latter stream. Hence reservoirs of double the capacity I have supposed would have been necessary upon the tributaries of that river, to prevent the injurious effects of the inundation. It is evident that the construction of reservoirs of such magnitude for such a purpose is financially, if not physically, impracticable, and when we take into account a point I have just suggested, namely, that the reservoirs must be empty at all times of apprehended flood, and, of course, their utility limited almost solely to the single object of preventing inundations, the total inapplicability of such a measure in this particular case becomes still more glaringly manifest.
Another not less conclusive fact is, that the valleys of all the upland tributaries of the Ardeche descend so rapidly, and have so little lateral expansion, as to render the construction of capacious reservoirs in them quite impracticable. Indeed, engineers have found but two points in the whole basin suitable for that purpose, and the reservoirs admissible at these would have only a joint capacity of about 70,000,000 cubic yards, or less than one-ninth part of what I suppose to be required. The case of the Ardeche is no doubt an extreme one, both in the topographical character of its basin and in its exposure to excessive rains; but all destructive inundations are, in a certain sense, extreme cases also, and this of the Ardeche serves to show that the construction of reservoirs is not by any means to be regarded as a universal panacea against floods.
Nor, on the other hand, is this measure to be summarily rejected. Nature has adopted it on a great scale, on both flanks of the Alps, and on a smaller, on those of the Adirondacks and of many lower chains. The quantity of water which, in great rains or sudden thaws, rushes down the steep declivities of the Alps, is so vast that the channels of the Swiss and Italian rivers would be totally incompetent to carry it off as rapidly as it would pour into them, were it not absorbed by the capacious basins which nature has scooped out for its reception, freed from the transported material which adds immensely both to the volume and to the force of its current, and then, after some reduction by evaporation and infiltration, gradually discharged into the beds of the rivers. In the inundation of 1829 the water discharged into Lake Como from the 15th to the 20th of September amounted to 2,600 cubic yards the second, while the outflow from the lake during the same period was only at the rate of about 1,050 cubic yards to the second. In those five days, then, the lake accumulated 670,000,000 cubic yards of superfluous water, and of course diminished by so much the quantity to be disposed of by the Po. In the flood of October, 1868, the surface of Lago Maggiore was raised twenty-five feet above low-water mark in the course of a few hours. There can be no doubt that without such detention of water by the Lakes Como, Maggiore, Garda, and other subalpine basins, almost the whole of Lombardy would have been irrecoverably desolated, or rather, its great plain would never have become anything but a vast expanse of river-beds and marshes; for the annual floods would always have prevented the possibility of its improvement by man.
Lake Bourget in Savoy, once much more extensive than it is at present, served, and indeed still serves, a similar purpose in the economy of nature. In a flood of the Rhone, in 1863, this lake received from the overflow of that river, which does not pass through it, 72,000,000 cubic yards of water, and of course moderated, to that extent, the effects of the inundation below.
In fact, the alluvial plains which border the course of most considerable streams, and are overflowed in their inundations, either by the rise of the water to a higher level than that of their banks, or by the bursting of their dikes, serve as safety-valves for the escape of their superfluous waters. The current of the Po, spreading over the whole space between its widely separated embankments, takes up so much water in its inundations, that, while a little below the outlet of the Ticino the discharge of the channel is sometimes not less than 19,500 cubic yards to the second, it has never exceeded 6,730 yards at Ponte Lagoscuro, near Ferrara. The currents of the Mississippi, the Rhone, and of many other large rivers, are modified in the same way. In the flood of 1858, the delivery of the Mississippi, a little below the month of the Ohio, was 52,000 cubic yards to the second, but at Baton Rouge, though of course increased by the waters of the Arkansas, the Yazoo, and other smaller tributaries, the discharge was reduced to 46,760 cubic yards. We rarely err when we cautiously imitate the processes of nature, and there are doubtless many cases where artificial basins of reception and lateral expansions of river-beds might be employed with advantage. Many upland streams present points where none of the objections usually urged against artificial reservoirs, except those of expense and of danger from the breaking of dams, could have any application. Reservoirs may be so constructed as to retain the entire precipitation of the heaviest thaws and rains, leaving only the ordinary quantity to flow along the channel; they may be raised to such a height as only partially to obstruct the surface drainage; or they may be provided with sluices by means of which their whole contents can be discharged in the dry season and a summer crop be grown upon the ground they cover at high water. The expediency of employing them and the mode of construction depend on local conditions, and no rules of universal applicability can be laid down on the subject.
It is remarkable that nations which we, in the inflated pride of our modern civilization, so generally regard as little less than barbarian, should have long preceded Christian Europe in the systematic employment of great artificial basins for the various purposes they are calculated to subserve. The ancient Peruvians built strong walls, of excellent workmanship, across the channels of the mountain sources of important streams, and the Arabs executed immense works of similar description, both in the great Arabian peninsula and in all the provinces of Spain which had the good fortune to fall under their sway. The Spaniards of the fifteenth and sixteenth centuries, who, in many points of true civilization and culture, were far inferior to the races they subdued, wantonly destroyed these noble monuments of social and political wisdom, or suffered them to perish, because they were too ignorant to appreciate their value, or too unskilful as practical engineers to be able to maintain them, and some of their most important territories were soon reduced to sterility and poverty in consequence.
Diversion of Rivers
Another method of preventing or diminishing the evils of inundation by torrents and mountain rivers, analogous to that employed for the drainage of lakes, consists in the permanent or occasional diversion of their surplus waters, or of their entire currents, from their natural courses, by tunnels or open channels cut through their banks. Nature, in many cases, resorts to a similar process. Most great rivers divide themselves into several arms in their lower course, and enter the sea by different mouths. There are also cases where rivers send off lateral branches to convey a part of their waters into the channel of other streams. The most remarkable of these is the junction between the Amazon and the Orinoco by the natural canal of the Cassiquiare and the Rio Negro. In India, the Cambodja and the Menam are connected by the Anam; the Saluen and the Irawaddi by the Panlaun. There are similar examples, though on a much smaller scale, in Europe. The Tornea, and the Calix rivers in Lapland communicate by the Tarando, and in Westphalia, the Else, an arm of the Haase, falls into the Weser.
The change of bed in rivers by gradual erosion of their banks is familiar to all, but instances of the sudden abandonment of a primitive channel are by no means wanting. At a period of unknown antiquity, the Ardeche pierced a tunnel 200 feet wide and 100 high, through a rock, and sent its whole current through it, deserting its former bed, which gradually filled up, though its course remained traceable. In the great inundation of 1827, the tunnel proved insufficient for the discharge of the water, and the river burst through the obstructions which had now choked up its ancient channel, and resumed its original course.
It was probably such facts as these that suggested to ancient engineers the possibility of like artificial operations, and there are numerous instances of the execution of works for this purpose in very remote ages. The Bahr Jusef, the great stream which supplies the Fayoum with water from the Nile, has been supposed, by some writers, to be a natural channel; but both it and the Bahr el Wady are almost certainly artificial canals constructed to water that basin, to regulate the level of Lake Meeris, and possibly, also, to diminish the dangers resulting from excessive inundations of the Nile, by serving as waste-weirs to discharge a part of its overflowing waters. Several of the seven ancient mouths of the Nile are believed to be artificial channels, and Herodotus even asserts that King Menes diverted the entire course of that river from the Libyan to the Arabian side of the valley. There are traces of an ancient river-bed along the western mountains, which give eome countenance to this statement. But it is much more probable that the works of Menes were designed rather to prevent a natural, than to produce an artificial, change in the channel of the river.
Two of the most celebrated cascades in Europe, those of the Teverone at Tivoli and of the Velino at Terni, owe, if not their existence, at least their position and character, to the diversion of their waters from their natural beds into new channels, in order to obviate the evils produced by their frequent floods. Remarkable works of the same sort have been executed in Switzerland, in very recent times. Until the year 1714, the Kander, which drains several large Alpine valleys, ran, for a considerable distance, parallel with the Lake of Thun, and a few miles below the city of that name emptied into the river Aar. It frequently flooded the flats along the lower part of its course, and it was determined to divert it into the Lake of Thun. For this purpose, two parallel tunnels were cut through the intervening rock, and the river turned into them. The violence of the current burst up the roof of the tunnels, and, in a very short time, wore the new channel down not less than one hundred feet, and even deepened the former bed at least fifty feet, for a distance of two or three miles above the tunnel. The lake was two hundred feet deep at the point where the river was conducted into it, but the gravel and sand carried down by the Kander has formed at its mouth a delta containing more than a hundred acres, which is still advancing at the rate of several yards a year. The Linth, which formerly sent its waters directly to the Lake of Zurich, and often produced very destructive inundations, was turned into the Wallensee about fifty years ago, and in both these cases a great quantity of valuable land was rescued both from flood and from insalubrity.
In Switzerland, the most terrible inundations often result from the damming up of deep valleys by ice-slips or by the gradual advance of glaciers, and the accumulation of great masses of water above the obstructions. The ice is finally dissolved by the heat of summer or the flow of warm waters, and when it bursts, the lake formed above is discharged almost in an instant, and all below is swept down to certain destruction. In 1595, about a hundred and fifty lives and a great amount of property were lost by the eruption of a lake formed by the descent of a glacier into the valley of the Drance, and a similar calamity laid waste a considerable extent of soil in the year 1818. On this latter occasion, the barrier of ice and snow was 3,000 feet long, 600 thick, and 400 high, and the lake which had formed above it contained not less than 800,000,000 cubic feet. A tunnel was driven through the ice, and about 300,000,000 cubic feet of water safely drawn off by it, but the thawing of the walls of the tunnel rapidly enlarged it, and before the lake was half drained, the barrier gave way and the remaining 500,000,000 cubic feet of water were discharged in half an hour. The recurrence of these floods has since been prevented by directing streams of water, warmed by the sun, upon the ice in the bed of the valley, and thus thawing it before it accumulates in sufficient mass to form a new barrier and threaten serious danger. In the cases of diversion of streams above mentioned, important geographical changes have been directly produced by those operations. By the rarer process of draining glacier lakes, natural eruptions of water, which would have occasioned not less important changes in the face of the earth, have been prevented by human agency. River Embankments. The most obvious and doubtless earliest method of preventing the escape of river-waters from their natural channels, and the overflow of fields and towns by their spread, is that of raised embankments along their course. The necessity of such embankments usually arises from the gradual elevation of the bed of running streams in consequence of the deposit of the earth and gravel they are charged with in high water; and, as we have seen, this elevation is rapidly accelerated when the highlands around the headwaters of rivers are cleared of their forests. When a river is embanked at a given point, and, consequently, the water of its floods, which would otherwise spread over a wide surface, is confined within narrow limits, the velocity of the current and its transporting power are augmented, and its burden of sand and gravel is deposited at some lower point, where the rapidity of its flow is checked by a dam or other artificial obstruction, by a diminution in the inclination of the bed, by a wider channel, or finally by a lacustrine or marine basin which receives its waters. Wherever it lets fall solid material, its channel is raised in consequence, and the declivity of the whole bed between the head of the embankment and the slack of the stream is reduced. Hence the current, at first accelerated by confinement, is afterwards checked by the mechanical resistance of the matter deposited, and by the diminished inclination of its channel, and then begins again to let fall the earth it holds in suspension, and to raise its bed at the point where its overflow had been before prevented by embankment. The bank must now be raised in proportion, and these processes would be repeated and repeated indefinitely, had not nature provided a remedy in floods, which sweep out recent deposits, burst the bonds of the river and overwhelm the adjacent country with final desolation, or divert the current into a new channel, destined to become, in its turn, the scene of a similar struggle between man and the waters.
But here, as in so many other fields where nature is brought into conflict with man, she first resists his attempts at interference with her operations, then, finding him the stronger, quietly submits to his rule, and ends by contributing her aid to strengthen the walls and shackles by which he essays to confine her. If, by assiduous repair of his dikes, he, for a considerable time, restrains the floods of a river within new bounds, nature, by a series of ingenious compensations, brings the fluctuating bed of the stream to a substantially constant level, and when his ramparts have been, by his toil, raised to a certain height and widened to a certain thickness, she, by her laws of gravitation and cohesion, consolidates their material until it becomes almost as hard, as indissoluble, and as impervious as the rock.
But, though man may press the forces of nature into his service, there is a limit to the extent of his dominion over them, and unless future generations shall discover new modes of controlling those forces, or new remedies against their action, he must at last succumb in the struggle. When the marine estuaries and other basins of reception shall be filled up with the sedimentary debris of the mountains, or when the lower course of the rivers shall be raised or prolonged by their own deposits until they have, no longer, such a descent that gravitation and the momentum of the current can overcome the frictional resistance of the bed and banks, the water will, in spite of all obstacles, diffuse itself laterally and for a time raise the level of the champaign land upon its borders, and at last convert it into morasses. It is for this reason that Lombardini advises that a considerable space along the lower course of rivers be left undiked, and the water allowed to spread itself over its banks and gradually raise them by its deposits. This would, indeed, be a palliative, but only a palliative. For the present, however, we have nothing better, and here, as often in political economy, we must content ourselves with "apres nous le deluge," allowing posterity to suffer the penalty of our improvidence and our ignorance, or to devise means for itself to ward off the consequences of them.
The deposit of slime by rivers upon the flats along their banks not only contributes greatly to the fertility of the soil thus flowed, but it subserves a still more important purpose in the general economy of nature. All running streams begin with excavating channels for themselves, or deepening the natural depressions in which they flow; but in proportion as their outlets are raised by the solid material transported by their currents, their velocity is diminished, they deposit gravel and sand at constantly higher and higher points, and so at last elevate, in the middle and lower part of their course, the beds they had previously scooped out. The raising of the channels is compensated in part by the simultaneous elevation of their banks and the flats adjoining them, from the deposit of the finer particles of earth and vegetable mould brought down from the mountains, without which elevation the low grounds bordering all rivers would be, as in many cases they in fact are, mere morasses.
All arrangements which tend to obstruct this process of raising the flats adjacent to the channel, whether consisting in dikes which confine the waters, and, at the same time, augment the velocity of the current, or in other means of producing the last-mentioned effect, interfere with the restorative economy of nature, and at last occasion the formation of marshes where, if left to herself, she might have accumulated inexhaustible stores of the richest soil, and spread them out in plains above the reach of ordinary floods.
Dikes, which, as we have seen, are the means most frequently employed to prevent damage by inundation, are generally parallel to each other and separated by a distance not very much greater than the natural width of the bed. If such walls are high enough to confine the water and strong enough to resist its pressure, they secure the lands behind them from all the evils of inundation except those resulting from filtration; but such ramparts are enormously costly in original construction and in maintenance, and, as has been already shown, the filling up of the bed of the river in its lower course, by sand and gravel, often involves the necessity of incurring new expenditures in increasing the height of the banks. They are attended, too, with some collateral disadvantages. They deprive the earth of the fertilizing deposits of the waters, which are powerful natural restoratives of soils exhausted by cultivation; they accelerate the rapidity and transporting power of the current at high water by confining it to a narrower channel, and it consequently conveys to the sea the earthy matter it holds in suspension, and chokes up harbors with a deposit which it would otherwise have spread over a wider surface; they interfere with roads and the convenience of river navigation, and no amount of cost or care can secure them from occasional rupture, in case of which the rush of the waters through the breach is more destructive than the natural flow of the highest inundation.
For these reasons, many experienced engineers are of opinion that the system of longitudinal dikes is fundamentally wrong, and it has been argued that if the Po, the Adige, and the Brenta had been left unconfined, as the Nile formerly was, and allowed to spread their muddy waters at will, according to the laws of nature, the sediment they have carried to the coast would have been chiefly distributed over the plains of Lombardy. Their banks, it is supposed, would have risen as fast as their beds, the coast-line would not have been extended so far into the Adriatic, and, the current of the streams being consequently shorter, the inclination of their channel and the rapidity of their flow would not have been so greatly diminished. Had man, too, spared a reasonable proportion of the forests of the Alps, and not attempted to control the natural drainage of the surface, the Po, it has been said, would resemble the Nile in all its essential characteristics, and, in spite of the difference of climate, perhaps be regarded as the friend and ally, not the enemy and the invader, of the population which dwells upon its banks.
But it has been shown by Humphreys and Abbot that the system of longitudinal dikes is the only one susceptible of advantageous application to the Mississippi, and if we knew the primitive geography and hydrography of the basin of the Po as well as wo do those of the valley of the great American river, we should very probably find that the condemnation of the plan pursued by the ancient inhabitants of Lombardy is a too hasty generalization, and that the case of the Nile is an exception, not an example of the normal regime and condition of a great river.
But in any event, these theoretical objections are counsels apres coup. The dikes of the Po and probably of some of its tributaries were begun before we have any trustworthy physical or political annals of the provinces they water. The civilization of the valley has accommodated itself to these arrangements, and the interests which might be sacrificed by a change of system are too vast to be hazarded by what, in the present state of our knowledge, can be only considered as a doubtful experiment.
The embankments of the Po, though they are of vast extent and have employed centuries in their construction, are inferior in magnitude to the dikes or levees of the Mississippi, which are the work of scarcely a hundred years, and of a comparatively sparse population. On the right or western bank of the river, the levee extends, with only occasional interruptions from high bluffs and the mouths of rivers, for a distance of more than eleven hundred miles. The left bank is, in general, higher than the right, and upon that side a continuous embankment is not needed; but the total length of the dikes of the Mississippi, including those of the lower course of its tributaries and of its bayous or natural emissaries, is not less than 2,500 miles. They constitute, therefore, not only one of the greatest material achievements of the American people, but one of the most remarkable systems of physical improvement which has been anywhere accomplished in modern times.
Those who condemn the system of longitudinal embankments have often advised that, in cases where that system cannot be abandoned without involving too great a sacrifice of existing interests, the elevation of the dikes should be much reduced, so as to present no obstruction to the lateral spread of extraordinary floods, and that they should be provided with sluices to admit the water without violence whenever they are likely to be overflowed. Where dikes have not been erected, or where they have been reduced in height, it is proposed to construct, at convenient intervals, transverse embankments of moderate height running from the banks of the river across the plains to the hills which bound them. These measures, it is argued, will diminish the violence of inundations by permitting the waters to extend themselves over a greater surface, and by thus retarding the flow of the river currents, will, at the same time, secure the deposit of fertilizing slime upon all the soil covered by the flood.
Rozet, an eminent French engineer, has proposed a method of diminishing the ravages of inundations, which aims to combine the advantages of all other systems, and at the same time to obviate the objections to which they are all more or less liable. The plan of Rozet is recommended by its simplicity and cheapness as well as its facility and rapidity of execution, and is looked upon with favor by many persons very competent to judge in such matters. It is, however, by no means capable of universal application, though it would often doubtless prove highly useful in connection with the measures now employed in South-eastern France. He proposes to commence with the amphitheatres in which mountain torrents so often rise, by covering their slopes and filling their beds with loose blocks of rock, and by constructing at their outlets, and at other narrow points in the channels of the torrents, permeable barriers of the same material promiscuously heaped up, much according to the method employed by the ancient Romans in their northern provinces for a similar purpose. By this means, he supposes, the rapidity of the current would be checked, and the quantity of transported pebbles and gravel--which, by increasing the mechanical force of the water, greatly aggravate the damage by floods--much diminished. When the stream has reached that part of its course where it is bordered by soil capable of cultivation, and worth the expense of protection, he proposes to place along one or both banks, according to circumstances, a line of cubical blocks of stone or pillars of masonry three or four feet high and wide, and at the distance of about eleven yards from each other. The space between the two lines, or between a line and the opposite high bank, would, of course, be determined by observation of the width of the swift-water current at high floods. As an auxiliary measure, small ditches and banks, or low walls of pebbles, should be constructed from the line of blocks across the grounds to be protected, nearly at right angles to the current, but slightly inclining downwards, and at convenient distances from each other. Rozet thinks the proper interval would be 300 yards, and it is evident that, if he is right in his main principle, hedges, rows of trees, or even common fences, would in many cases answer as good a purpose as banks and trenches or low walls. The blocks or pillars of stone would, he contends, check the lateral currents so as to compel them to let fall all their pebbles and gravel in the main channel--where they would be rolled along until ground down to sand or silt--and the transverse obstructions would detain the water upon the soil long enough to secure the deposit of its fertilizing slime. Numerous facts are cited in support of the author's views, and I imagine there are few residents of rural districts whose own observation will not furnish testimony confirmatory of their soundness.
Removal of Obstructions
The removal of obstructions in the beds of rivers dredging the bottom or blasting rocks, the washing out of deposits and locally increasing the depth of water by narrowing the channel by moans of spurs or other constructions projecting from the banks, and, finally, the cutting off of bends and thus shortening the course of the stream, diminishing the resistance of its shores and bottom and giving the bed a more rapid declivity, have all been employed not only to facilitate navigation, but as auxiliaries to more effectual modes of preventing inundations. But a bar removed from one point is almost sure to re-form at the same or another, spurs occasion injurious eddies and unforeseen diversions of the current, and the cutting off of bends, though occasionally effected by nature herself, and sometimes advantageous in torrential streams whose banks are secured by solid walls of stone or other artificial constructions, seldom establishes a permanent channel, and besides, the increased rapidity of the flow through the new cut often injuriously affects the regime of the river for a considerable distance below.
