Zhang Heng (78 A.D.-139 A.D.), a Chinese mathematician and scientist who lived during the Han Dynasty. Heng is thought to have built the first seismoscope or seismometer in A. D. 132.
The device worked on the principle of inertia wherein a mass is suspended. An earthquake shakes the vessel, causing a slight displacement between the mass and the vessel; this movement is transmitted via levers and gears to push out a ball. The surface of the vessel was decorated with motifs of mountains, tortoises, birds, animals, and antique writing. Around the vessel were eight dragons whose mouths held a ball. Around the base of the vessel sat eight corresponding toads with their mouths open, looking upwards. The sign for an earthquake was the falling of a ball from the dragon into the mouth of a toad. The direction of the earthquake was indicated by the position of the toad that swallowed the ball.