Evangelista Torricelli, born in Faenza, part of the Papal States, had been left fatherless at an early age and educated under the care of his uncle, a Camaldolese monk, who had first entered young Torricelli into a Jesuit College in 1624 to study mathematics and philosophy until 1626, when he had sent Torricelli to Rome in 1627 to study science under the Benedictine Benedetto Castelli, professor of mathematics at the Collegio della Sapienza in Pisa.
Shortly after the publication of Galileo's Dialogues of the New Science, Torricelli had written to Galileo in 1632 of reading it "with the delight [...] of one who, having already practiced all of geometry most diligently [...] and having studied Ptolemy and seen almost everything of Tycho Brahe, Kepler and Longomontanus, finally, forced by the many congruences, came to adhere to Copernicus, and was a Galileian in profession and sect".
(The Vatican had condemned Galileo in June of the following year, and this was the only known occasion on which Torricelli had openly declared himself to hold the Copernican view.)
Aside from several letters, little is known of Torricelli's activities in the years between 1632 and 1641, when Castelli sent Torricelli's monograph of the path of projectiles to Galileo, then a prisoner in his villa at Arcetri.
Although Galileo had promptly invited Torricelli to visit, he did not accept until just three months before Galileo's death on January 8, 1642; during his stay, he written out Galileo's Discourse of the Fifth day.
Succeeding Galileo, at the request of Grand Duke Ferdinando II de' Medici, as the grand-ducal mathematician and professor of mathematics in the University of Pisa, Toriceli solves some of the great mathematical problems of the day, such as finding a cycloid's area and center of gravity; he also designs and builds a number of telescopes and simple microscopes; several large lenses, engraved with his name, are still preserved at Florence.
Torricelli’s accidental invention of the mercury barometer arose from solving a practical problem: The Grand Duke's pump makers, in attempting to raise water to a height of twelve meters or more, have found that ten meters is the limit with a suction pump.
Employing mercury, fourteen times heavier than water, Toricelli n 1643 creates a tube approximately one meter long and sealed at the top, fills it with mercury, and sets it vertically into a basin of mercury.
The column of mercury falls to about 76 cm, leaving a Torricellian vacuum above.
As we now know, the column's height fluctuates with changing atmospheric pressure; this is the first barometer.
This discovery perpetuates his fame, and the Torr, a unit used in vacuum measurements, is named for him.
Torricelli also discovers Torricelli's Law, regarding the speed of a fluid flowing out of an opening, which will later be shown to be a particular case of Bernoulli's principle.