Combination of Methods
Upon the whole, it is obvious that no one of the methods heretofore practised or proposed for averting the evils resulting from river inundations is capable of universal application. Each of them is specially suited to a special case. But the hydrography of almost every considerable river and its tributaries will be found to embrace most special cases, most known forms of superficial fluid circulation. For rivers, in general, begin in the mountains, traverse the plains, and end in the sea; they are torrents at their sources, swelling streams in their middle course, placid currents, flowing molli flumine, at their termination. Hence in the different parts of their course the different methods of controlling and utilizing them may successively find application, and there is every reason to believe that by a judicious application of all, every great river may, in a considerable degree, be deprived of its powers of evil and rendered subservient to the use, the convenience, and the dominion of man.
Dikes of the Nile
"History tells us," says Mengotti, "that the Nile became terrible and destructive to ancient Egypt, in consequence of being confined within elevated dikes, from the borders of Nubia to the sea. It being impossible for these barriers to resist the pressure of its waters at such a height, its floods burst its ramparts, sometimes on one side, sometimes on the other, and deluged the plains, which lay far below the level of its current. . . . In one of its formidable inundations the Nile overwhelmed and drowned a large part of the population. The Egyptians then perceived that they were struggling against nature in vain, and they resolved to remove the dikes, and permit the river to expand itself laterally and raise by its deposits the surface of the fields which border its channel."
The original texts of the passages cited by Mengotti, from Latin translations of Diodorus Siculus and Plutarch and from Pliny the Elder, do not by any means confirm this statement, though the most important of them, that from Diodorus Siculus, is, perhaps, not irreconcilable with it. Not one of them speaks of the removal of the dikes, and I understand them all as relating to the mixed system of embankments, reservoirs, and canals which have been employed in Egypt through the whole period concerning which we have clear information. I suppose that the disastrous inundations referred to by the authors in question were simply extraordinary floods of the same character as those which have been frequent at later periods of Egyptian history, and I find nothing in support of the proposition that continuous embankments along the banks of the Nile ever existed until such were constructed by Mehemet Ali.
The object of the dikes of the Po, and, with few exceptions, of those of other European river[s, has always been to confine the waters of[ flood]]s and the solid material transported by them within as narrow a channel as possible, and entirely to prevent them from flowing over the adjacent plains. The object of the Egyptian dikes and canals is the reverse, namely, to diffuse the swelling waters and their sediment over as wide a surface as possible, to store them up until the soil they cover has them thoroughly saturated and enriched, and then to conduct them over other grounds requiring a longer or a second submersion, and, in general, to suffer none of the precious fluid to escape except by evaporation and infiltration.
Lake Moeris, whether wholly an artificial excavation, or a natural basin converted by embankments into a reservoir, was designed chiefly for the same purpose as the barrage built by Mougel Bey across the two great arms which enclose the Delta, namely, as a magazine to furnish a perennial supply of water to the thirsty soil. But these artificial arrangements alone did not suffice. Canals were dug to receive the water at lower stages of the river and conduct it far into the interior, and as all this was still not enough, hundreds of thousands of wells were sunk to bring up from the subsoil, and spread over the surface, the water which, by means of infiltration from the river-bed, pervades the inferior strata of the whole valley.
If a system of lofty continuous dikes, like those of the Po, had really been adopted in Egypt, in the early dynasties when the power and the will to undertake the most stupendous material enterprises were so eminently characteristic of the government of that country, and persevered in through later ages, and the waters of the annual inundation had thus been permanently prevented from flooding the land, it is conceivable that the productiveness of the small area of cultivable soil in the Nile valley might have been long kept up by artificial irrigation and the application of manures. But nature would have rebelled at last, and centuries before our time the mighty river would have burst the fetters by which impotent man had vainly striven to bind his swelling floods, the fertile fields of Egypt would have been converted into dank morasses, and then, perhaps, in some distant future, when the expulsion of man should have allowed the gradual restoration of the primitive equilibrium, would be again transformed into luxuriant garden and plough land. Fortunately, the sapientia AEgyptiorum, the wisdom of the Egyptians, taught them better things. They invited and welcomed, not repulsed, the slimy embraces of Nilus, and his favors have been, from the hoariest antiquity, the greatest material blessing that nature ever bestowed upon a people.
Deposits of the Nile
The Nile is larger than all the rivers of Lombardy together, it drains a basin fifty, possibly even a hundred, times as extensive, its banks have been occupied by man probably twice as long. But its geographical character has not been much changed in the whole period of recorded history, and, though its outlets have somewhat fluctuated in number and position, its historically known encroachments upon the sea are trifling compared with those of the Po and the neighboring streams. The deposits of the Nile are naturally greater in Upper than in Lower Egypt. They are found to have raised the soil at Thebes about seven feet within the last seventeen hundred years, and in the Delta the rise has been certainly more than half as great.
We shall, therefore, probably not exceed the truth if we suppose the annually inundated surface of Egypt to have been elevated, upon an average, ten feet, within the last 5,000 years, or twice and a half the period during which the history of the Po is known to us.
As I have observed, the area of cultivated soil is much less extensive now than under the dynasties of the Pharaohs and the Ptolemies; for--though, in consequence of the elevation of the river-bed, the inundations now have a wider NATURAL spread--the industry of the ancient Egyptians conducted the Nile water over a great surface which it does not now reach.
Had the Nile been banked in, like the Po, all this deposit, except that contained in the water diverted by canals or otherwise drawn from the river for irrigation and other purposes, would have soon carried out to sea. This would have been a considerable quantity; for the Nile holds some earth in suspension at all seasons except at the very lowest water, a much larger proportion during the flood, and irrigation must have been carried on during the whole year. The precise amount of sediment which would have been thus distributed over the soil is matter of conjecture, but though large, it would have been much less than the inundations have deposited, and continuous longitudinal embankments would have compelled the Nile to transport to the Mediterranean an immense quantity over and above what it has actually deposited in that sea. The Mediterranean is shoal for some miles out to sea along the whole coast of the Delta, and the large bays or lagoons within the coast-line, which communicate both with the river and the sea, have little depth of water. These lagoons the river deposits would have filled up, and there would still have been surplus earth enough to extend the Delta far into the Mediterranean.
Obstruction of River Mouths
The mouths of a large proportion of the streams known to ancient navigation are already blocked up by sand-bars or fluviatile deposits, and the maritime approaches to river harbors frequented by the ships of Phenicia and Carthage and Greece and Rome are shoaled to a considerable distance out to sea. The inclination of the lower course of almost every known river bed has been considerably reduced within the historical period, and nothing but great volume of water, or exceptional rapidity of flow, now enables a few large streams like the Amazon, the La Plata, the Ganges, and, in a loss degree, the Mississippi, to carry their own deposits far enough out into deep water to prevent the formation of serious obstructions to navigation. But the degradation of their banks, and the transportation of earthy matter to the sea by their currents, are gradually filling up the estuaries even of those mighty floods, and unless the threatened evil shall be averted by the action of geological forces, or by artificial contrivances more efficient than dredging-machines, the destruction of every harbor in the world which receives a considerable river must inevitably take place at no very distant date.
This result would, perhaps, have followed in some incalculably distant future, if man had not come to inhabit the earth as soon as the natural forces which had formed its surface had arrived at such an approximate equilibrium that his existence on the globe was possible; but the general effect of his industrial operations has been to accelerate it immensely. Rivers, in countries planted by nature with forests and never inhabited by man, employ the little earth and gravel they transport chiefly to raise their own beds and to form plains in their basins. In their upper course, where the current is swiftest, they are most heavily charged with coarse rolled or suspended matter, and this, in floods, they deposit on their shores in the mountain valleys where they rise; in their middle course, a lighter earth is spread over the bottom of their widening basins, and forms plains of moderate extent; the fine silt which floats farther is deposited over a still broader area, or, if carried out to sea, is in great part quickly swept far off by marine currents and dropped at last in deep water. Man's "improvement" of the soil increased the erosion from its surface; his arrangements for confining the lateral spread of the water in floods compel the rivers to transport to their mouths the earth derived from that erosion even in their upper course; and, consequently, the sediment they deposit at their outlets is not only much larger in quantity, but composed of heavier materials, which sink more readily to the bottom of the sea and are less easily removed by marine currents.
The tidal movement of the ocean, deep-sea currents, and the agitation of inland waters by the wind, lift up the sands strewn over the bottom by diluvial streams or sent down by mountain torrents, and throw them up on dry land, or deposit them in sheltered bays and nooks of the coast--for the flowing is stronger than the ebbing tide, the affluent than the refluent wave. This cause of injury to harbors it is not in man's power to resist by any means at present available; but, as we have seen, something can be done to prevent the degradation of high grounds, and to diminish the quantity of earth which is annually abstracted from the mountains, from table-lands, and from river-banks, to raise the bottom of the sea.
This latter cause of harbor obstruction, though an active agent, is, nevertheless, in many cases, the less powerful of the two. The earth suspended in the lower course of fluviatile currents is lighter than sea-sand, river water lighter than sea water, and hence, if a land stream enters the sea with a considerable volume, its water flows over that of the sea, and bears its slime with it until it lets it fall far from shore, or, as is more frequently the case, mingles with some marine current and transports its sediment to a remote point of deposit. The earth borne out of the mouths of the Nile is in part carried over the waves which throw up sea-sand on the beach, and deposited in deep water, in part drifted by the current, which sweeps east and north along the coasts of Egypt and Syria, and lodged in every nook along the shore--and among others, to the great detriment of the Suez Canal, in the artificial harbor at its northern terminus--and in part borne along until it finds a final resting-place in the north-eastern angle of the Mediterranean.
Thus the earth loosened by the rude Abyssinian ploughshare, and washed down by the rain from the hills of Ethiopia which man has stripped of their protecting forests, contributes to raise the plains of Egypt, to shoal the maritime channels which lead to the city built by Alexander near the mouth of the Nile, to obstruct the artificial communication between the Mediterranean and the Red Sea, and to fill up the harbors made famous by Phenician commerce.
Deposits of the Tuscan Rivers
The Arno, and all the rivers rising on the western slopes and spurs of the Apennines, carry down immense quantities of mud to the Mediterranean. There can be no doubt that the volume of earth so transported is very much greater than it would have been had the soil about the headwaters of those rivers continued to be protected from wash by forests; and there is as little question that the quantity borne out to sea by the rivers of Western Italy is much increased by artificial embankments, because they are thereby prevented from spreading over the surface the sedimentary matter with which they are charged. The western coast of Tuscany has advanced some miles seawards within a very few centuries. The bed of the sea, for a long distance, has been raised, and of course the relative elevation of the land above it lessened; harbors have been filled up and destroyed; long lines of coast dunes have been formed, and the diminished inclination of the beds of the rivers near their outlets has caused their waters to overflow their banks and convert them into pestilential marshes. The territorial extent of Western Italy has thus been considerably increased, but the amount of soil habitable and cultivable by man has been, in a still higher proportion, diminished. The coast of ancient Etruria was filled with great commercial towns, and their rural environs were occupied by a large and prosperous population. But maritime Tuscany has long been one of the most unhealthy districts in Christendom; the famous Etruscan mart of Populonia has scarcely an inhabitant; the coast is almost absolutely depopulated, and the malarious fevers have extended their ravages far into the interior.
These results are certainly not to be ascribed wholly to human action. They are, in a large proportion, due to geological causes over which man has no control. The soil of much of Tuscany becomes pasty, almost fluid even, as soon as it is moistened, and when thoroughly saturated with water, it flows like a river. Such a soil as this would not be completely protected by woods, and, indeed, it would now be difficult to confine it long enough to allow it to cover itself with forest vegetation. Nevertheless, it certainly was once chiefly wooded, and the rivers which flow through it must then have been much less charged with earthy matter than at present, and they must have carried into the sea a smaller proportion of their sediment when they were free to deposit it on their banks than since they have been confined by dikes.
It is, in general, true, that the intervention of man has hitherto seemed to insure the final exhaustion, ruin, and desolation of every province of nature which he has reduced to his dominion. Attila was only giving an energetic and picturesque expression to the tendencies of human action, as personified in himself, when he said that "no grass grew where his horse's hoofs had trod." The instances are few, where a second civilization has flourished upon the ruins of an ancient culture, and lands once rendered uninhabitable by human acts or neglect have generally been forever abandoned as hopelessly irreclaimable. It is, as I have before remarked, a question of vast importance, how far it is practicable to restore the garden we have wasted, and it is a problem on which experience throws little light, because few deliberate attempts have yet been made at the work of physical regeneration, on a scale large enough to warrant general conclusions in any one class of cases.
The valleys and shores of Tuscany form, however, a striking exception to this remark. The succcess with which human guidance has made the operations of nature herself available for the restoration of her disturbed harmonies, in the Val di Chiana and the Tuscan Maremma, is among the noblest, if not the most brilliant achievements of modern engineering, and, regarded in all its bearings on the great question of which I have just spoken, it is, as an example, of more importance to the general interests of humanity than the proudest work of internal improvement that mechanical means have yet constructed. The operations in the Val di Chiana have consisted chiefly in so regulating the flow of the surface-waters into and through it, as to compel them to deposit their sedimentary matter at the will of the engineers, and thereby to raise grounds rendered insalubrious and unfit for agricultural use by stagnating water; the improvements in the Maremma have embraced both this method of elevating the level of the soil, and the prevention of the mixture of salt-water with fresh in the coast marshes and shallow bays, which is regarded as a very active cause of the development of malarious influences.
Improvements in the Tuscan Maremma
In the improvements of the Tuscan Maremma, formidable difficulties have been encountered. The territory to be reclaimed was extensive; the salubrious places of retreat for laborers and inspectors were remote; the courses of the rivers to be controlled were long and their natural inclination not rapid; some of them, rising in wooded regions, transported comparatively little earthy matter, and above all, the coast, which is a recent deposit of the waters, is little elevated above the sea, and admits into its lagoons and the mouths of its rivers floods of salt-water with every western wind, every rising tide.
The western coast of Tuscany is not supposed to have been an unhealthy region before the conquest of Etruria by the Romans, but it certainly became so within a few centuries after that event. This was a natural consequence of the neglect or wanton destruction of the public improvements, and especially the hydraulic works in which the Etruscans were so skilful, and of the felling of the upland forests, to satisfy the demand for wood at Rome for domestic, industrial, and military purposes. After the downfall of the Roman empire, the incursions of the barbarians, and then feudalism, foreign domination, intestine wars, and temporal and spiritual tyrannies, aggravated still more cruelly the moral and physical evils which Tuscany and the other Italian States were doomed to suffer, and from which they have enjoyed but brief respites during the whole period of modern history. The Maremma was already proverbially unhealthy in the time of Dante, who refers to the fact in several familiar passages, and the petty tyrants upon its borders often sent criminals to places of confinement in its territory, as a slow but certain mode of execution. Ignorance of the causes of the insalubrity, and often the interference of private rights, prevented the adoption of measures to remove it, and the growing political and commercial importance of the large towns in more healthful localities absorbed the attention of Government, and deprived the Maremma of its just share in the systems of physical improvement which were successfully adopted in interior and Northern Italy.
Before any serious attempts were made to drain or fill up the marshes of the Maremma, various other sanitary experiments were tried. It was generally believed that the insalubrity of the province was the consequence, not the cause, of its depopulation, and that, if it were once densely inhabited, the ordinary operations of agriculture, and especially the maintenance of numerous domestic fires, would restore it to its ancient healthfulness. In accordance with these views, settlers were invited from various parts of Italy, from Greece, and, after the accession of the Lorraine princes, from that country also, and colonized in the Maremma. To strangers coming from soils and skies so unlike those of the Tuscan marshes, the climate was more fatal than to the inhabitants of the neighboring districts, whose constitutions had become in some degree inured to the local influences, or who at least knew better how to guard against them. The consequence very naturally was that the experiment totally failed to produce the desired effects, and was attended with a great sacrifice of life and a heavy loss to the treasury of the state.
The territory known as the Tuscan Maremma, ora maritime, or Maremme--for the plural form is most generally used--lies upon and near the western coast of Tuscany, and comprises about 1,900 square miles English, of which 500 square miles, or 320,000 acres, are plain and marsh including 45,500 acres of water surface, and about 290,000 acres are forest. One of the mountain peaks, that of Mount Amiata, rises to the height of 6,280 feet. The mountains of the Maremma are healthy, the lower hills much less so, as the malaria is felt at some points at the height of 1,000 feet, and the plains, with the exception of a few localities favorably situated on the seacoast, are in a high degree pestilential. The fixed population is about 80,000, of whom one-sixth live on the plains in the winter and about one-tenth in the summer. Nine or ten thousand laborers come down from the mountains of the Maremma and the neighboring provinces into the plain, during the latter season, to cultivate and gather the crops.
Out of this small number of inhabitants and strangers, 35,619 were ill enough to require medical treatment between the 1st of June, 1840, and the 1st of June, 1841, and more than one-half the cases were of intermittent, malignant, gastric, or catarrhal fever. Very few agricultural laborers escaped fever, though the disease did not always manifest itself until they had returned to the mountains. In the province of Grosseto, which embraces nearly the whole of the Maremma, the annual mortality was 3.92 per cent., the average duration of life but 23.18 years, and 75 per cent. of the deaths were among persons engaged in agriculture.
The filling up of the low grounds and the partial separation of the waters of the sea and the land, which had been in progress since the year 1827, now began to show very decided effects upon the sanitary condition of the population. In the year ending June 1st, 1842, the number of the sick was reduced by more than 2,000, and the cases of fever by more than 4,000. The next year the cases of fever fell to 10,500, and in that ending June 1st, 1844, to 9,200. The political events of 1848, and the preceding and following years, occasioned the suspension of the works of improvement in the Maremma, but they were resumed after the revolution of 1859. I have spoken with some detail of the improvements in the Tuscan Maremma, because of their great relative importance, and because their history is well known; but like operations have been executed in the territory of Pisa and upon the coast of the duchy of Lucca. In the latter case they were confined principally to prevention of the intermixing of fresh water with that of the sea. In 1741 sluices or lock-gates were constructed for this purpose, and the following year the fevers, which had been destructive to the coast population for a long time previous, disappeared altogether. In 1768 and 1769, the works having fallen to decay, the fevers returned in a very malignant form, but the rebuilding of the gates again restored the healthfulness of the shore. Similar facts recurred in 1784 and 1785, and again from 1804 to 1821. This long and repeated experience has at last impressed upon the people the necessity of vigilant attention to the sluices, which are now kept in constant repair. The health of the coast is uninterrupted, and Viareggio, the capital town of the district, is now much frequented for its sea-baths and its general salubrity, at a season when formerly it was justly shunned as the abode of disease and death.
Improvements in the Val di Chiana
For twenty miles or more after the remotest headwaters of the Arno have united to form a considerable stream, this river flows south-eastwards to the vicinity of Arezzo. It here sweeps round to the north-west, and follows that course to near its junction with the Sieve, a few miles above Florence, from which point its general direction is westward to the sea. From the bend at Arezzo, a depression called the Val di Chiana runs south-eastwards until it strikes into the valley of the Paglia, a tributary of the Tiber, and thus connects the basin of the latter river with that of the Arno. In the Middle Ages, and down to the eighteenth century, the Val di Chiana was often overflowed and devastated by the torrents which poured down from the highlands, transporting great quantities of slime with their currents, stagnating upon its surface, and gradually converting it into a marshy and unhealthy district, which was at last very greatly reduced in population and productiveness. It had, in fact, become so desolate that even the swallow had deserted it.
The bed of the Arno near Arezzo and that of the Paglia at the southern extremity of the Val di Chiana did not differ much in level. The general inclination of the valley was therefore small; it does not appear to have ever been divided into opposite slopes by a true watershed, and the position of the summit seems to have shifted according to the varying amount and place of deposit of the sediment brought down by the lateral streams which emptied into it. The length of its principal channel of drainage, and even the direction of its flow at any given point, were therefore fluctuating. Hence, much difference of opinion was entertained at different times with regard to the normal course of this stream, and, consequently, to the question whether it was to be regarded as properly an affluent of the Tiber or of the Arno.
The bed of the latter river at the bend has been eroded to the depth of thirty or forty feet, and that, apparently, at no very remote period. If it were elevated to what was evidently original height, the current of the Arno would be so much above that of the Paglia as to allow of a regular flow from its channel to the latter stream, through the Val di Chiana, provided the bed of the valley had remained at the level which excavations prove it to have had a few centuries ago, before it was raised by the deposits I have mentioned. These facts, together with the testimony of ancient geographers which scarcely admits of any other explanation, are thought to prove that all the waters of the Upper Arno were originally discharged through the Val di Chiana into the Tiber, and that a part of them still continued to flow, at least occasionally, in that direction down to the days of the Roman empire, and perhaps for some time later. The depression of the bed of the Arno, and the raising of that of the valley by the deposits of the lateral torrents, finally cut off the branch of the river which had flowed to the Tiber, and all its waters were turned into its present channel, though the drainage of the principal part of the Val di Chiana appears to have been in a south-eastwardly direction until within a comparatively recent period.
