Torricelli , Evangelista

(1608–1647) Italian physicist

Born at Faenza in Italy, Torricelli was educated at the Sapienza College, Rome. His De motu (1641; On Movement) attracted the attention of Galileo who invited him to come to Florence to work and live with him. After the death of Galileo in 1642 Torricelli was appointed professor of mathematics in Florence, where he remained until his death.

He had been introduced by Galileo to the problem of why water, in the duke of Tuscany's well, could not be raised higher than 30 feet (9 m). Dissatisfied with earlier explanations, he used Galileo's earlier demonstration that the atmosphere has weight to offer a more satisfactory account. He argued that as the atmosphere has weight, it must also have pressure that can force water up a pipe but only until the weight of the water produced an equivalent counterpressure. 30 feet of water was equal to the pressure exerted by the atmosphere.

Torricelli realized that he could test this argument by substituting mercury for water. A tube of mercury inverted over a dish – given that mercury is 14 times heavier than water – should be supported by the atmosphere to only one-fourteenth the height of an equivalent amount of water. This was confirmed by his pupil Viviani in 1643. Torricelli noticed that over time there was a variation in the height of the mercury in the tube, and reasoned that this was due to variations in the pressure of the atmosphere. This led to his construction of the barometer in 1644. The vacuum above the mercury in a closed tube is called a Torricellian vacuum and the unit of pressure, the torr, was named in his honor. Torricelli also made advances in pure mathematics and geometry, in particular in his calculations on the cycloid.