An Italian scientist and mathematician who was professor of mathematics in Pisa, and did work on the thermal expansion of liquids. For a short time he was an assistant to Galileo and started building telescopes after the death of his teacher. He discovered the attraction between the Earth and air molecules that results in pressure and concluded by saying ‘We live submerged at the bottom of an ocean of the element air’. He wrote on a number of subjects in applied mathematics, including the movement of a stream of water through a small hole in the side of a container. The Torricelli theorem, which he proposed in 1643, is named in honour of him: $v=\sqrt{2gh}$, where v is the velocity of the discharging jet of liquid and is proportional to the square root of the supplied head, h. The equation was derived from a balance between potential and kinetic energies for which energy losses are neglected. His most important development was the mercury barometer in 1643 with the help of his pupil Vincenzo Viviani (1622–1703) who demonstrated the existence of a vacuum. He was able to show that the pressure of the atmosphere varied with weather conditions. This important work led to the development of meteorology for weather prediction. Pascal’s siphon, Samuel Morland’s diagonal barometer, and Robert Hooke’s quadrant barometer are all variations and improvements on Torricelli’s instrument.