In the sixteenth century the elevation of the bed of the valley had become so considerable, that in 1551, at a point about ten miles south of the Arno, it was found to be not less than one hundred and thirty feet above that river; then followed a level of ten miles, and then a continuous descent to the Paglia. Along the level portion of the valley was a boatable channel, and lakes, sometimes a mile or even two miles in breadth, had formed at various points farther south. At this period the drainage of the summit level might easily have been determined in either direction, and the opposite descents of the valley made to culminate at the north or at the south end of the level. In the former case, the watershed would have been ten miles south of the Arno; in the latter, twenty miles, and the division of the valley into two opposite slopes would have been not very unequal.
Various schemes were suggested at this time for drawing off the stagnant waters, as well as for the future regular drainage of the valley, and small operations for those purposes were undertaken with partial success; but it was feared that the discharge of the accumulated waters into the Tiber would produce a dangerous inundation, while the diversion of the drainage into the Arno would increase the violence of the floods to which that river was very subject, and no decisive steps were taken. In 1606 an engineer, whose name has not been preserved, proposed, as the only possible method of improvement, the piercing of a tunnel through the hills bounding the valley on the west to convey its waters to the Ombrone, but the expense and other objections prevented the adoption of this scheme. The fears of the Roman Government for the safety of the basin of the Tiber had induced it to construct embankments across the portion of the valley lying within its territory, and these obstructions, though not specifically intended for that purpose, naturally promoted the deposit of sediment and the elevation of the bed of the valley in their neighborhood. The effect of this measure and of the continued spontaneous action of the torrents was, that the northern slope, which in 1551 had commenced at the distance of ten miles from the Arno, was found in 1605 to begin nearly thirty miles south of that river, and in 1645 it had been removed about six miles farther in the same direction.
In the seventeenth century the Tuscan and Papal Governments consulted Galileo, Torricelli, Castelli, Cassini, Viviani, and other distinguished philosophers and engineers, on the possibility of reclaiming the valley by a regular artificial drainage. Most of these eminent physicists were of opinion that the measure was impracticable, though not altogether for the same reasons; but they seem to have agreed in thinking that the opening of such channels, in either direction, as would give the current a flow sufficiently rapid to drain the lands properly, would dangerously augment the inundations of the river--whether the Tiber or the Arno--into which the waters should be turned. The general improvement of the valley was now for a long time abandoned, and the waters were allowed to spread and stagnate until carried off by partial drainage, infiltration, and evaporation. Torricelli had contended that the slope of a large part of the valley was too small to allow it to be drained by ordinary methods, and that no practicable depth and width of canal would suffice for that purpose. It could be laid dry, he thought, only by converting its surface into an inclined plane, and he suggested that this might be accomplished by controlling the flow of the numerous torrents which pour into it, so as to force them to deposit their sediment at the pleasure of the engineer, and, consequently, to elevate the level of the area over which it should be spread. This plan did not meet with immediate general acceptance, but it was soon adopted for local purposes at some points in the southern part of the valley, and it gradually grew in public favor and was extended in application until its final triumph a hundred years later.
In spite of these encouraging successes, however, the fear of danger to the valley of the Arno and the Tiber, and the difficulty of an agreement between Tuscany and Rome--the boundary between which states crossed the Val di Chiana not far from the half-way point between the two rivers--and of reconciling other conflicting interests, prevented the resumption of the projects for the general drainage of the valley until after the middle of the eighteenth century. In the meantime the science of hydraulics had become better understood, and the establishment of the natural law according to which the velocity of a current of water, and of course the proportional quantity discharged by it in a given time, are increased by increasing its mass, had diminished if not dissipated the fear of exposing the banks of the Arno to greater danger from inundations by draining the Val di China into it. The suggestion of Torricelli was finally adopted as the basis of a comprehensive system of improvement, and it was decided to continue and extend the inversion of the original flow of the waters, and to turn them into the Arno from a point as far to the south as should be found practicable. The conduct of the works was committed to a succession of able engineers who, for a long series of years, were under the general direction of the celebrated philosopher and statesman Fossombroni, and the success has fully justified the expectations of the most sanguine advocates of the scheme. The plan of improvement embraced two branches: the one, the removal of obstructions in the bed of the Arno, and, consequently, the further depression of the channel of that river, in certain places, with the view of increasing the rapidity of its current; the other, the gradual filling up of the ponds and swamps, and raising of the lower grounds of the Val di Chiana, by directing to convenient points the flow of the streams which pour down into it, and there confining their waters by temporary dams until the sediment was deposited where it was needed. The economical result of these operations has been, that in 1835 an area of more than four hundred and fifty square miles of pond, marsh, and damp, sickly low grounds had been converted into fertile, healthy, and well-drained soil, and, consequently, that so much territory has been added to the agricultural domain of Tuscany. But in our present view of the subject, the geographical revolution which has been accomplished is still more interesting. The climatic influence of the elevation and draining of the soil must have been considerable, though I do not know that an increase or a diminution of the mean temperature or precipitation in the valley has been established by meteorological observation. There is, however, in the improvement of the sanitary condition of the Val di Chiana, which was formerly extremely unhealthy, satisfactory proof of a beneficial climatic change. The fevers, which not only decimated the population of the low grounds but infested the adjacent hills, have ceased their ravages, and are now not more frequent than in other parts of Tuscany. The strictly topographical effect of the operations in question, besides the conversion of marsh into dry surface, has been the inversion of the inclination of the valley for a distance of thirty-five miles, so that this great plain which, within a comparatively short period, sloped and drained its waters to the south, now inclines and sends its drainage to the north. The reversal of the currents of the valley has added to the Arno a new tributary equal to the largest of its former affluents, and a most important circumstance connected with this latter fact is, that the increase of the volume of its waters has accelerated their velocity in a still greater proportion, and, instead of augmenting the danger from its inundations, has almost wholly obviated that source of apprehension.
Between the beginning of the fifteenth century and the year 1761, thirty-one destructive floods of the Arno are recorded; between 1761, when the principal streams of the Val di Chiana were diverted into that river, and 1835, not one.
Results of Operations
It is now a hundred years since the commencement of the improvements in the Val di Chiana, and those of the Maremma have been in more or less continued operation for above a generation. They have, as we have seen, produced important geographical changes in the surface of the earth and in the flow of considerable rivers, and their effects have been not less conspicuous in preventing other changes, of a more or less deleterious character, which would infallibly have taken place if they had not been arrested by the improvements in question.
The sediment washed into the marshes of the Maremma is not less than 12,000,000 cubic yards per annum. The escape of this quantity into the sea, which, is now almost wholly prevented, would be sufficient to advance the coast-line fourteen yards per year, for a distance of forty miles, computing the mean depth of the sea near the shore at twelve yards. It is true that in this case, as well as in that of other rivers, the sedimentary matter would not be distributed equally along the shore, and much of it would be carried out into deep water, or perhaps transported by the currents to distant coasts. The immediate effects of the deposit in the sea, therefore, would not be so palpable as they appear in this numerical form, but they would be equally certain, and would infallibly manifest themselves, first, perhaps, at some remote point, and afterwards more energetically at or near the outlets of the rivers which produced them. The elevation of the bottom of the sea would diminish the inclination of the beds of the rivers discharging themselves into it on that coast, and of course their tendency to overflow their banks and extend still further the domain of the marshes which border them would be increased in proportion.
It has been already stated that, in order to prevent the overflow of the valley of the Tiber by freely draining the Val di Chiana into it, the Papal authorities, long before the commencement of the Tuscan works, constructed strong barriers near the southern end of the valley, which detained the waters of the wet season until they could be gradually drawn off into the Paglia. They consequently deposited most of their sediment in the Val di Chiana and carried down comparatively little earth to the Tiber. The lateral streams contributing the largest quantities of sedimentary matter to the Val di Chiana originally flowed into that valley near its northern end; and the change of their channels and outlets in a southern direction, so as to raise that part of the valley by their deposits and thereby reverse its drainage, was one of the principal steps in the process of improvement.
We have seen that the north end of the Val di Chiana near the Arno had been raised by spontaneous deposit of sediment to such a height as to interpose a sufficient obstacle to all flow in that direction. If, then, the Roman dam had not been erected, or the works of the Tuscan Government undertaken, the whole of the earth, which has been arrested by those works and employed to raise the bed and reverse the declivity of the valley, would have been carried down to the Tiber and thence into the sea. The deposit thus created would, of course, have contributed to increase the advance of the shore at the mouth of that river, which has long been going on at the rate of three metres and nine-tenths (twelve feet and nine inches) per annum. It is evident that a quantity of earth, sufficient to effect the immense changes I have described in a wide valley more than thirty miles long, if deposited at the outlet of the Tiber, would have very considerably modified the outline of the coast, and have exerted no unimportant influence on the flow of that river, by raising its point of discharge and lengthening its channel.
The Coast of the Netherlands. It has been shown in a former section that the dikes of the Netherlands and the adjacent states have protected a considerable extent of coast from the encroachments of the sea, an have won a large tract of cultivable land from the dominion of the ocean waters. The immense results obtained from the operations of the Tuscan engineers in the Val di Chiana, and the Maremma have suggested the question, whether a different method of accomplishing these objects might not have been adopted with advantage. It has been argued, as in the case of the Po, that a system of transverse inland dikes and canals, upon the principle of those which have been so successfully employed in the Val di Chiana and in Egypt, might have elevated the low grounds above the ocean tides, by spreading over them the sediment brought down by the Rhine, the Maes, and the Scheld. If this process had been introduced in the Middle Ages, and constantly pursued to our times, the superficial and coast geography, as well as the hydrography of the countries in question, would undoubtedly have presented an aspect very different from their present condition; and by combining the process with a system of maritime dikes, which would have been necessary, both to resist the advance of the sea and to retain the slime deposited by river overflows, it is, indeed, possible that the territory of those states would have been as extensive as it now is, and, at the same time, somewhat elevated above its natural level.
The argument in favor of that method rests on the assumption that all the sea-washed earth, which the tides have let fall upon the shallow coast of the Netherlands, has been brought down by the rivers which empty upon those shores, and could have been secured by allowing those rivers to spread over the flats and deposit their sediment in still-water pools formed by cross-dikes like those of Egypt.
But we are ignorant of the proportions in which the marine deposits that form the soil of the polders have been derived from materials brought down by these rivers, or from other more remote sources. Much of the river slime has, no doubt, been transported by marine currents quite beyond the reach of returning streams, and it is uncertain how far this loss has been balanced by earth washed by the sea from distant shores and let fall on the coasts of the Netherlands and other neighboring countries.
We know little or nothing of the quantity of solid matter brought down by the rivers of Western [[[Europe]] in early ages, but, as the banks of those rivers are now generally better secured against wash and abrasion than in former centuries, the sediment transported by them must be less than at periods nearer the removal of the primitive forests of their valleys, though certainly greater than it was before those forests were felled. Kladen informs us that the sedimentary matter transported to the sea by the Rhine would amount to a cubic geographical mile in five thousand years. The proportion of this suspended matter which, with our present means, could be arrested and precipitated upon the ground, is almost infinitesimal, for only the surface-water, which carries much less sediment than that at the bottom of the channel, would flow over the banks, and as the movement of this water, if not checked altogether, would be greatly retarded by the proposed cross-dikes, the quantity of solid matter which would be conveyed to a given portion of land during a single inundation would be extremely small. Inundations of the Rhine occur but once or twice a year, and high water continues but a few days, or even hours; the flood-tide of the sea happens seven hundred times in a year, and at the turn of the tide the water is brought to almost absolute rest. Hence, small as is the proportion of suspended matter in the tide-water, the deposit probably amounts to far more in a year than would be let fall upon the same area by the Rhine.
This argument, except as to the comparison between river and tide water, applies to the Mississippi, the Po, and most other great rivers. Hence, until that distant day when man shall devise means of extracting from rivers at flood, the whole volume of their suspended material and of depositing it at the same time on their banks, the system of cross-dikes and COLMATAGE must be limited to torrential streams transporting large proportions of sediment, and to the rivers of hot countries, like the Nile, where the saturation of the soil with water, and the securing of a supply for irrigation afterwards, are the main objects, while raising the level of the banks is a secondary consideration.
1. ^It is possible that the weight of the sediment let fall at the mouths of great rivers, like the Ganges, the Mississippi, and the Po, may cause the depression of the strata on which they are deposited, and hence if man promotes the erosion and transport of earthy material by rivers, he augments the weight of the sediment they convey into their estuaries, and consequently his action tends to accelerate such depression. There are, however, cases where, in spite of great deposits of sediment by rivers, the coast is rising. Further, the manifestation of the internal heat of the earth at any given point is conditioned by the thickness of the crust at such point. The deposits of rivers tend to augment that thickness at their estuaries. The sediment of slowly-flowing rivers emptying into shallow seas is spread over so great a surface that we can hardly imagine the foot or two of slime they let fall over a wide area in a century to form an element among even the infinitesimal quantities which compose the terms of the equations of nature. But some swift rivers, rolling mountains of fine earth, discharge themselves into deeply scooped gulfs or bays, and in such cases the deposit amounts, in the course of a few years, to a mass the transfer of which from the surface of a large basin, and its accumulation at a single point, may be supposed to produce other effects than those measurable by the sounding-line. Now, almost all the operations of rural life, as I have abundantly shown, increase the liability of the soil to erosion by water. Hence, the clearing of the valley of the Ganges, for example, by man, must have much augmented the quantity of earth transported by that river to the sea, and of course have strengthened the effects, whatever they may be, of thickening the crust of the earth in the Bay of Bengal. In such cases, then, human action must rank among geological influences.
To the geological effects of the thickening of the earth's crust in the Bay of Bengal, are to be added those of thinning it on the highlands where the Ganges rises. The same action may, as a learned friend suggests to me, even have a cosmical influence. The great rivers of the earth, taken as a whole, transport sediment from the polar regions in an equatorial direction, and hence tend to increase the equatorial diameter, and at the same time, by their inequality of action, to a continual displacement of the centre of gravity, of the earth. The motion of the globe, and of all bodies affected by its attraction, is modified by every change of its form, and in this case we are not authorized to say that such effects are in any way compensated.
2. ^It is, nevertheless, remarkable that in the particular branch of coast engineering where great improvements are most urgently needed, comparatively little has been accomplished. I refer to the creation of artificial harbors, and of facilities for loading and discharging ships. The whole coast of Italy is, one may almost say, harborless and even, wharfless, and there are many thousands of miles of coast in rich commercial countries in Europe, where vessels can neither lie in safety for a single day, nor even, in better protected heavens, ship or land their passengers or cargoes except by the help of lighters, and other not less clumsy contrivances. It is strange that such enormous inconveniences are borne with so little effort to remove them, and especially that break-waters are rarely constructed by Governments except for the benefit of the military marine.
3. ^Some notice of great works executed by man in foreign lands, and probably not generally familiar to my readers, may, however, prove not uninteresting.
The desaguadero, or canal constructed by the Viceroy Revillagigedo to prevent the inundation of the city of Mexico by the lakes in its vicinity, besides subsidiary works of great extent, has a cutting half a mile long, 1,000 feet wide, and from 150 to 200 feet deep.--Hoffmann, Encyclopaedie, art. Mexico.
The adit which drains the mines of Gwennap in Cornwall, with its branches, is thirty miles long. Those of the silver mines of Saxony are scarcely less extensive, and the Ernst-August-Stollen, or great drain of the mines of the Harz, is fifteen miles long.
The excavation for the Suez Canal were computed at 75,000,000 cubic metres, or about 100,000,000 cubic yards, and those of the Ganges Canal, which, with its branches, had a length of 3,000 miles, amount to nearly the same quantity.
The quarries at Maestricht have undermined a space of sixteen miles by six, or more than two American townships, and the catacombs of Rome, in part, at least, originally quarries, have a lineal extent of five hundred and fifty miles. The catacombs of Paris required the excavation of 13,000,000 cubic yards of stone, or more than four times the volume of the great pyramid.
The excavation for the Mt. Cenis tunnel, eight miles in length, wholly through solid rock, amounted to more than 900,000 cubic yards, and 16,000,000 of brick were employed for the lining.
In an article on recent internal improvements in England, in the London Quarterly Review for January, 1858, it is stated that in a single rock-cutting on the Liverpool and Manchester railway, 480,000 cubic yards of stone were removed; that the earth excavated in the construction of English railways up to that date amounted to a hundred and fifty million cubic yards, and that at the Round Down Cliff, near Dover, a single blast of nineteen thousand pounds of powder blew down a thousand million tons of chalk, and covered fifteen acres of land with the fragments.
In 1869, a mass of marble equal to one and a half times the cubical contents of the Duomo at Florence, or about 450,000 cubic yards, was thrown down at Carrara by one blast, and two hours after, another equal mass, which had been loosened by the explosion, fell of itself. Zolfanelli, La Lunigiana, p. 43.
The coal yearly extracted from the mines of England averages not less than 100,000,000 tons. The specific gravity of British coal ranges from 1.20 to 1.35, and consequently we may allow a cubic yard to the ton. If we add the earth and rock removed in order to reach the coal, we shall have a yearly amount of excavation for this one object equal to more than thirty times the volume of the pyramid of Cheops. These are wonderful achievements of human industry; but the rebuilding of Chicago within a single year after the great fire--not to speak of the extraordinary material improvements previously executed at that city--surpasses them all, and it probably involved the expenditure of a sum of muscular and of moral energy which has never before been exerted in the accomplishment of a single material object, within a like period.
4. ^It has often been alleged by eminent writers that a part of the fens in Lincolnshire was reclaimed by sea-dikes under the government of the Romans. I have found no ancient authority in support of this assertion, nor can I refer to any passage in Roman literature in which sea-dikes are expressly mentioned otherwise than as walls or piers, except that in Pliny (Hist. Nat. xxxvi. 24), where it is said that the Tyrrhenian Sea was excluded from the Lucrino Lake by dikes. Dugdale, whose enthusiasm for his subject led him to believe that recovering from the sea land subject to be flooded by it, was of divine appointment, because God said: "Let the waters under the heavens be gathered together unto one place and let the dry land appear," unhesitatingly ascribes the reclamation of the Lincolnshire fens to the Romans, though he is able to cite but one authority, a passage in Tacitus's Life of Agricola which certainly has no such meaning, in support of the assertion.--History of Embankment and Drainage, 2d edition, 1772.
5. ^It is worth mentioning, as an illustration of the applicability of military instrumentalities to pacific art, that the sale of gunpowder in the United States was smaller during the late rebellion than before, because the war caused the suspension of many public and private improvements, in the execution of which great quantities of powder were used for blasting.
The same observation was made in France during the Crimean war, and it is alleged that, in general, not ten per cent. of the powder manufactured on either either side of the Atlantic is employed for military purposes.
The blasting for the Mount Cenis tunnel consumed gunpowder enough to fill more than 200,000,000 musket cartridges. It is a fact not creditable to the moral sense of modern civilization, that very many of the most important improvements in machinery and the working of metals have originated in the necessities of war, and that man's highest ingenuity has been shown, and many of his most remarkable triumphs over natural forces achieved, in the contrivance of engines for the destruction of his fellow-man. The military material employed by the first Napoleon has become, in less than two generations, nearly as obsolete as the sling and stone of the shepherd, and attack and defence now begin at distances to which, half a century ago, military reconnaissances hardly extended. Upon a partial view of the subject, the human race seems destined to become its own executioner--on the one hand, exhausting the capacity of the earth to furnish sustenance to her taskmaster; on the other, compensating diminished production by inventing more efficient methods of exterminating the consumer. At the present moment, at an epoch of universal peace, the whole civilized world with the happy exception of our own country, is devoting its utmost energies, applying the highest exercise of inventive genius, to the production of new engines of war; and the last extraordinary rise in the price of iron and copper is in great part due to the consumption of these metals in the fabrication of arms and armed vessels. The simple substitution of sheet-copper for paper and other materials in the manufacture of cartridges has increased the market-price of copper by a large percentage on its former cost.
But war develops great civil virtues, and brings into action a degree and kind of physical energy which seldom fails to awaken a new intellectual life in a people that achieves great moral and political results through great heroism and endurance and perseverance. Domestic corruption has destroyed more nations than foreign invasion, and a people is rarely conquered till it has deserved subjugation.
6. ^Staring, Voormaals en Thans, p. 150.
7. ^Idem, p. 163. Much the largest proportion of the lands so reclaimed, though for the most part lying above low-water tidemark, are at a lower level than the Lincolnshire fens, and more subject to inundation from the irruptions of the sea.
8. ^Die Inseln und Marschen der Herzogthamer Schleswig und Holstein, iii., p. 151.
9. ^The purely agricultural island of Pelworm, off the coast of Schleswig, containing about 10,000 acres, annually expends for the maintenance of its dikes not less than L6,000 sterling, or nearly $30,000.--J. G. Kohl, Inseln und Marschen Schleswig's und Holstein's, ii., p. 394.
