Lezioni accademiche
Evangelista Torricelli(1608-1647)
 In his later years, Galileo investigated the reason why the pump was incapable of sucking water more than 32 feet high. Although he had already confirmed that air has its own weight in an experiment which compared the weight of compressed air with that of normal air with the same volume, he had not correlated this with enough factual evidence. Galileo thought that the phenomenon of the pump was caused by Aristotele’s dictum of “horror vacui”, the belief that nature has an aversion to a vacuum.
 This problem was taken up by Torricelli, the most eminent pupil, secretary and close friend of Galileo in his final years. Torricelli conjectured that the weight of water in the pipe equilibrated with the weight of air. In 1643, with Vincenzo Viviani, he intended to verify this by experiment employing mercury instead of water, since the former is 14 times as heavy as the latter, thus allowing him to make the pipe length one fourteenth shorter. First he filled a long glass pipe with one end closed with mercury. Then he capped the open end with his thumb, turned the pipe upside down, immersed the capped end in a pan filled with mercury and then uncapped it. The mercury flowed partially from the pipe. The mercury column maintained a height of approximately 30 inches and a vacuum space was left at the top of the pipe.
 The “Torricelli vacuum”, the first man-made vacuum, ultimately exploded the Aristotelian theory of motion. Torricelli concluded the weight of air raised the mercury column and equilibrated with the weight of the column at 30 inches. Consequently, this device became a measuring instrument for the weight of air (atmospheric pressure), that is, he had invented the barometer. The account of the barometer experiment was not publicly reported before his death but the firs announcement of that was written up in private letters to his friend Filaleti. It was published in an anonymous pamphlet in 1663.
 Torricelli achieved many other contributions in hydraulics, mechanics, optics, geometry, differential and integral calculus, in spite of his short life. The above book was the first published, collected volume of his lectures, manuscripts, and reports done 68 years after his death. It contains the two letters to Michelangelo Ricci which reported the barometer experiment, dated June 11 and June 28, 1644 and also contains a letter to Galileo. The selections in the book were arranged and edited by Tomasso Bonaventura, who wrote a factual and extensive preface concerned much with the life of Torricelli.