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a copy of Torricelli's original mercury barometerEvangelista Torricelli

"discoverer" of atmospheric pressure

Evangelista Torricelli

Evangelista Torricelli was born in Faenza, Italy, on October 15, 1608. He gained his initial scientific inspiration from the writings of Galileo Galilei. These led him to write Concerning Movement (1641), which in turn led to his being offered a post as Galileo's secretary and assistant.

At this time Galileo was trying to prove or disprove the Aristotelian assertion that no vacuum could exist in nature. This assertion was being challenged by miners working suction pumps to prevent flooding in mines. These pumps depended upon the vacuum principle to draw up water. The miners found that it was impossible to lift water higher than 33 feet above its natural level, and that this limit was constant regardless of how large or powerful the pump. Galileo theorized that there was a limit to how much nature disliked a vacuum, while Torricelli wondered what would happen if the same phenomenon was tested with a heavier and denser liquid such as mercury, which is 13.5 times denser than water. Galileo died only three months after Torricelli's arrival, however, without ever testing his ideas on vaccums.

In 1644, two years after Galileo's death, Torricelli carried out an experiment designed to prove or disprove Aristotle's statement. He filled a 4-foot-long glass tube with mercury, sealed it at one end, and inverted it in a dish of mercury. Some of the mercury flowed out of the tube, but in the space that remained a vacuum was created. This proved Aristotle right in the sense that vacuums must be created because there are no natural ones. He published his second book, De Sphaera, that same year.

right: a relica of Torricelli's original mercury barometer

title page of De Sphaera
title page of De Sphaera

Continuing his experiments over a period of time, Torricelli found that the height of the column of mercury in the tube was subject to fluctuations. He correctly concluded that these fluctuations are the result of variations in the pressure exerted by the atmosphere, and that it was the atmospheric pressure that supported the column of mercury in the tube. On this basis he began the first accurate measurement of the pressure (or weight) of air, which scientists had always assumed was weightless. Torricelli had accidentally devised the first rudimentary version of the instrument, now called a barometer, for measuring atmospheric pressure. Although the mercury barometer has gone through many refinements since Torricelli, it is still the most sensitive instrument available for measuring atmospheric pressure.

Torricelli was also interested in pure mathematics, the motions of fluids, and projectiles. His geometric theories contributed to the development of integral calculus. He also developed Torricelli's theorem, an equation to calculate the rate at which a liquid will flow out of a tank under gravity through an opening a given distance below the liquid's surface.

Evangelista Torricelli died of typhus on October 25, 1647.

Galileo Galilei

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This page was last updated on 09/22/2017.