The original cost of the dikes of Pelworm is not stated. "The greatest part of the province of Zeeland is protected by dikes measuring 250 miles in length, the maintenance of which costs, in ordinary years, more than a million guilders [above $400,000] ... The annual expenditure for dikes and hydraulic works in Holland is from five to seven million guilders" [$2,000,000 to $2,800,000].--Wild, Die Niederlande, i., p. 62.
One is not sorry to learn that the Spanish tyranny in the Netherlands had some compensations. The great chain of ring-dikes which surrounds a large part of Zeeland is due to the energy of Caspar de Robles, the Spanish governor of that province, who in 1570 ordered the construction of these works at the public expense, as a substitute for the private embankments which had previously partially served the same purpose.--Wild, Die Niederlande, i., p. 62.
10. ^Staring, Voormaals en Thans, p. 163.
11. ^Voormaals en Thans, pp. 150, 151. According to Reventlov, confercae first appear at the bottom in shoal water, then, after the deposit has risen above the surface, Salicornia herbacea. The Salicornia is followed by various sand-plants, and so the ground rises, by Poa distans and Poa maritum, and finally common grasses establish themselves.--Om Markdannelsen poa Vestkyeten of Slesvig, pp. 7, 8.
12. ^Staring, Voormaals en Thans, p, 152. Kohl states that the peninsula of Diksand on the coast of Holstein consisted, at the close of the last century, of several islands measuring together less than five thousand acres. In 1837 they had been connected with the mainland, and had nearly doubled in area.--Inseln u. Marschen Schlene, Holst., iii., p. 202
13. ^The inclination varies from one foot rise in four of base to one foot in fourteen.--Kohl, iii., p. 210.
14. ^The dikes are sometimes founded upon piles, and sometimes protected by one or more rows of piles driven deeply down into the bed of the sea in front of them. "Triple rows of piles of Scandinavian pine," says Wild, "have been driven down along the coast of Friesland, where there are no dunes, for a distance of one hundred and fifty miles. The piles are bound together by strong cross-timbers and iron clamps, and the interstices filled with stones. The ground adjacent to the piling is secured with fascines, and at exposed points heavy blocks of stone are heaped up as an additional protection. The earth-dike is built behind the mighty bulwark of this breakwater, and its foot also is fortified with stones." ... "The great Helder dike is about five miles long and forty feet wide at the top, along which runs a good road. It slopes down two hundred feet into the sea, at an angle of forty degrees. The highest waves do not reach the summit, the lowest always cover its base. At certain distances, immense buttresses, of a height and width proportioned to those of the dike, and even more strongly built, run several hundred feet out into the rolling sea. This gigantic artificial coast is entirely composed of Norwegian granite."--Wild, Die Niederlande, i., pp. 61, 62.
15. ^A similar subsidence of the surface is observed in the diked ground of the Lincolnshire fens, where there is no reason to suspect a general depression from geological causes.
16. ^The shaking of the ground, even when loaded with large buildings, by the passage of heavy carriages or artillery, or by the march of a body of cavalry or even infantry, shows that such causes may produce important mechanical effects on the condition of the soil. The bogs in the Netherlands, as in most other countries, contain large numbers of fallen trees, buried to a certain depth by earth and vegetable mould. When the bogs are dry enough to serve as pastures, it is observed that trunks of these ancient trees rise of themselves to the surface. Staring ascribes this singular phenomenon to the agitation of the ground by the tread of cattle. "When roadbeds," observes he, "are constructed of gravel and pebbles of different sizes, and these latter are placed at the bottom without being broken and rolled hard together, they are soon brought to the top by the effect of travel on the road. Lying loosely, they undergo some motion from the passage of every wagon-wheel and the tread of every horse that passes over them. This motion is an oscillation or partial rolling, and as one side of a pebble is raised, a little fine sand or earth is forced under it, and the frequent repetition of this process by cattle or carriages moving in opposite directions brings it at last to the surface. We may suppose that a similar effect is produced on the stems of trees in the bogs by the tread of animals."--De Bodem van Nederland, i., pp. 75, 76.
It is observed in the Northern United States, that when soils containing pebbles are cleared and cultivated, and the stones removed from the surface, new pebbles, and even bowlders of many pounds weight, continue to show themselves above the ground, every spring, for a long series of years. In clayey soils the fence-posts are thrown up in a similar way, and it is not uncommon to see the lower rail of a fence thus gradually raised a foot or even two feet above the ground. This rising of stones and fences is popularly ascribed to the action of the severe frosts of that climate. The expansion of the ground, in freezing, it is said, raises its surface, and, with the surface, objects lying near or connected with it. When the soil thaws in the spring, it settles back again to its former level, while the pebbles and posts are prevented from sinking as low as before by loose earth which has fallen under them. The fact that the elevation spoken of is observed only in the spring gives countenance to this theory, which is perhaps applicable also to the cases stated by Staring, and it is probable that the two causes above assigned concur in producing the effect.
The question of the subsidence of the Netherlandish coast has been much discussed. Not to mention earlier geologists, Venema, in several essays, and particularly in Het Dalen van de Noordelijke Kuststreken van ons Land, 1854, adduces many facts and arguments to prove a slow sinking of the northere provinces of Holland. Laveleye (Affaissement du sol at envasement des fleuves survenus dans les temps historiques, 1859), upon a still fuller investigation, arrives at the same conclusion. The eminent geologist Staring, however, who briefly refers to the subject in De Bodem van Nederland, i., p. 356 et seqq., does not consider the evidence sufficient to prove anything more than the sinking of the surface of the polders from drying and consolidation.--See Elisee Reclus, La Terre, vol. i., pp. 730, 732.
17. ^The elevation of the lands enclosed by dikes--or polders, as they are called in Holland--above low-water mark, depends upon the height of the tides or, in other words, upon the difference between ebb and flood. The tide cannot deposit earth higher than it flows, and after the ground is once enclosed, the decay of the vegetables grown upon it and the addition of manures do not compensate the depression occasional by drying and consolidation. On the coast of Zeeland and the islands of South Holland, the tides, and of course the surface of the lands deposited by them, are so high that the polders can be drained by ditching and sluices, but at other points, as in the enclosed grounds of North Holland on the Zuiderzee, where the tide rises but three feet or even less, pumping is necessary from the beginning.--Staring, Voormaals en Thans, p. 15.
19. ^In England and New England, where the marshes have been already drained or are of comparatively small extent, the existence of large floating islands seems incredible, and has sometimes been treated as a fable, but no geographical fact is better established. Kohl (Inseln und Marschen Schleswig-Holsteins, iii., p. 309) reminds us that Pliny mentions among the wonders of Germany the floating islands, covered with trees, which met the Roman fleets at the mouths of the Elbe and the Weser. Our author speaks also of having visited, in the territory of Bremen, floating moors, bearing not only houses but whole villages. At low stages of the water these moors rest upon a bed of sand, but are raised from six to ten feet by the high water of spring, and remain afloat until, in the course of the summer, the water beneath is exhausted by evaporation and drainage, when they sink down upon the sand again.
Staring explains, in an interesting way, the whole growth, formation, and functions of floating fens or bogs, in his very valuable work, De Bodem van Nederland, i., pp. 36-43. The substance of his account is as follows: The turf and the surface of the fens, is stillness of the water. Hence they are not found in running streams, nor in pools so large as to be subject to frequent agitation by the wind. For example, not a single plant grew in the open part of the Lake of Haarlem, and fens cease to form in all pools as soon as, by the cutting of the turf for fuel or other purposes, their area is sufficiently enlarged to be much acted on by wind. When still water above a yard deep is left undisturbed, aquatic plants of various genera, such as Nuphar, Nymphaea, Limnanthemum, Stratiotes, Polygonum, and Potamogeton, fill the bottom with roots and cover the surface with leaves. Many of the plants die every year, and prepare at the bottom a soil fit for the growth of a higher order of vegetation, Phragmites, Acorus, Sparganium, Rumex, Lythrum, Pedicularis, Spiraea, Polystichum, Comarum, Caltha, etc., etc. In the course of twenty or thirty years the muddy bottom is filled with roots of aquatic and marsh plants, which are lighter than water, and if the depth is great enough to give room for detaching this vegetable network, a couple of yard for example, it rises to the surface, bearing with it, of course, the soil formed above it by decay of stems and leaves. New genera now appear upon the mass, such a Carex, Menyanthes, and others, and soon thicky cover it. The turf has now acquired a thickness of from two to four feet, and is called in Groningen lad; in Friesland, til, tilland, or drifftil; in Overijsse, krag; and in Holland, rietzod. It floats about as driven by the wind, gradually increasing in thickness by the decay of its annual crops of vegetation, and in about half a century reaches the bottom and becomes fixed. If it has not been invaded in the meantime by men or cattle, trees and arborescent plants, Alnus, Salix, Myrica, etc., appear, and these contribute to hasten the attachment of the turf to the bottom, both by their weight and by sending their roots quite through into the ground."
This is the regular method employed by nature for the gradual filling up of shallow lakes and pools, and converting them first into morass and then into dry land. Whenever, therefore, man removes the peat or turf, he exerts an injurious geographical agency, and, as I have already said, there is no doubt that the immense extension of the inland seas of Holland in modern times is owing to this and other human imprudences. "Hundreds of hectares of floating pastures," says our author, "which have nothing in their appearance to distinguish them from grass-lands resting on solid bog, are found in Overijssel, in North Holland, and near Utrecht. In short, they occur in all deep bogs, and wherever deep water is left long undisturbed."
In one case a floating island, which had attached itself to the shore, continued to float about for a long time after it was torn off by a flood, and was solid enough to keep a pond of fresh water upon it sweet, though the water in which it was swimming had become brackish from the irruption of the sea. After the hay is cut, cattle are pastured, and occasionally root-crops grown upon these islands, and they sometimes have large trees growing upon them.
When the turf or peat has been cut, leaving water less than a yard deep, Equisetum limosum grows at once, and is followed by the second class of marsh plants mentioned above. Their roots do not become detached from the bottom in such shallow water, but form ordinary turf or peat. These processes are so rapid that a thickness of from three to six feet of turf is formed in half a century, and many men have lived to mow grass where they had fished in their boyhood, and to cut turf twice in the same spot. In Ireland the growth of peat is said to be much more rapid. Elisee Reclus, La Terre, i., 591, 592. But see Asbjornsen, Torv og Torvdrift, ii., 29, 30.
Captain Gilliss says that before Lake Taguataga in Chili was drained, there were in it islands composed of dead plants matted together to a thickness of from four to six feet, and with trees of medium size growing upon them. These islands floated before the wind "with their trees and browsing cattle."--United States Naval Astronomical Expedition to the Southern Hemisphere, i., pp. 16, 17.
20. ^The dependence of man upon the aid of spontaneous nature, in his most arduous material works, is curiously illustrated by the fact that one of the most serious difficulties to be encountered in executing this gigantic scheme is that of procuring brushwood for the fascines to be employed in the embankments. See Diggelen's pamphlet, "Groote Werken in Nederland."
21. ^The plan at present most in favor is that which proposes the drainage of only a portion of the southern half of the Zuiderzee, which covers not far from 400,000 acres. The project for the construction of a ship-canal directly from Amsterdam to the North Sea, now in course of execution, embraces the drainage of the Ij, a nearly land-locked basin communicating with the Zuiderzee and covering more than 12,000 acres. See official reports on these projects in Droogmaking vom het zuidelyk gedeelte der Zuiderzee, te s' Gravenhage, 1868, 4to.
23. ^One reason for the silence of Roman writers in respect to great material improvements which had no immediate relation to military or political objects, is doubtless the contempt in which mechanical operations and mechanical contrivances were held by that nation of spoilers. Even the engineer, upon whose skill the attack or defence of a great city depended, was only praefectus fabrum, the master-artisan, and had no military rank or command. This prejudice continued to a late period in the Middle Ages, and the chiefs of artillery were equally without grade or title as soldiers. "The occupations of all artisans," says Cicero, "are base, and the shop can have nothing of the respectable." De Officiis, 1, i., 42. The position of the surgeon relatively to the physician, in England, is a remnant of the same prejudice, which still survives in full vigor in Italy, with regard to both trade and industry. See p. 6, ante.
24. ^The fact alluded to in a note on p. 97, ante, that since the opening of a communication between Lake Celano and the Garigliano by the works noticed in the text, fish, of species common in the lake, but not previously found in the river, have become naturalized in the Garigliano, is a circumstance of some weight as evidence that the emissary was not actually open in ancient times; for if the waters had been really connected, the fish of the lake would naturally have followed the descending current and established themselves in the river as they have done now.
25. ^Springs rising in the bottom of the lake have materially impeded the process of drainage, and some engineers believe that they will render the complete discharge of the waters impossible. It appears that the earthy and rocky strata underlying the lake are extremely porous, and that the ground already laid dry on the surface absorbs an abnormally large proportion of the precipitation upon it. These strata, therefore, constitute a reservoir which contributes to maintain the spring fed chiefly, no doubt, by underground channels from the neighboring mountains. But it is highly probable that, after a certain time, the process of natural desiccation noticed in note to p. 20, ante, will drain this reservoir, and the entire removal of the surface-water will then become practicable.
26. ^The draining of Lake Celano was undertaken by a company, but Prince Alessandro Torlonia of Rome bought up the interest of all the shareholders and has executed the entire work at his own private expense. Montricher, the celebrated constructor of the great aqueduct of Marseilles, was the engineer who designed and partly carried out the plans, and after his lamentable death the work has been directed with equal ability by Bermont and Brisse.--See Leon De Rothou, Prosciugamento del Lago Fucino, 8vo. Firenza, 1871.
27. ^A considerable work of this character is mentioned by Captain Gilliss as having been executed in Chili, a country to which we should hardly have looked for an improvement of such a nature. The Lake Taguataga was partially drained by cutting through a narrow ridge of land, not at the natural outlet, but upon one side of the lake, and eight thousand acres of land covered by it were gained for cultivation.--U. S. Naval Astronomical Expedition to the Southern Hemisphere, i., pp. 16, 17.
Lake Balaton and the Neusiedler Sea in Hungary have lately been, at least partially, drained.
The lakes of Neuchatel, Bienne, and Morat, in Switzerland, have been connected and the common level of all of them lowered about four feet. The works now in operation will produce, in the course of the year 1874, a further depression of four feet, and recover for agricultural use more than twelve thousand acres of fertile soil.
28. ^In the course of the year 1864 there were slides of the banks of the Lake of Como, and in one case the grounds of a villa near the water suffered a considerable displacement. More important slips occurred at Fesiolo on the shore of Lago Maggiore in 1867 and 1869, and on the Lake of Orta in 1868. These occurrences excited some apprehensions in regard to the possible effects of projects then under discussion for lowering the level of some of the Italian lakes, to obtain an increased supply of water for irrigation and as a mechanical power, but as it was not proposed to depress the surface below the lowest natural low-water level, there seems to have been little ground for the fears expressed.
See, for important observations on the character and probable results of these projects, Tagliasecchi, Nostizie etc. del Canali dell' Alta Lombardia, Milano, 1871.
Jacini says: "A large proportion of the water of the lakes, instead of discharging itself by the Ticino, the Adda, the Oglio, the Mincio, filters through the silicious strata which underlie the hills, and follows subterranean channels to the plain, where it collects in the fontanili, and being thence conducted into the canals of irrigation, becomes a source of great fertility."--La Proprieta Fondiaria, etc., p.144. The quantity of water escaping from the lakes by infiltration depends much on the hydrostatic pressure on the bottom and the walls of the lake-basins, and consequently the depression of the lake surface, diminishing this pressure, would diminish the infiltration. Hence it is possible that the lowering of the level of these lakes would manifest itself in a decreased supply of water for the springs, fontanili, and wells of Lombardy.
29. ^Economie Rurale de la France, p. 289.
30. ^Very interesting and important experiments, on the practicability of washing out the salt from seacoast lands too highly impregnated with that mineral to be fit for cultivation, are now in progress near the mouth of the Rhone, where millions of acres of marshy soil can easily be recovered, if these experiments are successful.
See Duponchel, Traite d'Hydraulique et de Geologie agricoles. Paris, 1868, chap. xi. and xii.
In the neighborhood of Ferrara are pools and marshes covering nearly two hundred square miles, or a surface more than equal to eight American townships. Centrifugal steam-pumps, of 2,000 horse-power, capable of discharging more than six hundred and fifty millions of gallons of water per day have lately been constructed in England for draining these marshes. This discharge is equal to an area of 640 acres, or a mile square, with nearly three feet of water.
31. ^Physikalische Geographie, p. 288. This method is now frequently employed in France. Details as to the processes will be found in Mangon Pratique du Drainage, pp. 78 et seqq. Draining by driving down stakes mentioned in a note in the chapter on the Woods, ante, is a process of the same nature. In the United States, large tracts of marshy ground, and even shallow lakes of considerable extent, have been sufficiently drained not only for pasturage but for cultivation, without resort to any special measures for effecting that end. The ordinary processes of rural improvement in the vicinity, such as felling woods upon and around such grounds, and the construction of roads, the side ditches of which act as drains, over or near them, aided now and then by the removal of a fallen tree or other accidental obstruction in the beds of small streams which flow from them, often suffice to reclaim miles square of unproductive swamp and water. See notes on p. 20, and on cedar swamps, p. 208, ante.
32. ^Coste found, in his experiments on pisciculture, that the fermentation, which takes place in the water of the Seine in consequence of the discharge of the drains into the river, destroyed a large proportion of the eggs of fish in his breeding basins. Analysis of Seine water by Boussingault in 1855 detected a considerable quantity of ammonia.
33. ^The relative evaporating action of earth and water is a very complicated problem, and the results of observation on the subject are conflicting. Schubler found that at Geneva the evaporation from bare loose earth, in the months of December, January, and February, was from two and a half to nearly six times as great as from a like surface of water in the other months. The evaporation from water was from about once and a half to six times as great as from earth. Taking the whole year together, the evaporation from the two surfaces was 199 lines from earth and 536 lines from water. Experiments by Van der Steer, at the Helder, in the years 1861 and 1862, showed, for the former year, an evaporation of 602.9 millimetres from water, 1399.6 millimetres from ground covered with clover and other grasses; in 1862, the evaporation from water was 584.5 millimetres, from grassground, 875.5. --Wilhelm, Der Boden und das Wasser, p. 57; Krecke, Het Klimaat van Nederland, ii., p. 111. On the other hand, the evaporation from the Nile in Egypt and Nubia is stated to be three times as great as that from an equal surface of the soil which borders it.--Lombardini, Saggio Idrologico sul Nilo, Milano, 1864, and Appendix. The relative thermometrical conditions of land and water in the same vicinity are constantly varying, and the hygrometrical state of both is equally unstable. Consequently there is no general formula to express the proportionate evaporation from fluid and solid geographical surfaces.
34. ^"The simplest backwoodsman knows by experiences that all cultivation is impossible in the neighborhood of bogs and marshes. Why is a crop near the borders of a marsh out off by frost, while a field upon a hillock, a few stone's throws from it, is spared "--Lars Levi Laestadius, Om Uppoldingar Lappmarrken, pp. 69, 74.
35. ^Mangon thinks that the diminution of evaporation by agricultural drainage corresponds, in certain circumstances, to five per cent. of the heat received from the sun by the same surface in a year. He cites observations by Parkes, showing a difference in temperature of 5.5 degrees (centigrade) in favor of drained, as compared with undrained, ground in the same vicinity.--Instructions pratiques sur le Drainage, pp. 227, 228. The diminution of evaporation is not the only mode in which under-draining affects the temperature. The increased effective hygroscopicity of the soil increases its absorbent action, and the condensation of atmospheric vapor thus produced is attended with the manifestation of heat.
36. ^Krecke, Het Klimaat van Nederland, ii., p. 64.
37. ^Babinet condemns the general draining of marshes. "Draining," says he, "has been much in vogue for some years, and it has been a special object to dry and fertilize marshy grounds. I believe that excessive dryness is thus produced, and that other soils in the neighborhood are sterilized in proportion."--Etudes et Lectures, iv., p. 118.
"The extent of soil artificially dried by drainage is constantly increasing, and the water received by the surface from precipitation flows off by new channels, and is in general carried off more rapidly than before. Must not this fact exercise an influence on the regime of springs whose basin of supply thus undergoes a more or less complete transformation "--Bernhard Cotta, Preface to Paramelle, Quellenkunde, p. vii., viii.
The effects of agricultural drainage are perceptible at great depths. It has been observed in Cornwall that deep mines are more free from water in well-drained districts than in those where drainage is not generally practised.--Esquiros, Revue des Deux Mondes, 15 Nov., 1863, p. 430.
See also Asbjornsen, Torv og Torvdrift, p. 31.
38. ^See a very interesting paper on the Water-Supply of Constantinople, by Mr. Homes, of the New York State Library, in the Albany Argus of June 6, 1872. The system of aqueducts for the supply of water to that city was commenced by Constantine, and the great aqueduct, frequently ascribed to Justinian, which is 840 feet long and 112 feet high, is believed to have been constructed during the reign of the former emperor.
