Evangelista Torricelli (1608-1647) was an Italian physicist and mathematician who was a colleague of Galileo. He is known for inventing the barometer in 1643. His work in geometry aided in the eventual development of integral calculus. Following a suggestion by Galileo, he filled a 4 ft long glass tube with mercury and inverted the tube into a dish. Upon doing so, he correctly recognized the fact that the height of the mercury was only 1/14 that of a water barometer, thus proving that mercury is fourteen times as dense as water. He also noticed that the level of mercury varied from day to day and observed that the space above the mercury in the barometer must contain a vacuum.