<P> It was traditionally thought (especially by the Aristotelians) that the air did not have lateral weight: that is, that the kilometers of air above the surface did not exert any weight on the bodies below it . Even Galileo had accepted the weightlessness of air as a simple truth . Torricelli questioned that assumption, and instead proposed that air had weight and that it was the latter (not the attracting force of the vacuum) which held (or rather, pushed) up the column of water . He thought that the level the water stayed at (c. 10.3 m) was reflective of the force of the air's weight pushing on it (specifically, pushing on the water in the basin and thus limiting how much water can fall from the tube into it). In other words, he viewed the barometer as a balance, an instrument for measurement (as opposed to merely being an instrument to create a vacuum), and because he was the first to view it this way, he is traditionally considered the inventor of the barometer (in the sense in which we now use the term). </P> <P> Because of rumors circulating in Torricelli's gossipy Italian neighborhood, which included that he was engaged in some form of sorcery or witchcraft, Torricelli realized he had to keep his experiment secret to avoid the risk of being arrested . He needed to use a liquid that was heavier than water, and from his previous association and suggestions by Galileo, he deduced by using mercury, a shorter tube could be used . With mercury, which is about 14 times denser than water, a tube only 80 cm was now needed, not 10.5 m . </P> <P> In 1646, Blaise Pascal along with Pierre Petit, had repeated and perfected Torricelli's experiment after hearing about it from Marin Mersenne, who himself had been shown the experiment by Torricelli toward the end of 1644 . Pascal further devised an experiment to test the Aristotelian proposition that it was vapors from the liquid that filled the space in a barometer . His experiment compared water with wine, and since the latter was considered more "spiritous", the Aristotelians expected the wine to stand lower (since more vapors would mean more pushing down on the liquid column). Pascal performed the experiment publicly, inviting the Aristotelians to predict the outcome beforehand . The Aristotelians predicted the wine would stand lower . It did not . </P> <P> However, Pascal went even further to test the mechanical theory . If, as suspected by mechanical philosophers like Torricelli and Pascal, air had lateral weight, the weight of the air would be less at higher altitudes . Therefore, Pascal wrote to his brother - in - law, Florin Perier, who lived near a mountain called the Puy de Dome, asking him to perform a crucial experiment . Perier was to take a barometer up the Puy de Dome and make measurements along the way of the height of the column of mercury . He was then to compare it to measurements taken at the foot of the mountain to see if those measurements taken higher up were in fact smaller . In September 1648, Perier carefully and meticulously carried out the experiment, and found that Pascal's predictions had been correct . The mercury barometer stood lower the higher one went . </P>

Who invented the mercury barometer and what is it used to measure