39. ^The unhealthiness of the Roman Campagna is ascribed by many mediaeval as well as later writers to the escape of water from the ancient aqueducts, which had fallen out of repair from neglect, or been broken down by enemies in the sieges of Rome.
40. ^Sismondi, speaking of the Tuscan canals, observes: "But inundations are not the only damage caused by the waters to the plains of Tuscany. Raised, as the canals are, above the soil, the water percolates through their banks, penetrates every obstruction, and, in spite of all the efforts of industry, sterilizes and turns to morasses fields which nature and the richness of the soil seemed to have designed for the most abundant harvests. In ground thus pervaded with moisture, or rendered COLD, as the Tuscans express it, by the filtration of the canal-water, the vines and the mulberries, after having for a few years yielded fruit of a saltish taste, rot and perish. The wheat decays in the ground, or dies as soon as it sprouts. Winter crops are given up, and summer cultivation tried for a time; but the increasing humidity, and the saline matter communicated to the earth--which affects the taste of all its products, even to the grasses, which the cattle refuse to touch--at last compel the husbandman to abandon his fields and leave uncultivated a soil that no longer repays his labor."--Tableau de l'Agriculture Toscane, pp. 11, 12.
41. ^I ought perhaps to except the Mexicans and the Peruvians, whose arts and institutions are not yet shown to be historically connected with those of any more ancient people. The lamentable destruction of so many memorials of these tribes, by the ignorance and bigotry of the so-called Christian barbarians who conquered them, has left us much in the dark as to many points of their civilization; but they seem to have reached that stage where continued progress in knowledge and in power over nature is secure, and a few more centuries of independence might have brought them to originate for themselves most of the great inventions which the last four centuries have bestowed upon man.
42. ^In Egypt, evaporation and absorption by the earth are so rapid, that all annual crops require irrigation during the whole period of their growth. As fast as the water retires by the subsidence of the annual inundation, the seed is sown upon the still moist, uncovered soil, and irrigation begins at once. Upon the Nile, you hear the creaking of the water-wheels, and sometimes the movement of steam-pumps, through the whole night, while the poorer cultivators unceasingly ply the simple shadoof, or bucket-and-sweep, laboriously raising the water from trough to trough by as many as six or seven stages when the river is low. The bucket is of flexible leather, with a stiff rim, and is emptied into the trough, not by inverting it like a wooden bucket, but by putting the hand beneath and pushing the bottom up till the water all runs out over the brim, or, in other words, by turning the vessel inside out. The quantity of water thus withdrawn from the Nile is enormous. Most of this is evaporated directly from the surface or the superficial strata, but some moisture percolates down and oozes through the banks into the river again, while a larger quantity sinks till it joins the slow current of infiltration by which the Nile water pervades the earth of the valley to the distance, at some points, of not less than fifty miles.
43. ^"Forests," "woods," and "groves," are frequently mentioned in the Old Testament as existing at particular places, and they are often referred to by way of illustration, as familiar objects. "Wood" is twice spoken of as a material in the New Testament, but otherwise--at least according to Cruden--not one of the above words occurs in that volume. In like manner, while the box, the cedar, the fir, the oak, the pine, "beams," and "timber," are very frequently mentioned in the Old Testament, not one of these words is found in the New, EXCEPT the case of the "beam in the eye," in the parable in Matthew and Luke. This interesting fact, were other evidence wanting, would go far to prove that a great change had taken place in this respect between the periods when the Old Testament and the New were respectively composed; for the scriptural writers, and the speakers introduced into their narratives, are remarkable for their frequent allusions to the natural objects and the social and industrial habits which characterized their ages and their country.
44. ^One of these on Mount Hor, two stories deep, is in such good preservation, although probably not repaired for many centuries, that I found ten feet of water in it in June, 1851.
45. ^The present government of India obtains the same result more economically and advantageously by constructing in many provinces of that vast empire canals of great length and capacity, which not only furnish a greater supply of water than the old reservoirs, but so distribute it as to irrigate a larger area than could be watered by any system of artificial basins. The excavacations for the Ganges Canal were nearly equal to those for the Suez Canal, falling little short of 100,000 cubic yards, without counting feeders and accessory lines amounting to a length of 3,000 miles. This canal, according to a recent article in the London Times, waters a tract of land 320 miles long by 50 broad. The Jumna Canal, 130 miles long, with 608 miles of distributing branches, waters a territory 120 miles long with a breadth of 15 miles.
Other statements estimate the amount of land actually under irrigation in British India at 6,000,000 acres, and add that canals now in construction will water as much more. The Indian irrigation canals are generally navigable, some of them by boats of large tonnage, and the canals return a net revenue of from five to twenty per cent. on their cost.
46. ^The area which the waters of the Nile, left to themselves, would now cover is greater than it would have been in ancient times, because the bed of the river has been elevated, and consequently the lateral spread of the inundation increased. See Smith's Dictionary of Geography, article "Aegyptus". But the industry of the Egyptians in the days of the Pharaohs and the Ptolemies carried the Nile-water to large provinces, which have now been long abandoned and have relapsed into the condition of desert. "Anciently," observes the writer of the article "Egypt" in Smith's Dictionary of the Bible, "2,735 square miles more [about 3,700 square statute miles] may have been cultivated. In the best days of Egypt, probably all the land was cultivated that could be made available for agricultural purposes, and hence we may estimate the ancient arable area of that country at not less than 11,000 square statute miles, or fully double its present extent." According to an article in the Bollettino della Societa Geografica Italiana, vol. v., pt. iii., p. 210, the cultivated soil of Egypt in 1869 amounted to 4,500,000 acres, and the remaining soil capable of cultivation was estimated at 2,000,000 acres.
48. ^The date and the doum palm, the sont and many other acacias, the caroub, the sycamore and other trees grow in Egypt without irrigation, and would doubtless spread through the entire valley in a few years.
49. ^Wilkinson states that the total population, which, two hundred years ago, was estimated at 4,000,000, amounted till lately to only about 1,800,000 souls, having been reduced since the year 1800 from 2,500,000 to less than 2,000,000.--Handbook for Travellers in Egypt. p. 10. The population at the end of the year 1869 is computed at 5,215,000.--Bollettino della Soc. Geog. Ital., vol. v., pt. iii., p. 215. This estimate doubtless includes countries bordering on the upper Nile not embraced in Wilkinson's statistics.
50. ^Ritter supposes Egypt to have been a sandy desert when it was first occupied by man. "The first inhabitant of the sandy valley of the Nile was a desert-dweller, as his neighbors right and left, the Libyan, the nomade Arab, still are. But the civilized people of Egypt transformed, by canals, the waste into the richest granary of the world; they liberated themselves from the shackles of the rock and sand desert, in the midst of which, by a wise distribution of the fluid through the solid geographical form, by irrigation in short, they created a region of culture most rich in historical monuments."--Einleitung zur allgemeinen vergleichenden Geographie, pp. 165, 166.
This view seems to me highly improbable; for great rivers, in warm climates, are never bordered by sandy plains. A small stream may be swallowed up by sands, but if the volume of water is too large to be carried off by evaporation or drank up by absorption, it saturates its banks with moisture, and unless resisted by art, converts them into marshes covered with aquatio vegetation. By canals and embankments, man has done much to modify the natural distribution of the waters of the Nile; yet the annual inundation is not his work, and the river must have overflowed its banks and carried spontaneous vegetation with its waters, as well before as since Egypt was first occupied by the human family. There is, indeed, some reason to suppose that man lived upon the banks of the Nile when its channel was much lower, and the spread of its inundations much narrower, than at present; but wherever its flood reached, there the forest would propagate itself, and its shores would certainly have been morasses rather than sands.
The opinions of Ritter on this subject are not only improbable, but they are contradictory to the little historical testimony we possess. Herodotus informs us in Euterpe that except the province of Thebes, all Egypt, that is to say, the whole of the Delta and of middle Egypt extending to Hemopolis Magna in N. L. 27 degrees 45 minutes, was originally a morass. This morass was doubtless in great part covered with trees, and hence, in the most ancient hieroglyphical records, a tree is the sign for the cultivated land between the desert and the channel of the Nile. In all probability, the real change effected by human art in the superficial geography of Egypt is the conversion of pools and marshes into dry land, by a system of transverse dikes, which compelled the flood-water to deposit its sediment on the banks of the river instead of carrying it to the sea. The colmate of modern Italy were thus anticipated in ancient Egypt.
51. ^The mean annual precipitation in Lombardy is thirty-six inches, of which nearly two-thirds fall during the season of irrigation. The rain-fall is about the same in Piedmont, though the number of days in the year classed as "rainy" is said to be but twenty-four in the former province while it is seventy in the latter.--Baird Smith, Italian Irrigation, vol. i., p. 196.
The necessity of irrigation in the great alluivial plain of Northern Italy is partly explained by the fact that the superficial stratum of fine earth and vegetable mould is very extensively underlaid by beds of pebbles and gravel brought down by mountain torrents at a remote epoch. The water of the surface-soil drains rapdily down into these loose beds, and passes off by subterranean channels to some unknown point of discharge; but this circumstance alone is not a sufficient solution. It is not possible that the habits of vegetables, grown in countries where irrigation has been immemorially employed, have been so changed that they require water under conditions of soil and climate where their congeners, which have not been thus indulgently treated, do not It is a remarkable fact that during the season of irrigation, when large tracts of surface are almost constantly saturated with water, there is an extraordinary dryness in the atmosphere of Lombardy, the hygrometer standing for days together a few degrees only above zero, while in winter the instrument indicates extreme humidity of the air, approaching to total saturation.--Baird Smith, Italian Irrigation, i., p. 189.
There are some atmospheric phenomena in Northern Italy, which an American finds it hard to reconcile with what he has observed in the United States. To an American eye, for instance, the sky of Piedmont, Lombardy, and the northern coast of the Mediterranean, is always whitish and curdled, and it never has the intensity and fathomless depth of the blue of his native heavens. And yet the heat of the sun's rays, as measured by sensation, and, at the same time, the evaporation, are greater than they would be with the thermometer at the same point in America. I have frequently felt in Italy, with the mercury below 60 degrees Fahrenheit, and with a mottled and almost opaque sky, a heat of solar irradiation which I can compare to nothing but the scorching sensation experienced in America at a temperature twenty degrees higher, during the intervals between showers, or before a rain, when the clear blue of the sky seems infinite in depth and transparency. Such circumstances may create a necessity for irrigation where it would otherwise be superfluous, if not absolutely injurious.
In speaking of the superior apparent clearness of the SKY in America, I confine myself to the concave vault of the heavens, and do not mean to assert that terrestrial objects are generally visible at greater distances in the United States than in Italy. Indeed, I am rather disposed to maintain the contrary; for though I know that the lower strata of the atmosphere in Europe never equal in transparency the air near the earth in New Mexico, Peru, and Chili, yet I think the accidents of the coast-line of the Riviera, as, for example, between Nice and La Spezia, and those of the incomparable Alpine panorama seen from Turin, are distinguishable at greater distances than they would be in the United States.
52. ^In our comparatively rainless Western territory, irrigation is extensively and very beneficially employed. In the Salt Lake valley and in California, hundreds if not thousands of miles of irrigation canals have been constructed, and there is little doubt that artificially watering the soil will soon be largely resorted to in the older States. See valuable observations on this subject in Hayden, Preliminary Report on Geological Survey of Wyoming, 1870, pp. 194, 195, 258-261.
53. ^Memorie sui progetti per Pestensions dell' Irrigazione, etc., il Politecniso, for January, 1868, p. 6.
54. ^Niel, L'Agriculture des Etats Sardes, p. 232. This estimate, it will be observed, is 275,000 acres less than that of Lombardini.
55. ^In 1865 the total quantity of irrigated lands in the kingdom of Italy was estimated at 1,357,677 hectares, or 2,000,000 acres, of which one-half is supplied with water by artificial canals. The Canal Cavour adds 250,000 acres to the above amount. The extent of artificially watered ground in Italy is consequently equal to the entire area of the States of Delaware and Rhode Island.--See the official report, Sulle Bonificazione, Risaie, ed Irrigazioni, 1865, p. 269.
56. ^It belongs rather to agriculture than to geography to discuss the quality of the crops obtained by irrigation, or the permanent effects produced by it on the productiveness of the soil. There is no doubt, however, that all crops which can be raised without watering are superior in flavor and in nutritive power to those grown by the aid of irrigation. Garden vegetables, particularly, profusely watered, are so insipid as to be hardly eatable. Wherever irrigation is practised, there is an almost irresistible tendency, especially among ignorant cultivators, to carry it to excess; and in Piedmont and Lombardy, if the supply of water is abundant, it is so liberally applied as sometimes not only to injure the quality of the product, but to drown the plants and diminish the actual weight of the crop. Grass-lands are perhaps an exception to this remark, as it seems almost impossible to apply too much water to them, provided it be kept in motion and not allowed to stagnate on the surface. Protestor Liebig, in his Modern Agriculture, says: "There is not to be found in chemistry a more wonderful phenomenon, one which more confounds all human wisdom, than is presented by the soil of a garden or field. By the simplest experiment, any one may satisfy himself that rain-water filtered through field or garden soil does not dissolve out a trace of potash, silicic acid, ammonia, or phosphoric acid. The soil does not give up to the water one particle of the food of plants which it contains. The most continuous rains cannot remove from the field, except mechanically, any of the essential constituents of ite fertility." "The soil not only retains firmly all the food of plants which is actually in it, but its power to preserve all that may be useful to them extends much farther. If rain or other water holding in solution ammonia, potash, and phosphoric and silicic acids, be brought in contact with soil, these substances disappear almost immediately from the solution; the soil withdraws them from the water. Only such substances are completely withdrawn by the soil as are indispensable articles of food for plants; all others remain wholly or in part in solution."
These opinions were confirmed, soon after their promulgation, by the experimental researches of other chemists, but are now questioned, and they are not strictly in accordance with the alleged experience of agriculturists in those parts of Italy where irrigation is most successfully applied. They believe that the constituents of vegetable growth are washed out of the soil by excessive and long-continued watering. They consider it also established as a fact of observation, that water which has flowed through or over rich ground is more valuable for irrigation than water from the same source, which has not been impregnated with fertilizing substances by passing through soils containing them; and, on the other hand, that water, rich in the elements of vegetation, parts with them in serving to irrigate a poor soil, and is therefore less valuable as a fertilizer of lower grounds to which it may afterward be conducted. See Baird Smith, Italian Irrigation, i., p. 25; Scott Moncrieff, Irrigation in Southern Europe, pp. 34, 87, 89; Lombardini, Sulle Inondazioni etc., p. 73; Mangon, Les Irrigations, p. 48.
The practice of irrigation--except in mountainous countries where springs and rivulets are numerous--is attended with very serious economical, social, and political evils. The construction of canals and their immensely ramified branches, and the grading and scarping of the ground to be watered, are always expensive operations, and they very often require an amount of capital which can be commanded only by the state, by moneyed corporations, or by very wealthy proprietors; the capacity of the canals must be calculated with reference to the area intended to be irrigated, and when they and their branches are once constructed, it in very difficult to extend them, or to accommodate any of their original arrangements to changes in the condition of the soil, or in the modes or objects of cultivation; the flow of the water] being limited by the abundance of the source or the capacity of the canals, the individual proprietor cannot be allowed to withdraw water at will, according to his own private interest or convenience, but both the time and the quantity of supply must be regulated by a general system applicable, as far as may be, to the whole area irrigated by the same canal, and every cultivator must conform his industry to a plan which may be quite at variance with his special objects or with his views of good husbandry. The clashing interests and the jealousies of proprietors depending on the same means of supply are a source of incessant contention and litigation, and the caprices or partialities of the officers who control, or of contractors who farm, the canals, lead not unfrequently to ruinous injustice towards individual landholders. These circumstances discourage the division of the soil into small properties, and there is a constant tendency to the accumulation of large estates of irrigated land in the hands of great capitalists, and consequently to the dispossession of the small cultivators, who pass from the condition of owners of the land to that of hireling tillers.
Though farmers are no longer yeomen, but peasants. Having no interest in the soil which composes their country, they are virtually expatriated, and the middle class, which ought to constitute the real physical and moral strength of the land, ceases to exist as a rural estate, and is found only among the professional, the mercantile, and the industrial population of the cities.--See, on the difficulty of regulating irrigation by law, Negri, Idea su una Legge in materia di Acqua, 1864; and Agmard, Irrigations du Midi de L'Europe' where curious and important remarks on the laws and usages of the Spanish Moors and the Spaniards, in respect to irrigation, will be found. The Moors were so careful in maintaining the details of their system, that they kept in publio offices bronze models of their dams and sluices, as guides for repairs and rebuilding. Some of these models are still preserved. --Ibidem, pp. 204, 205. For an account of recent irrigation works in Spain, see Spon, Dictionary of Engineering, article Irrigation. As near as can be ascertained, the amount of water applied to irrigated lands is scarcely anywhere less than the total precipitation during the season of vegetable growth, and in general it much exceeds that quantity. In grass-grounds and in field-culture it ranges from 27 or 28 to 60 inches, while in smaller crops, tilled by hand-labor, it is sometimes carried as high as 300 inches.
57. ^Niel, Agriculture des Etata Sardes, p. 237. Lombardini's computation just given allows eighty-one cubic metres per day to the hectare [two hundred and sixty cubic yards to the acre], which, supposing the season of irrigation to be one hundred days, in equal to a precipitation of thirty-two inches. But in Lombardy, water in applied to some crops during a longer period than one hundred days; and in the marcite it flows over the ground even in winter. According to Boussingault (Economie Rurale, ii., p. 240), grass-grounds ought to receive, in Germany, twenty-one centimetres of water per week, and with less than half that quantity it is not advisable to incur the expense of supplying it. The ground is irrigated twenty-five or thirty times, and if the full quantity of twenty-one centimetres is applied, it receives more than two hundred inches of water, or six times the total amount of precipitation. Puvis, quoted by Boussingault, after much research comes to the conclusion that a proper quantity is twenty centimetres [eight inches] applied twenty-five or thirty times, which corresponds with the estimate just stated. Puvis adds--and, as our author thinks, with reason--that this amount might be doubled without disadvantage.--Ibidem, ii., p. 248, 249. In some parts of France this quantity is immensely exceeded, and it is very important to observe, with reference to the employment of irrigation in our Northern States, that water is most freely supplied in the COLDER provinces. Thus, in the Vosges, meadows are literally flooded for weeks together, and while in the department of Vancluse a meadow may receive, in five waterings of six and a half hours each, twenty-one inches of wnter, in the Vosges it might be deluged for twenty-four hundred hours in six applications, the enormous quantity of thirteen hundred feet of water flowing over it. See the important work of Herve Mangon, Sur l'emploi des eaux dans les Irrigations, chap. ix. Boussingault observes that rain-water is vastly more fertilizing than the water of irrigating canals, and therefore the supply of the latter must be greater. This is explained partly by the different character of the substances held in solution or suspension by the waters of the earth and of the sky, partly by the higher temperature of the latter, and, possibly, partly also by the mode of application--the rain being finely divided in its fall or by striking plants on the ground, river-water flowing in a continuous sheet.
The temperature of the water is thought even more important than its composition. The sources which irrigate the marcite of Lombardy--meadows so fertile that less than an acre furnishes grass for a cow the whole year--are very warm. The ground watered by them never freezes, and a first crop, for soiling, is cut from it in January or February. The Canal Cavour--which takes its supply chiefly from the Po at Chivasso, fourteen or fifteen miles below Turin--furnishes water of much higher fertilizing power than that derived from the Dora Baltea and the Sesia, both because it is warmer, and because it transports a more abundant and a richer sediment than the latter streams, which are fed by Alpine ice-fields and melting snows, and which flow, for long distances, in channels ground smooth and bare by ancient glaciers and not now contributing much vegetable mould or fine slime to their waters.
58. ^About one-seventh of the water which flows over the marcite is absorbed by the soil of those meadows or evaporated from their surface, and consequently six-sevenths of the supply remain for use on ground at lower levels.
59. ^On the pluviometric effect of irrigation, see Lombardini, Sulle Inondazioni, etc., p. 72, 74; the same author, Essai Hydrologique sur le Nil, p. 32; Messedaglia, Analisi dell' opera di Champion, pp. 96, 97, note; and Baird Smith, Italian Irrigation, i., pp. 189, 190. In an article in Aus der Natur, vol. 57, p. 443, it is stated that the rain on the Isthmus of Suez has increased since the opening of the canal, and has enlarged the evaporable surface of the country; but this cannot be accepted as an established fact without further evidence.
60. ^Is not the mottled appearance of the upper atmosphere in Italy, which I have already noticed, perhaps due in part to the condensation of the aqueous vapor exhaled by watered ground.
61. ^I do not know that observations have been made on the thermometric influence of irrigation, but I have often noticed that, on the irrigated plains of Piedmont ten miles south of Turin, the morning temperature in summer was several degrees below that marked at the Observatory in the city.
62. ^The proportion of the waters of the Nile withdrawn for irrigation is greater than this calculation makes it. The quantity required for an acre is less in the Delta than in Upper Egypt, both because the soil of the Delta, to which Linant Bey's estimate applies, lies little higher than the surface of the river, and is partly saturated by infiltration, and because near the sea, in N. L. 30 degrees, evaporation is much less rapid than it is several degrees southwards and in the vicinity of a parched desert.
63. ^See Vigan, Etude sur les Irrigations, Paris, 1867; and Scott Moncrieff, Irrigation in Southern Europe, pp. 89, 90.
The brook Ain Musa, which runs through the ruined city of Petra and finally disappears in the sands of Wadi el Araba, is a considerable stream in winter, and the inhabitants of that town were obliged to excavate a tunnel through the rock near the right bank, just above the upper entrance of the narrow Sik, to discharge a part of its swollen current. The sagacity of Dr. Robinson detected the necessity of this measure, though the tunnel, the mouth of which was hidden by brushwood, was not discovered till some time after his visit. I even noticed, near the arch that crosses the Sik, unequivocal remains of a sluice by which the water was diverted to the tunnel. Immense labor was also expended in widening the natural channel at several points below the town, to prevent the damming up and setting back of the water--a fact I believe not hitherto noticed by travellers.
The Fellahheen above Petra still employ the waters of Ain Musa for irrigation, and in summer the superficial current is wholly diverted from its natural channel for that purpose. At this season, the bed of the brook, which is composed of pebbles, gravel, and sand, is dry in the Sik and through the town; but the infiltration is such that water is generally found by digging to a small depth in the channel. Observing these facts in a visit to Petra in the summer, I was curious to know whether the subterranean waters escaped again to daylight, and I followed the ravine below the town for a long distance. Not very far from the upper entrance of the ravine, arborescent vegetation appeared upon its bottom, and as soon as the ground was well shaded, a thread of water burst out. This was joined by others a little lower down, and, at the distance of a mile from the town, a strong current was formed and ran down towards Wadi el Araba.
Similar facts are observed in all countries where the superficial current of water-courses is diverted from their bed for irrigation, but this case is of special interest because it shows the extent of absorption and infiltration even in the torrid climate of Arabia. See Baird Smith, Italian Irrigation, vol. i., pp. 172, 386 and 387.
64. ^Escourrou-Milliago, D'Italie a propos de l'Exposition de Paris, 1856, p. 92. According to an article in the Gazzetto di Torino for the 17th of January, 1869, the deaths from malarious fever in the Canavese district--which is asserted to have been altogether free from this disease before the recent introduction of rice-culture--between the 1st of January and the 15th of October, 1868, were two thousand two hundred and fifty. The extent of the injurious influence of this very lucrative branch of rural industry in Italy is contested by the rice-growers. But see Secondo Laura, Le Risaje, Torino, 1869; Selmi, Il Miasma Palustre, p. 89; and especially Carlo Livi, Della coltivazione del Riso in Italia, in the Nuova Antologia for July, 1871, p. 599 et seqq. According to official statistics, the rice-grounds of Italy, including the islands, amounted in 1866 to 450,000 acres. It is an interesting fact in relation to geographical and climatic conditions, that while little rice is cultivated SOUTH of N. L. 44 degrees in Italy, little is grown in the United States NORTH of 35 degrees. To the southward of the great alluvial plain of the Po, the surface is in general too much broken to admit of the formation of level fields of much extent, and where the ground is suitable, the supply of water is often insufficient.
The Moors introduced the cultivation of rice into Spain at an early period of their dominion in that country. The Spaniards sowed rice in Lombardy and in the Neapolitan territory in the 16th century; but besides the want of water and of level ground convenient for irrigation, rice-husbandry has proved so much more pestilential in Southern than in Northern Italy that it has long been discouraged by the Neapolitan government.
65. ^The great limestone plateau of the Karst in Carniola is completely honey-combed by caves through which the drainage of that region is conducted. Rivers of considerable volume pour into some of these caves and can be traced underground to their exit. Thus the Recca has been satisfactorily identified with a stream flowing through the cave of Trebich, and with the Timavo--the Timavus of Virgil and the ancient geographers--which empties through several mouths into the Adriatic between Trieste and Aquileia. The city of Trieste is very insufficiently supplied with fresh water. It has been thought practicable to supply this want by tunnelling through the wall of the plateau, which rises abruptly in the rear of the town, until some subterranean stream is encountered, the current of which can be conducted to the city. More visionary projectors have gone further, and imagined that advantage might be taken of the natural tunnels under the Karst for the passage of roads, railways, and even navigable canals. But however chimerical these latter schemes may seem, there is every reason to believe that art might avail itself of these galleries for improving the imperfect drainage of the champaign country bounded by the Karst, and that stopping or opening the natural channels might very much modify the hydrography of an extensive region. See in Aus des Natur, xx., pp. 250-254, 263-266, two interesting articles founded on the researches of Schmidt.
The cases are certainly not numerous where marine currents are known to pour continuously into cavities beneath the surface of the earth, but there is at least one well-authenticated instance of this sort--that of the mill-stream at Argostoli in the island of Cephalonia. It had been long observed that the sea-water flowed into several rifts and cavities in the limestone rocks of the coast, but the phenomenon has excited little attention until very recently. In 1833, three of the entrances were closed, and a regular channel, sixteen feet long and three feet wide, with a fall of three feet, was cut into the mouth of a larger cavity. The sea-water flowed into this canal, and could be followed eighteen or twenty feet beyond its inner terminus, when it disappeared in holes and clefts in the rock.
In 1858 the canal had been enlarged to thewidth of five feet and a half, and a depth of a foot. The water pours rapidly through the canal into an irregular depression and forms a pool, the surface of which is three or four feet below the adjacent soil, and about two and a half or three feet below the level of the sea. From this pool it escapes through several holes and clefts in the rock, and has not yet been found to emerge elsewhere.
There is a tide at Argostoli of about six inches in still weather, but it is considerably higher with a south wind. I do not find it stated whether water flows through the canal into the cavity at low tide, but it distinctly appears that there is no refluent current, as of course there could not be from a base so much below the sea. Mousson found the delivery through the canal to be at the rate of 24.88 cubic feet to the second; at what stage of the tide does not appear. Other mills of the same sort have been erected, and there appear to be several points on the coast where the sea flows into the land.
Various hypotheses have been suggested to explain this phenomenon, some of which assume that the water descends to a great depth beneath the crust of the earth; but the supposition of a difference of level in the surface of the sea on the opposite sides of the island, which seems confirmed by other circumstances, is the most obvious method of explaining these singular facts. If we suppose the level of the water on one side of the island to be raised by the action of currents three or four feet higher than on the other, the existence of cavities and channels in the rock would easily account for a subterranean current beneath the island, and the apertures of escape might be so deep or so small as to elude observation. See Aus der Natur, vol. xix., pp. 129 et seqq. I have lately been informed by a resident of the Ionian Islands, who is familiar with the locality, that the sea flows uninterruptedly into the sub-insular cavities, at all stages of the tide.
66. ^"The affluents received by the Seine below Rouen are so inconsiderable, that the augmentation of the volume of that river must be ascribed principally to springs rising in its bed. This is a point of which engineers now take notice, and M. Belgrand, the able officer charged with the improvement of the navigation of the Seine between Paris and Rouen, has devoted much attention to it."--Babinet, Etudes et Lectures, iii., p. 185.
On page 232 of the volume just quoted, the same author observes: "In the lower part of its course, from the falls of the Oise, the Seine receives so few important affluents, that evaporation alone would suffice to exhaust all the water which passes under the bridges of Paris."
This supposes a much greater amount of evaporation than has been usually computed, but I believe it is well settled that the Seine conveys to the sea much more water than is discharged into it by all its superficial branches. Babinet states the evaporation from the surface of water at Paris to be twice as great as the precipitation.
Belgrand supposes that the floods of the Seine at Paris are not produced by the superficial flow of the water of precipitation into its channel, but from the augmented discharge of its remote mountain sources, when swollen by the rains and melted snows which percolate through the permeable strata in its upper course.--Annales des Ponts et Chaussees, 1851, vol. i.
67. ^Physicalische Geographie, p. 286. It does not appear whether this inference is Mariotte's or Wittwer's. I suppose it is a conclusion of the latter. According to Valles, the Seine discharges into the sea thirty per cent. of the precipitation in its valley, while the Po delivers into the Adriatic two-thirds and perhaps even three-quarters of the total down-fall of its basin. The differences between the tributaries of the Mississippi in this respect are remarkable, the Missouri discharging only fifteen per cent., the Yazoo not less than ninety. The explanation of these facts is found in the geographical and geological character of the valleys of these rivers. The Missouri flows with a rapid current through an irregular country, the Yazoo has a very slow flow through a low, alluvial region which is kept constantly almost saturated by infiltration.
69. ^In the low peninsula of Florida, rivers, which must have their sources in mountains hundreds of miles distant, pour forth from the earth with a volume sufficient to permit steamboats to ascend to their basins of eruption. In January, 1857, a submarine fresh-water river burst from the bottom of the sea not far from the southern extremity of the peninsula, and for a whole month discharged a current not inferior in volume to the river Mississippi, or eleven times the mean delivery of the Po, and more than six times that of the Nile. We can explain this phenomenon only by supposing that the bed of the sea was suddenly burst up by the hydrostatic upward pressure of the water in a deep reservoir communicating with some great subterranean river or receptacle in the mountains of Georgia or of Cuba, or perhaps even in the valley of the Mississippi.--Thomassy, Essai sur l'Hydrologie. Late southern journals inform us that the creek under the Natural Bridge in Virginia has suddenly disappeared, being swallowed up by newly formed fissures, of unknown depth, in its channel. It does not appear that an outlet for the waters thus absorbed has been discovered, and it is not improbable that they are filling some underground cavity like that which supplied the submarine river just mentioned.
70. ^See especially Stoppani, Corso di Geologia, i., pp. 270 et seqq.
71. ^Paramelle, Quellenkunde, mit einem Vorwort von B. Cotta. 1856.
72. ^See Lombardini, Importanza degli studi sulla Statistica da Fiumi, p. 27; also, same author, Sulle Inondazioni avvenute in Francia, etc., p. 29.
73. ^The roots of trees planted in towns do not depend exclusively on infiltration for their supply of water, for they receive a certain amount of both moisture and air through the interstices between the paving-stones; but where wide surfaces of streets and courts are paved with air and water tight asphaltum, as in Paris, trees suffer from the diminished supply of these necessary elements.
74. ^See the interesting observations of Krieck on this subject, Schriften zur allgemeinen Erdkunde, cap. iii., Section 6, and especially the passages in Ritter s Erdkunde, vol. i., there referred to.
The tenacity with which the parched soil of Egypt retains the supply of moisture it receives from the Nile is well illustrated by observations of Girard cited by Lombardini from the Memoires de l'Academie des Sciences, t. ii., 1817. Girard dug wells at distances of 3,200, 1,800, and 1,200 metres from the Nile, and after three months of low water in the river, found water in the most remote well, at 4m. 97, in the next at 4m. 23, and in that nearest the bank at 3m. 44 above the surface of the Nile. The fact that the water was highest in the most distant well appears to show that it was derived from the inundation and not, by lateral infiltration, from the river. But water is found beneath the sands at points far above and beyond the reach of the inundations, and can be accounted for only by subterranean percolation from the Nile. At high flood, the hydrostatic pressure on the banks, combined with capillary attribution, sends water to great horizontal distances through the loose soil; at low water the current is reversed, and the moisture received from the river is partly returned, and may often be seen oozing from the banks into the river.--Clot Bey, Apercu sur l'Egypte, i., 128.
Laurent (Memoires sur le Sahara Oriental, pp. 8, 9), in speaking of a river at El-Faid, "which, like all those of the desert, is, most of the time, without water," observes, that many wells are dug in the bed of the river in the dry season, and that the subterranean supply of water thus reached extends itself laterally, at about the same level, at least a kilometre from the river, as water is found by digging to the depth of twelve or fifteen metres at a village situated at that distance from the bank.
Recent experiments, however, have shown that in the case of rivers flowing through thickly peopled regions, and especially where the refuse from industrial establishments is discharged into them, the finely comminuted material received from sewers and factories sometimes clogs up the interstices between the particles of sand and gravel which compose the bed and banks, and the water is consequently confined to the channel and no longer diffuses itself laterally through the adjacent soil. This obstruction of course acts in both directions, according to circumstances. In one case, it prevents the escape of river-water and tends to maintain a full flow of the current; in another it intercepts the supply the river might otherwise receive by infiltration from the land, and thus tends to reduce the volume of the stream. In some instances pits have been sunk along the banks of large rivers and the water which filters into them pumped up to supply aqueducts. This method often succeeds, but where the bed of the stream has been rendered impervious by the discharge of impurities into it, it cannot be depended upon.
The tubular wells generally known as the American wells furnish another proof of the free diffusion and circulation of water through the soil. I do not know the date of the first employment of these tubes in the United States, but as early as 1861, the Chevalier Calandra used wooden tubes for this pose in Piedmont, with complete success. See the interesting pamphlet, Sulla Estrazione delle Acque Sotterrance, by C. Calandra. Torino, 1867.
The most remarkable case of infiltration known to me by personal observation is the occurrence of fresh water in the beach-sand on the eastern side of the Gulf of Akaba, the eastern arm of the Red Sea. If you dig a cavity in the beach near the sea-level, it soon fills with water so fresh as not to be undrinkable, though the sea-water two or three yards from it contains even more than the average quantity of salt. It cannot be maintained that this is sea-water freshed by filtration through a few feet or inches of sand, for salt-water cannot be deprived of its salt by that process. It can only come from the highlands of Arabia, and it would seem that there must exist some large reservoir in the interior to furnish a supply which, in spite of evaporation, holds out for months after the last rains of winter, and perhaps even through the year. I observed the fact in the month of June. See Robinson, Biblical Researches, 1857, vol i., p. 167.
The precipitation in the mountains that border the Red Sea is not known by pluviometric measurement, but the mass of debris brought down the ravines by the torrents proves that their volume must be large. The proportion of surface covered by sand and absorbent earth, in Arabia Petraea and the neighboring countries, is small, and the mountains drain themselves rapidly into the wadies or ravines where the torrents are formed; but the beds of earth and disintegrated rock at the bottom of the valleys are of so loose and porous texture, that a great quantity of water is absorbed in saturating them before a visible current is formed on their surface. In a heavy thunder-storm, accompanied by a deluging rain, which I witnessed at Mount Sinai in the month of May, a large stream of water poured, in an almost continuous cascade, down the steep ravine north of the convent, by which travellers sometimes descend from the plateau between the two peaks, but after reaching the foot of the mountain, it flowed but a few yards before it was swallowed up in the sands.
Fresh-water wells are not unfrequently found upon the borders of ocean beaches. In the dry summer of 1870, drinkable water was procured in many places on the coast of Liguria by digging to the depth of a yard in the beach-sands. Tubular wells reach fresh water at twelve or fifteen feet below the surface on the sandy plains of Cape Cod. In this latter case, the supply is more probably derived directly from precipitation than from lateral infiltration.
75. ^Charles Martins, Le Sahara, in Revue des Deux Mondes, Sept. 1, 1864, p. 619; Stoppani, Corso di Geologia, i., 281; Desor, Die Sahara, Basel, 1871, pp. 50, 51.
76. ^It is conceivable that in shallow subterranean basins superincumbered mineral strata may rest upon the water and be partly supported by it. In such case the weight of such strata would be an additional, if not the sole, cause of the ascent of the water through the tubes of artesian wells. The ascent of petroleum in the artesian oil-wells in Pennsylvania, and, in many cases, of salt-water in similar tubes, can hardly be ascribed to hydrostatic pressure, and there is much difficulty in accounting for the rise of water in artesian wells in many parts of the African desert on that principle. Perhaps the elasticity of gases, which probably aids in forcing up petroleum and saline waters, may be, not unfrequently, an agency in causing the flow of water in common artesian borings. It is said that artesian wells lately bored in Chicago, some to the depth of 1,600 feet, raise water to the height of 100 feet above the surface. What is the source of the pressure.
77. ^Many more or less probable conjectures have been made on this subject but thus far I am not aware that any of the apprehended results have been actually shown to have happened. In an article in the Annales des Ponts et Chaussees for July and August, 1839, p. 131, it was suggested that the sinking of the piers of a bridge at Tours in France was occasioned by the abstraction of water from the earth by artesian wells, and the consequent withdrawal of the mechanical support it had previously given to the strata containing it. A reply to this article will be found in Viollet, Theorie des Puits Artesiens, p. 217.
In some instances the water has rushed up with a force which seemed to threaten the inundation of the neighborhood, and even the washing away of much soil; but in those cases the partial exhaustion of the supply, or the relief of hydrostatic or elastic pressure, has generally produced a diminution of theflow in a short time, and I do not know that any serious evil has everbeen occasioned in this way.In April, 1866, a case of this sort occurred in boring an artesian well near the church of St. Agnes at Venice. When the drill reached the depth of 160 feet, a jet of mud and water was shot up to the height of 130 feet above the surface, and continued to flow with gradually diminishing force for about eight hours.
78. ^See a very interesting account of these wells, and of the workmen who clean them out when obstructed by sand brought up with the water, in Laurent's memoir on the artesian wells recently bored by the French Government in the Algerian desert. Mimoire sur le Sahara Oriental, etc., pp. et seqq. Some of the men remained under water from two minutes to two minutes and forty seconds. Several officers are quoted as having observed immersions of three minutes' duration, and M. Berbrugger witnessed ona of six minutes and five seconds and another of five minutes and fifty-five seconds. The shortest of these periods is longer than the best pearl-diver can remain below the surface of salt-water. The wells of the Sahara are from twenty to eighty metres deep.-Desor, Die Sahara, Basel, 1871, p. 43.
The ancient Egyptians were acquainted with the art of boring artesian wells. Ayme, a French engineer in the service of the Pacha of Egypt, found several of these old wells, a few years ago, in the oases. They differed little from modern artesian wells, but were provided with pear-shaped valves of stone for closing them when water was not needed. When freed from the sand and rubbish with which they were choked, they flowed freely and threw up fish large enough for the table. The fish were not blind, as cave-fish often are, but were provided with eyes, and belonged to species common in the Nile. The sand, too, brought up with them resembled that of the bed of that river. Hence it is probable that they were carried to the oases by subterranean channels from the Nile.--Desor, Die Sahara, Basel, 1871, p. 28; Stoppani, Corso di Geologia, i., p. 281. Barth speaks of common wells in Northern Africa from 200 to 360 feet deep.--Reisen in Africa, ii., p. 180.
It is certain that artesian wells have been common in China from a very remote antiquity, and the simple method used by the Chinese--where the drill is raised and let fall by a rope, instead of a rigid rod--has lately been employed in Europe with advantage. Some of the Chinese wells are said to be 3,000 feet deep; that of Neusalzwerk in Silesia is 2,300. A well was bored at St. Louis, in Missouri, a few years ago, to supply a sugar refinery, to the depth of 2,199 feet. This was executed by a private firm in three years, at the expense of only $10,000. Four years since the boring was recommenced in this well and reached a depth of 3,150 feet, but without a satisfactory result. Another artesian well was sunk at Columbus, in Ohio, to the depth of 2,500 feet, but without obtaining the desired supply of water. Perhaps, however, the artesian well of the greatest depth ever executed until very recently, is that bored within the last six or seven years, for the use of an Insane Asylum near St. Louis. This well descends to the depth of three thousand eight hundred and forty-three feet, but the water which it furnishes is small in quantity and of a quality that cannot be used for ordinary domestic purposes. The bore has a diameter of six inches to the depth of 425 feet, and after that it is reduced to four inches. For about three thousand feet the strata penetrated were of carboniferous and magnesian limestone alternating with sandstone. The remainder of the well passes through igneous rock. At St. Louis the Missouri and Mississippi rivers are not more than twenty miles distant from each other, and it is worthy of note that the waters of neither of those two rivers appear to have opened for themselves a considerable subterranean passage through the rocky strata of the peninsula which separates them.
When in boring an artesian well water is not reached at a moderate depth, it is not always certain that it will be found by driving the drill still lower. In certain formations, water diminshes as we descend, and it seems probable that, except in case of caverns and deep fissures, the weight of the superincumbent mineral strata so compresses the underlying ones, at no very great distance below the surface, as to render them impermeable to water and consequently altogether dry. See London Quarterly Journal of Science, No. xvii., Jan., 1868, p. 18, 19.
In the silver mines of Nevada water is scarcely found at depths below 1,000 feet, and at 1,200 feet from the surface the earth is quite dry.--American Annual of Scientific Discovery for 1870, p. 75.
Similar facts are observed in Australia. The Pleasant Creek News writes: "A singular and unaccountable feature in connection with our deep quartz mines is being developed daily, which must surprise those well experienced in mining matters. It is the decrease of water as the greater depths are reached. In the Magdala shaft at 950 ft. the water has decreased to a MINIMUM; in the Crown Cross Reef Company's shaft, at 800 ft., notwithstanding the two reefs recently struck, no extra water has been met with; and in the long drive of the Extended Cross Reef Company, at a depth of over 800 ft., the water is lighter than it was nearer the surface."
Boring has been carried to a great depth at Sperenberg near Berlin, where, in 1871, the drill had descended 5,500 feet below the surface, passing through a stratum of salt for the last 3,200 feet; but the drilling was still in progress, the whole thickness of the salt-bed not having been penetrated.--Aus der Natur, vol. 55, p. 208.
The facts that there are mines extending two miles under the bed of the sea, which are not particularly subject to inconvenience from water, that little water was encountered in the Mt. Cenis tunnel, 3500 feet below the surface, and that at Scarpa, not far from Tivoli, there is an ancient well 1700 feet deep with but eighteen feet of water, may also be cited as proofs that water is not universally diffused at great distances beneath the surface.
79. ^"In the anticipation of our success at Oum-Thiour, everything had been prepared to take advantage of this new source of wealth without a moment's delay. A division of the tribe of the Selmia, and their sheikh, Aissa ben Sha, laid the foundation of a village as soon as the water flowed, and planted twelve hundred date-palms, renouncing their wandering life to attach themselves to the soil. In this arid spot, life had taken the place of solitude, and presented itself, with its smiling images, to the astonished traveller. Young girls were drawing water at the fountain; the flocks, the great dromedaries with their slow pace, the horses led by the halter, were moving to the watering trough; the hounds and the falcons enlivened the group of party-colored tents, and living voices and animated movement had succeeded to silence and desolation."--Laurent, Memoires sur le Sahara, p. 85. Between 1856 and 1864 the French engineers had bored 83 wells in the Hodna and the Sahara of the Province of Constantine, yielding, all together, 9,000 gallons a minute, and irrigating more than 125,000 date-palms. Reclus, La Terre, i., p. 110.
80. ^The variety of hues and tones in the local color of the desert is, I think, one of the phenomena which most surprise and interest a stranger to those regions. In England and the United States, rock is so generally covered with moss or earth, and earth with vegetation, that untravelled Englishmen and Americans are not very familiar with naked rock as a conspicuous element of landscape. Hence, in their conception of a bare cliff or precipice, they hardly ascribe definite color to it, but depict it to their imagination as wearing a neutral tint not assimilable to any of the hues with which nature tinges her atmospheric or paints her organic creations. There are certainly extensive desert ranges, chiefly limestone formations, where the surface is either white, or has weathered down to a dull uniformity of tone which can hardly be called color at all; and there are sand plains and drifting hills of wearisome monotony of tint. But the chemistry of the air, though it may tame the glitter of the limestone to a dusky gray, brings out the green and brown and purple of the igneous rocks, and the white and red and blue and violet and yellow of the sandstone. Many a cliff in Arabia Petraea is as manifold in color as the rainbow, and the veins are so variable in thickness and inclination, so contorted and involved in arrangement, as to bewilder the eye of the spectator like a disk of party-colored glass in rapid evolution. In the narrower wadies the mirage is not common; but on broad expanses, as at many points between Cairo and Suez, and in Wadi el Araba, it mocks you with lakes and land-locked bays, studded with inlands and fringed with trees, all painted with an illusory truth of representation absolutely indistinguishable from the reality. The checkered earth, too, is canopied with a heaven as variegated as itself. You see, high up in the sky, rosy clouds at noonday, colored probably by reflection from the ruddy mountains, while near the horizon float cumuli of a transparent, ethereal blue, seemingly balled up out of the clear cerulean substance of the firmament, and detached from the heavenly vault, not by color or consistence, but solely by the light and shade of their salient and retreating outlines.
81. ^Oeuvres de Palissy, Des Eaux et Fontaines, p. 157.
82. ^Id., p. 166. Palissy's method has recently been tried with good success in various parts of France.
83. ^Babinet, Etudes et Lectures sur les Sciences d'Observation, ii., p. 225. Our author precedes his account of his method with a complaint which most men who indulge in thinking have occasion to repeat many times in the course of their lives. "I will explain to my readers the construction of artificial fountains according to the plan of the famous Bernard de Palissy, who, a hundred and fifty [three hundred] years ago, came and took away from me, a humble academician of the nineteenth century, this discovery which I had taken a great deal of pains to make. It is enough to discourage all invention when one finds plagiarists in the past as well as in the future!" (P. 224.)
84. ^M. G. Dumas, La Science des Fontaines, 1857.
85. ^All the arrangements of rural husbandry, and we might say of civilised occupancy of the earth, are such as necessarily to increase the danger and the range of floods by promoting the rapid discharge of the waters of precipitation. Superficial, if not subterranean, drainage is a necessary condition of all agriculture. There is no field which has not some artificial disposition for this purpose, and even the furrows of ploughed land, if the surface is inclined, and especially when it if frozen, serve rather to carry off than to retain water. As Bacquerel has observed, common road and railway ditches are among the most efficient conduits for the discharge of surface-water which man has yet constructed, and of course they are powerful agents in causing river inundations. All these channels are, indeed, necessary for the convenience of man, but this convenience, like every other interference with the order of nature, must often be purchased at a heavy cost.
86. ^Champion, Les Inondations en France, iii., p.156, note.
87. ^Notwithstanding this favorable circumstance, the damage done by the inundation of 1840 in the valley of the Rhone was estimated at seventy-two millions of francs.--Champion, Les Inondations en France, iv., p. 124.
Several smaller floods of the Rhone, experienced at a somewhat earlier season of the year in 1846, occasioned a loss of forty-five millions of francs. "What if," says Dumont, "instead of happening in October, that is, between harvest and seedtime, they had occurred before the crops were secured The damage would have been counted by hundreds of millions."--Des Travaux Publics, p. 99, note.
88. ^On the construction of temporary and more permanent barriera to the curreuts of torrents and rivulets, see Marchand, Les Torrents des Alpes, in Recue des Eaux et Forets for October and November, 1871.
89. ^In reference to the utilization of artificial as well as natural reservoirs, see Ackerhof, Die Nutruny der Teiche und Gewasser, Quadlinburg, 1869.
90. ^For accounts of damage from the bursting of reservoirs, see Vallee, Memoire sur les Reservoir d'Alimentation des Canaux, Annales des Ponts et Chaussees, 1833, 1er semestre, p.261. The dam of the reservoir of Puentes in Spain, which was one hundred and sixty feet high, after having discharged its functions for eleven years, burst, in 1802, in consequence of a defect in its foundations, and the eruption of the water destroyed or seriously injured eight hundred houses, and produced damage to the amount of more than a million dollars.--Aynard, Irrigations du Midi d l'Europe, pp. 257-259.
91. ^Baird Smith, Italian Irrigation, i., p. 176.
92. ^Bollettino della Societa Geog. Italiana, iii., p. 466.
93. ^See, as to the probable effects of certain proposed hydraulic works at the outlet of Lake Maggiore on the action of the lake as a regulating reservoir, Tagliasecchi, Notizie sui Canali dell' Alta Lombardia, Milano, 1869.
94. ^Elisee Recluse, La Terre, i., p. 460.
95. ^The insufficiency of artificial basins of reception as a means of averting the evils resulting from the floods of great rivers has been conclusively shown, in reference to a most important particular case--that of the Mississippi--by Humphreys and Abbot, in their admirable monograph of that river.
96. ^Some geographical writers apply the term bifurcation exclusively to this intercommunication of rivers; others, with more etymological propriety, use it to express the division of great rivers into branches at the head of their deltas. A technical word is wanting to designate the phenomenon mentioned in the text, and there is no valid objection to the employment of the anatomical term anastomosis for this purpose.
97. ^The division of the currents of rivers, as a means of preventing the overflow of their banks, is by no means a remedy capable of general application, even when local conditions are favorable to the construction of an emissary. The velocity of a stream, and consequently its delivery in a given time, are frequently diminished in proportion to the diminution of the volume by diversion; and on the other hand, the increase of volume by the admission of a new tributary increases proportionally the velocity and the quantity of water delivered. Emissaries may, nevertheless, often be useful in carrying off water which has already escaped from the channel and which would otherwise become stagnant and prevent further lateral discharge from the main current, and it is upon this principle that Humphreys and Abbot think a canal of diversion at Lake Providence might be advisable. Emissaries serve an important purpose in the lower course of rivers where the bed is nearly a dead level and the water moves from previously acquired momentum and the pressure of the current above, rather than by the force of gravitation, and it is, in general, only under such circumstances, as for example in the deltas at the mouths of great rivers, that nature employs them.
98. ^Mardigny, Memoire sur les Inondations de l'Ardeche, p. 13.
99. ^The starting-points of these anals were far up the Nile, and of course at a comparatively high level, and it is probable that they received water only during the inundation. Linant Bey calculates the capacity of Lake Moeris at 3,686,667 cubic yards and the water received by it at high Nile at 465 cubic yards the second.
100. ^In 1845 a similar lake was formed by the extension of the Vernagt glacier[. When the ice barrier gave way, 3,000,000 cubic yards of water were discharged in an hour.--Sonklar, Die Oetzthaler Gebirgsgruppe, section 167.
101. ^Riparian embankments are a real, if not a conscious, imitation of a natural process. The waters of rivers which flow down planes of gentle inclination deposit, in their inundations, the largest proportion of their [[[sediment]] as soon as, by overflowing their banks, they escape from the swift current of the channel. The immediate borders of such rivers consequently become higher than the grounds lying further from the stream, and constitute, of themselves, a sort of natural dike of small elevation. In the "intervales" or "bottoms" of the great North American rivers the alluvial banks are elevated and dry, the flats more remote from the river lower and swampy. This is generally observable in Egypt (see Figari Bey, Studi Scientifici sull' Egitto, i, p. 87), though less so than in the valley of the Mississippi, where the alluvial banks form natural glacis, descending as you recede from the river, and in some places, as below Cape Girardeau, at the rate of seven feet in the first mile. Humphreys and Abbott, Report, pp. 96, 97. In fact, rivers, like mountain torrents, often run for a long distance on the summit of a ridge built up by their own deposits. The delta of the Mississippi is a regular cone, or rather mountain, of dejection, extending far out into the Gulf of Mexico, along the crest of which the river flows, sending off here and there, as it approaches the sea, a system of lateral streams resembling the fan-shaped discharge of a torrent.
102. ^In proportion as the dikes are improved, and breaches and the escape of the water through them are less frequent, the height of the annual inundations is increased. Some towns on the banks of the Po, and of course within the system of parallel embankments, were formerly secure from flood by the height of the artificial mounds on which they were built; but they have recently been obliged to construct ring-dikes for their protection.
Lombardini lays down the following general statement of the effects of river embankments:
"The immediate effect of embanking a river is generally an increase in the height of its floods, but, at the same time, a depression of its bed, by reason of the increased force, and consequently excavating action, of the current.
"It is true that coarser material may hence be carried further, and at the same time deposit itself on a reduced slope.
"The embankment of the upper branches of a river increases the volume, and therefore the height of the floods in the lower course, in consequence of the more rapid discharge of its affluents into it.
"When, in consequence of the flow of a river channel through an alluvial soil not yet REGULATED, or, in other words, which has not acquired its normal inclination, the course of the river has not become established, it is natural that its bed should rise more rapidly after its embankment. ...
"The embankment of the lower course of a river, near its discharge into the sea, causes the elevation of the bed of the next reach above, both because the swelling of the current, in consequence of its lateral confinement, occasions eddies, and of course deposits, and because the prolongation of the course of the stream, or the advance of its delta into the sea, is accelerated."--Dei congiamenti cia soggiacque l'idraulica condizione del Po, etc., pp. 41, 42.
Del Noce states that in the levellings for the proposed Leopolda railway, he found that the bed of the Sieue had been permanently elevated two yards between 1708 and 1844, and that of the Fosso di San Gaudenzio more than a yard and a half between 1752 and 1845. Those, indeed, are not rivers of the rank of the Po; but neither are they what are technically called torrents or mountain streams, whose flow is only an occasional effect of heavy rains or melting snow.--Trattato delle Macchie e Foreste di Tuscana, Firenze, 1857, p. 29.
103. ^The Noang-ho has repeatedly burst its dikes and changed the channel of its lower course, sometimes delivering its waters into the sea to the north, sometimes to the south of the peninsula of Chan-tung, thus varying its point of discharge by a distance of 220 miles.--Elisee Reclus, La Terre, t. i, p. 477.
Sec interesting notices of the lower course of the Noang-ho in Nature, Nov. 25, 1869.
The frequent changes of channel and mouth in the deltas of great rivers are by no means always an effect of diking. The mere accumulation of deposits in the beds of rivers which transport much sediment compels them continually to seek new outlets, and it is only by great effort that art can keep their points of discharge pproximately constant. The common delta of the Ganges and the Brahmapootra is in a state of incessant change, and the latter river is said to have shifted its main channel 200 miles to the west since 1785, the revolution having been principally accomplished between 1810 and 1830.
104. ^This method has been adopted on the lower course of the Lamone, and a considerable extent of low ground adjacent to that river has been raised by spontaneous deposit to a sufficient height to admit of profitable cultivation.
105. ^I do not mean to say that all rivers excavate their own valleys, for I have no doubt that in the majority of cases such depressions of the surface originate in higher geological causes, such as the fissures and other irregularities of surface which could not fail to accompany upheaval, and hence the valley makes the river, not the river the valley. But even if we suppose a basin of the hardest rock to be elevated at once, completely formed, from the submarine abyss where it was fashioned, the first shower of rain that falls upon it, after it rises to the air, will discharge its waters along the lowest lines of the surface, and cut those lines deeper, and so on with every successive rain. The disintegrated rock from the upper part of the basin forms the lower by alluvial deposit, which is constantly transported farther and farther until the resistance of gravitation and cohesion balances the mechanical force of the running water. Thus plains, more or less steeply inclined, are formed, in which the river is constantly changing its bed, according to the perpetually varying force and direction of its currents, modified as they are by ever-fluctuating conditions. Thus the Po is said to have long inclined to move its channel southwards, at certain points, in consequence of the mechanical force of its northern affluents. A diversion of these tributaries from their present beds, so that they should enter the main stream at other points and in different directions, might modify the whole course of that great river. But the mechanical force of the tributary is not the only element of its influence on the course of the principal stream. The deposits it lodges in the bed of the latter, acting as simple obstructions or causes of diversion, are not less important agents of change.
106. ^The distance to which a new obstruction to the flow of a river, whether by a dam or by a deposit in its channel, will retard its current, or, in popular phrase, "set back the water," is a problem of more difficult practical solution than almost any other in hydraulics. The elements--such as straightness or crookedness of channel, character of bottom and banks, volume and previous velocity of current, mass of water far above the obstruction, extraordinary drought or humidity of seasons, relative extent to which the river may be affected by the precipitation in its own basin, and by supplies received through subterranean channels from sources so distant as to be exposed to very different meteorological influences, effects of clearing and other improvements always going on in new countries--are all extremely difficult, and some of them impossible, to be known and measured. In the American States, very numerous water-mills have been erected within a few years, and there is scarcely a stream in the settled portion of the country which has not several mill-dams upon it. When a dam is raised--a process which the gradual diminution of the summer currents renders frequently necessary--or when a new dam is built, it often happens that the meadows above are flowed, or that the retardation of the stream extends back to the dam next above. This leads to frequent law-suits. From the great uncertainty of the facts, the testimony is more conflicting in these than in any other class of cases, and the obstinacy with which "water causes" are disputed has become proverbial.
107. ^The sediment of the Po has filled up some lagoons and swamps in its delta, and converted them into comparatively dry land; but, on the other hand, the retardation of the current from the lengthening of its course, and the diminution of its velocity by the deposits at its mouth, have forced its waters at some higher points to spread in spite of embankments, and thus fertile fields have been turned into unhealthy and unproductive marshes.--See Botter, Sulla condizione dei Terreni Maremmani nel Ferraress. Annali di Agricoltura, etc., Fasc. v., 1863.
108. ^In the case of rivers flowing through wide alluvial plains and much inclined to shift their beds, like the Po, the embankments often leave a very wide space between them. The dikes of the Po are sometimes three or four miles apart.
109. ^It appears from the investigations of Lombardini that the rate of elevation of the bed of the Po has been much exaggerated by earlier writers, and in some parts of its course the change is so slow that its level may be regarded as nearly constant. Observation has established a similar constancy in the bed of the Rhone and of many other important rivers, while, on the other hand, the beds of the Adige and the Brenta, streams of a more torrential character, are raised considerably above the level of the adjacent fields.
The length of the lower course of the Po having been considerably increased by the filling up of the Adriatic with its deposits, the velocity of the current ought, prima facie, to have been diminished and its bed raised in proportion. There are abundant grounds for believing that this has happened in the case of the Nile, and one reason why the same effect has not been more sensibly perceptible in the Po is, that the confinement of the current by continuous embankements gives it a high-water velocity sufficient to sweep out deposits let fall at lower stages and slower movements of the water. Torrential streams tend to excavate or to raise their beds according to the inclination, and to the character of the material they transport. No general law on this point can be laid down in relation to the middle and lower courses of rivers. The conditions which determine the question of the depression or elevation of a river-bed are too multifarious, variable, and complex, to be subjected to formulae, and they can scarcely even be enumerated.
The following observation, however, though apparently too unconditionally stated, is too important to be omitted.
Rivers which transport sand, gravel, pebbles, heavy mineral matter in short, tend to raise their own beds; those charged only with fine, light earth, to cut them deeper. The prairie rivers of the western United States have deep channels, because the mineral matter they carry down is not heavy enough to resist the impulse of even a moderate current, and those tributaries of the Po which deposit their sediment in the lakes--the Ticino, the Adda, the Oglio, and the Mincio--flow in deep cuts, for the same reason.--Baumgarten, p. 132.
In regard to the level of the bed of the Po, there is another weighty consideration which does not seem to have received the attention it deserves. refer to the secular depression of the western coast of the Adriatic, which is computed at the rate of fifteen or twenty centimetres in a century, and which of course increases the inclination of the bed, and the velocity and transporting power of the current of the Po, UNLESS we assume that the whole course of the river, from the sea to its sources, shares in the depression. Of this assumption there is no proof, and the probability is to the contrary. For the evidence, though not conclusive, perhaps, tends to show an elevation of the Tuscan coast, and even of the Ligurian shore at points lying farther west than the sources of the Po. The level of certain parts of the bed of the river referred to by Lombardini as constant, is not their elevation as compared with points nearer the sea, but relatively to the adjacent plains, and there is every reason to believe that the depression of the Adriatic coast, whether, as is conceivable, occasioned by the mere weight of the fluviatile deposits or by more general geological causes, has increased the slope of the bed of the river between the points in question and the sea. In this instance, then, the relative permanency of the river level at certain points may be, not the ordinary case of a natural equilibrium, but the negative effect of an increased velocity of current which prevents deposits where they would otherwise have happened.
110. ^To secure the city of Sacramento, in California, from the inundations to which it is subject, a dike or levee was built upon the bank of the river and raised to an elevation above that of the highest known floods, and it was connected, below the town, with grounds lying considerably above the river. On one occasion a breach in the dike occurred above the town at a very high stage of the flood. The water poured in behind it, and overflowed the lower part of the city, which remained submerged for some time after the river had retired to its ordinary level, because the dike, which had been built to keep the water OUT, now kept it IN.
According to Arthur Young, on the lower Po, where the surface of the river at high water has been elevated considerably above the level of the adjacent fields by diking, the peasants in his time frequently endeavored to secure their grounds against threatened devastation through the bursting of the dikes, by crossing the river when the danger became imminent and opening a cut in the opposite bank, thus saving their own property by flooding their neighbors'. He adds, that at high water the navigation of the river was absolutely interdicted, except to mail and passenger boats, and that the guards fired upon all others; the object of the prohibition being to prevent the peasants from resorting to this measure of self-defence.--Travels in Italy and Spain, Nov. 7, 1789.
In a flood of the Po in 1839, a breach of the embankment took place at Bonizzo. The water poured through and inundated 116,000 acres, or 181 square miles, of the plain to the depth of from twenty to twenty-three feet, in the lower parts. The inundation of May, 1872, a giant breach occurred in the dike near Ferrara, and 170,000 acres of cultivated land were overflowed, and a population of 30,000 souls driven from their homes. In the flood of October in the same year, in consequence of a breach of the dike at Revere, 250,000 acres of cultivated soil were overflowed, and 60,000 persons were made homeless. The dikes were seriously injured at more than forty points. See page 279, ante. In the flood of 1856, the Loire made seventy-three breaches in its dikes, and thus, instead of a comparatively gradual rise and gentle expansion of its waters, it created seventy-three impetuous torrents, which inflicted infinitely greater mischief than a simply natural overflow would have done. The dikes or levees of the Mississippi, being of more recent construction than those of the Po, are not yet well consolidated and fortified, and for this reason crevasses which occasion destructive inundations are of very frequent occurrence.
111. ^Embankments have been employed on the lower course of the Po for at least two thousand years, and for some centuries they have been connected in a continuous chain from the sea to the vicinity of Cremona. From early ages the Italian hydrographers have stood in the front rank of their profession, and the Italian literature of this branch of material improvement is exceedingly voluminous, exhaustive, and complete.
"The science of rivers after the barbarous ages," says Mengotti, "may be said to have been born and perfected in Italy." The eminent Italian engineer Lombardini published in 1870, under the title of Guida allo studio dell' idrologia fluviale e dell' Idraulica practica, which serves both as a summary of the recent progress of that science and as an index to the literature of the subject. The professional student, therefore, as well as the geographer, will have very frequent occasion to consult Italian authorities, and in the very valuable Report of Humphreys and Abbot on the Mississippi, America has lately made a contribution to our potamological knowledge, which, in scientific interest and practical utility, does not fall short of the ablest European productions in the same branch of inquiry.
112. ^Dupenchel advised a resort to the "heroic remedy" of sacrificing, or converting into cellars, the lower storeys of houses in cities exposed to river inundation, filling up the streets, and admitting the water of floods freely over the adjacent country, and thus allowing it to raise the level of the soil to that of the highest inundations.--Traite d'Hydraulique et de Geologie Agricole, Paris, 1868, p. 241.
114. ^Moyens de forcer les Torrents de rendre une partie du sol qu'ils ravagent, et d'empecher les grandes Inondations.
115. ^The effect of trees and other detached obstructions in checking the flow of water is particularly noticed by Palissy in his essay on Waters and Fountains, p. 173, edition of 1844. "There be," says he, "in divers parts of France, and specially at Nantes, wooden bridges, where, to break the force of the waters and of the floating ice, which might endamage the piers of the said bridges, they have driven upright timbers into the bed of the rivers above the said piers, without the which they should abide but little. And in like wise, the trees which be planted along the mountains do much deaden the violence of the waters that flow from them."
Lombardini attaches great importance to the planting of rows of trees transversely to the current on grounds subject to overflow.--Esame degli Studi sul Tevere, Section 53, and Appendice, Sections 33, 34.
116. ^The introduction of a new system of spurs with parabolic curves has been attended with giant advantage in France.--Annales du Genie Civil, Mai, 1863.
117. ^This practice has sometimes been resorted to on the Mississippi with advantage to navigation, but it is quite another question whether that advantage has not been too dearly purchased by the injury to the banks at lower points. If we suppose a river to have a navigable course of 1,600 miles as measured by its natural channel, with a descent of 800 feet, we shall have a fall of six inches to the mile. If the length of channel be reduced to 1,200 miles by cutting off bends, the fall is increased to eight inches per mile. The augmentation of velocity consequent upon this increase of inclination is not computable without taking into account other elements, such as depth and volume of water, diminution of direct resistance, and the like, but in almost any supposable case, it would be sufficient to produce great effects on the height of floods, the deposit of sediment in the channel, on the shores, and at the outlet, the erosion of banks and other points of much geographical importance.
The Po, in those parts of its course where the embankments leave a wide space between, often cuts off bends in its channel and straightens its course. These short cuts are called salti, or leaps, and sometimes abridge the distance between their termini by several miles. In 1777, the salto of Cottaro shortened a distance of 7,000 metres by 5,000, or, in other words, reduced the length of the river by five kilometres, or about three miles, and in 1807 and 1810 the two salti of Mozzanone effected a still greater reduction.
118. ^On the remedies against inundation, see the valuable paper of Lombardini, Sulle Inondazioni avvenute in questi ultimi tempi in Francia. Milano, 1858.
There can be no doubt that in the case of rivers which receive their supply in a large measure from mountain streams, the methods described in a former chapter as recently employed in South-eastern France to arrest the formation and lessen the force of torrents, would prove equally useful as a preventive remedy against inundations. They would both retard the delivery of surface-water and diminish the discharge of sediment into rivers, thus operating at once against the two most efficient causes of destructive floods. See Chapter III., pp. 316 at seqq.
119. ^Idraulica Fisica e Sperimentale. 2d edizione, vol. i., pp. 131, 133.
120. ^The gradual elevation of the bed of the Nile from sedimentary deposit, from the prolongation of the Delta and consequent reduction of the inclination of the river-bed, or, as has been supposed by some, though without probability, from a secular rise of the coast, rendered necessary some change in the hydraulic arrangements of Egypt. Mehemet Ali was advised to adopt a system of longitudinal levees, and he embanked the river from Jebel Silsileh to the sea with dikes six or seven feet high and twenty feet thick. Similar embankments were made around the Delta. These dikes are provided with transverse embankments, with sluices for admitting and canals for distributing the water, and they serve rather to retain the water and control its flow than to exclude it. Clot Bey, Apercu sur l'Egypte, ii., 437.
121. ^It is said that in the Delta alone 50,000 wells are employed for irrigation.
122. ^Deep borings have not detected any essential difference in the quantity or quality of the deposits of the Nile for forty or fifty, or, as some compute, for a hundred centuries. From what vast store of rich earth does this river derive the three or four inches of fertilizing material which it spreads over the soil of Egypt every hundred years Not from the White Nile, for that river drops nearly all its suspended matter in the broad expansions and slow current of its channel south of the tenth degree of north latitude. Nor does it appear that much sediment is contributed by the Bahr-el-Azrek, which flows through forests for a great part of its course. I have been informed by an old European resident of Egypt who is very familiar with the Upper Nile, that almost the whole of the earth with which its waters are charged is brought down by the Takazze.
123. ^From daily measurements during a period of fourteen years--1827 to 1840--the mean delivery of the Po at Ponte Lagoscuro, below the entrance of its last tributary, is found to be 1,720 cubic metres, or 60,745 cubic feet, per second. Its smallest delivery is 186 cubic metres, or 6,569 cubic feet, its greatest 5,156 cubic metres, or 152,094 cubic feet. The average delivery of the Nile being 101,000 cubic feet per second, it follows that the Po contributes to the Adriatic[ rather more than six-tenths as much water as the Nile to the Mediterranean--a result which will surprise most readers.
It is worth remembering that the mean delivery of the Rhone is almost identical with that of the Po, and that of the Rhine is very nearly the same. Though the Po receives four-tenths of its water from lakes, in which the streams that empty into them let fall the solid material they bring down from the mountains, its deposits in the Adriatic are at least sixty or seventy per cent. greater than those transported to the Mediterranean by the Rhone, which derives most of its supply from mountain and torrential tributaries. Those tributaries lodge much sediment in the Lake of Geneva and the Lac de Bourget, but the total erosion of the Po and its affluents must be considerably greater than that of the Rhone system. The Rhine conveys to the sea much less sediment than either of the other two rivers.--Lombardini, Cargiamenti nella condizione del Po, pp. 29, 39.
The mean discharge of the Mississippi is 675,000 cubic feet per second, and, accordingly, that river contributes to the sea about eleven times as much water as the Po, and more than six and a half times as much as the Nile. The discharge of the Mississippi is estimated at one-fourth of the precipitation in its basin--certainly a very large proportion, when we consider the rapidity of evaporation in many parts of the basin, and the probable loss by infiltration.--Humphreys and Abbott'S Report, p. 93.
The basin of the Mississippi has an area forty-six times as large as that of the Po, with a mean annual precipitation of thirty inches, while that of the Po, at least according to official statistics, has a precipitation of forty inches. Hence the down-fall in the former is one-fourth less than in the latter. Besides this, the Mississippi loses little or nothing by the diversion of its waters for irrigation. Consequently the measured discharge of the Mississippi is proportionally much less than that of the Po, and we are authorized to conclude that the difference is partly due to the escape of water from the bed, or at least the basin of the Mississippi, by subterranean channels.
These comparisons are interesting in reference to the supply received by the sea directly from great rivers, but they fail to give a true idea of the real volume of the latter. To take the case of the Nile and the Po: we have reason to suppose that comparatively little water is diverted from the tributaries of the former for irrigation, but enormous quantities are drawn from its main trunk for that purpose, below the point where it receives its last affluent. This quantity is now increasing in so rapid a proportion, that Elisee Reclus foresees the day when the entire low-water current will be absorbed by new arrangements to meet the needs of extended and improved agriculture. On the other hand, while the affluents of the Po send off a great quantity of water into canals of irrigation, the main trunk loses little or nothing in that way except at Chivasso. Trustworthy data are wanting to enable us to estimate how far these different modes of utilizing the water balance each other in the case under consideration. Perhaps the Canal Cavour, and other irrigating canals now proposed, may one day intercept as large a proportion of the supply of the lower Po as Egyptian dikes, canals, shadoofs, and steam-pumps do of that of the Nile.
Another circumstance is important to be considered in comparing the character of these three rivers. The Po runs nearly east and west, and it and its tributaries are exposed to no other difference of meterological conditions than those which always subsist between the mountains and the plains. The course of the Nile and the Mississippi is mainly north and south. The sources of the Nile are in a very humid region, its lower course for many hundred miles in almost rainless latitudes with enormous evaporating power, while the precipitation is large throughout the Mississippi system, except in the basins of some of its western affluents.
124. ^Fraas and Eyth maintain that we have no trustworthy data for calculating the annual or secular elevation of the soil of Egypt by the sediment of the Nile. The deposit, they say, is variable from irregularity of current, and especially from the interference of man with the operations of nature, to a degree which renders any probable computation of the amount quite impossible.--Fraas, Aus dem Orient, pp. 212, 213.
The sedimentary matter transported by the Nile might doubtless be estimated with approximate precision by careful observation of the proportion of suspended slime and water at different stations and seasons for a few successive years. Figari Bey states that at low stages the water of the Nile contains little or no sediment, and that the greatest proportion occurs about the end of July, and of course, while the river is still rising. Experiments at Khartum at that season showed solid matter in the proportion of one to a thousand by weight. The quantity is relatively greater at Cairo, a fact which shows that the river receives more earth from the erosion of its banks than it deposits at its own bottom, and it must consequently widen its channel unless we suppose a secular depression of the coast at the mouth of the Nile which produces an increased inclination of the bed of the river, and consequently an augmented velocity of flow sufficient to sweep out earth from the bottom and mix it with the current.
Herschell states the Nile sediment at 1 in 633 by weight, and computes the entire annual quantity at 140 millions of tons.--Physical Geography, p. 231.
The mean proportion of sedimentary material in the waters of the Mississippi is calculated at 1 to 1,500 by weight, and 1 to 2,900 in volume, and the total annual quantity at 812,500,000,000 pounds, which would cover one square mile to the depth of 214 feet.--Humphreys and Abbott, Report, p. 140.
125. ^We are quite safe in supposing that the valley of the Nile has been occupied by man at least 5,000 years. The dates of Egyptian chronology are uncertain, but I believe no inquirer estimates the age of the great pyramids at less than forty centuries, and the construction of such works implies an already ancient civilization.
It is an interesting fact that the old Egyptian system of embankments and canals is probably more ancient than the geological changes which have converted the Mississippi from a limpid to a turbid stream, and occasioned the formation of the vast delta at the mouth of that river. Humphreys and Abbot conclude that the delta of the Mississippi began its encroachments on the Gulf of Mexico not more than 4,400 years ago, before which period they suppose the Mississippi to have been "a comparatively clear stream," conveying very little sediment to the sea. The present rate of advance of the delta is 262 feet a year, and there are reasons for thinking that the amount of deposit has long been approximately constant.--Report, pp. 435, 436.
126. ^The present annual extension of the Delta is, if perceptible, at all events very small. According to some authorities, a few hectares are added every year at each Nile mouth. Others, among whom I may mention Fraas, deny that there is any extension at all, the deposit being balanced by a secular depression of the coast.
Elisee Reclus states that the Delta advances about 40 inches per year.--La Terre, i., p. 500.
127. ^"The stream carries this mud, etc., at first farther to the east, and only lets it fall where the force of the current becomes weakened. This explains the continual advance of the land seaward along the Syrian coast, in consequence of which Tyre and Sideon no longer lie on the shore, but some distance inland. That the Nile contributes to this deposit may easily be seen, even by the unscientific observer, from the stained and turbid character of the water for many miles from its mouths. Ships often encounter floating masses of Nile mud, and Dr. Clarke thus describes a case of this sort:
"While we were at table, we heard the sailors who were throwing the lead suddenly cry out: 'Three and a half!' The ship slackened her way, and veered about. As she came round, the whole surface of the water was seen to be covered with thick, black mud, which extended so far that it appeared like an island. At the same time, actual land was nowhere to be seen--not even from the mast-head--nor was any notice of such a shoal to be found or any chart on board. The fact is, as we learned afterwards, that a stratum of mud, stretching from the mouths of the Nile for many miles out into the open sea, forms a movable deposit along the Egyptian coast. If this deposit is driven forwards by powerful currents, it sometimes rises to the surface, and disturbs the mariner by the sudden appearance of shoals where the charts lead him to expect a considerable depth of water. But these strata of mud are, in reality, not in the least dangerous. As soon as a ship strikes them they break up at once, and a frigate may hold her course in perfect safety where an inexperienced pilot, misled by his soundings, would every moment expect to be stranded."--Bottger, Das Mittchneer, pp. 188, 189.
This phenomenon is not peculiar to the locality in question, and it is frequently observed in the Gulf of Bengal, and other great marine estuaries.
128. ^The fact that the mixing of salt and fresh water in coast marsh[es and lagoons is deleterious to the sanitary condition of the vicinity, has been generally admitted, though the precise reason why a mixture of both should be more injurious than either alone, is not altogether clear. It has been suggested that the admission of salt-water to the lagoons and rivers kills many fresh-water plants and animals, while the fresh water is equally fatal to many marine organisms, and that the decomposition of the remains originates poisonous minsmata. Other theories, however, have been proposed. The whole subject is fully and ably discussed by Dr. Salvagnoli Marchetti in the appendix to his valuable Rapporto aul Bonificamento delle Maremme Toscane. See also the Memorie Economico-Statistiche sulle Maremme Toscane, of the same author. A different view of this subject is taken by Raffanini and Orlandini in Analisi, Storico-Fisico-Economica sulli insolubrita nelle Maremme Toscane, Firenze, 1869. See also the important memoir of D. Pantaleoni, Del miasma vegetale e delle Malattie Miasmatiche, in which the views of Salvagnoli on this point are combated.
129. ^This difficulty has been remedied--though with doubtful general advantage--as to one important river of the Maremma, the Pecora, by clearings recently executed along its upper course. "The condition of this marsh and of its affluents are now, November, 1859, much changed, and it is advisable to prosecute its improvement by deposits. In consequence of the extensive felling of the woods upon the plains, hills, and mountains of the territory of Massa and Scarlino, within the last ten years, the Pecora and other affluents of the marsh receive, during the rains, water abundantly charged with slime, so that the deposits within the first division of the marsh are already considerable, and we may now hope to see the whole marsh and pond filled up in a much shorter time than we had a right to expect before 1850. This circumstance totally changes the terms of the question, because the filling of the marsh and pond, which then seemed almost impossible on account of the small amount of [[sediment deposited by the Pecora, has now become practicable."--Salvagnoli, Rapporto sul Bonificamento delle Maremme Toscane, pp.li., lii. Between 1830 and 1859 more than 36,000,000 cubic yards of sediment were deposited in the marsh and shoal-water lake of Castiglione alone.--Salvagnoli, Raccolta di Documenti, pp. 74, 75.
On the tides of the Mediterranean, see Bottger, Das Mittelmeer, p. 190.
131. ^In Catholic countries, the discipline of the church requires a meagre diet at certain seasons, and as fish is not flesh, there is a great demand for that article of food at those periods. For the convenience of monasteries and their patrons, and as a source of pecuniary emolument to ecclesiastical establishments and sometimes to lay proprietors, great numbers of artificial fish-ponds were created during the Middle Ages. They were generally shallow pools formed by damming up the outlet of marshes, and they were among the most fruitful sources of endemic disease, and of the peculiar malignity of the epidemics which so often ravaged Europe in those centuries. These ponds, in religious hands, were too sacred to be infringed upon for sanitary purposes, and when belonging to powerful lay lords they were almost an inviolable. The rights of fishery were a standing obstacle to every proposal of hydralic improvement, and to this day large and fertile districts in Southern Europe remain sickly and almost unimproved and uninhabited, because the draining of the ponds upon them would reduce the income of proprietors who derive large profits by supplying the faithful, in Lent, with fish, and with various species of waterfowl which, though very fat, are, ecclesiastically speaking, meagre.
133. ^Giorgini, Sur les causes de l'Insalubrite de l'air dans le voisinage des marais, etc., lue a l'Academie des Sciences a Paris, le 12 Juillet, 1825. Reprinted in Salvagnoli, Rapporto, etc., appendice, p. 5, et seqq.
134. ^This curious fact is thus stated in the preface to Fossombroni (Memorie sopra la Val di Chiana, edition of 1835, p. xiii.), from which also I borrow most of the data hereafter given with respect to that valley: "It is perhaps not universally known, that the swallows, which come from the north [south] to spend the summer in our climate, do not frequent marshy districts with a malarious atmosphere. A proof of the restoration of salubrity in the Val di Chiana is furnished by these aerial visitors, which had never before been seen in those low grounds, but which have appeared within a few years at Forano and other points similarly situated."
Is the air of swamps destructive to the swallows, or is their absence in such localities merely due to the want of human habitations, near which this half-domestic bird loves to breed, perhaps because the house-fly and other insects which follow man are found only in the vicinity of his dwellings In almoust all European countries the swallow is protected, by popular opinion or superstition, from the persecution to which almost all other birds are subject. It is possible that this respect for the swallow is founded upon ancient observation of the fact just stated on the authority of Fossombroni. Ignorance mistakes the effect for the cause, and the absence of this bird may have been supposed to be the occasion, not the consequence, of the unhealthiness of particular localities. This opinion once adopted, the swallow would become a sacred bird, and in process of time fables and legends would be invented to give additional sanction to the prejudices which protected it. The Romans considered the swallow as consecrated to the Penates, or household gods, and according to Peretti (Le Serate del Villaggio, p. 168) the Lombard peasantry think it a sin to kill them, because they are le gallinelle del Signore, the chickens of the Lord.
135. ^Able geologists infer from recent investigations, that, although the Arno flowed to the south within the pliocenic period, the direction of its course was changed at an earlier epoch than that supposed in the text.
136. ^Morozzi, Dello stato dell' Arno, ii., pp. 39, 40.
137. ^Morozzi, Dello stato, etc., dell' Arno, ii., pp. 39, 40.
138. ^Torricelli thus expressed himself on this point: "If we content ourselves with what nature has made practicable to human industry, we shall endeavor to control, as far as possible, the outlets of these streams, which, by raising the bed of the valley with their deposits, will realize the fable of the Tagus and the Pactolus, and truly roll golden sands for him that is wise enough to avail himself of them."--Fossombroni, Memoris sopra la Val di China, p. 219.
139. ^Arrian observes that at the junction of the Hydaspes and the Acesines, both of which are described as wide streams, "one very narrow river is formed of two confluents, and its current is very swift."--Arrian, Alex. Anab., vi., 4.
A like example is observed in the Anapus near Syracuse, which, below the junction of its two branches, is narrower, though swifter than either of them, and such cases are by no means unfrequent. The immediate effect of the confluence of two rivers upon the current below depends upon local circumstances, and especially upon the angle of incidence. If the two nearly coincide in direction, so as to include a small angle, the join current will have a greater velocity than the slower confluent, perhaps even than either of them. If the two rivers run in transverse, still more if they flow in more or less opposite, directions, the velocity of the principal branch will be retarded both above and below the junction, and at high water it may even set back the current of the affluent.
On the other hand, the diversion of a considerable branch from a river retards its velocity below the point of separation, and here a deposit of earth in its channel immediately begins, which has a tendency to turn the whole stream into the new bed. "Theory and the authority of all hydrographical writers combine to show that the channels of rivers undergo an elevation of bed below a canal of diversion."--Letter of Fossombroni, in Salvagnoli, Raccolta di Documenti, p. 32. See the early authorities and discussions on the principle stated in the text, in Frisi, Del modo di regolare i Fiumi e i Torrenti, libro iii., capit. i., and Mongotti, Idraulica, ii., pp. 88 et seqq., and see p. 498, note, ante.
In my account of these improvements I have chiefly followed Fossombroni, under whose direction they were principally executed. Many of Fossombroni's statements and opinions have been controverted, and in comparatively unimportant particulars they have been shown to be erroneous.--See Lombardini, Guida allo studio dell' Idrologia, cap. xviii., and same author, Esame degli Studi sul Tevere, Section 33.
140. ^Fossombroni, Memorie Idraulico-storiche, Introduzione, p. xvi. Between the years 1700 and 1799 the chroniclers record seventeen floods of the Arno, and twenty between 1800 and 1870, but none of these were of a properly destructive character except those in 1844, 1864, and 1870, and the ravages of this latter were chiefly confined to Pisa, and were occasioned by the bursting of a dike or wall. They are all three generally ascribed to extraordinary, if not unprecedented, rains and snows, but many inquirers attribute them to the felling of the woods in the valleys of the upper tributaries of the Arno since 1835. See a paper by Griffini, in the Italia Nuova, 18 Marzo, 1871.
141. ^See the careful estimates of Rozet, Moyens de forcer les Torrents, etc., pp. 42, 44.
142. ^Erdhunde, vol. i, p. 384. The Mississippi--a river "undercharged with sediment"--with a mean discharge of about ten times that of the Rhine, deposits a cubic geographical mile in thirty-three years.
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