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The story of the barometer begins, fittingly, with observation. It was noticed, by Giovanni Batista Baliani, among others, that pumps would not draw water higher than around thirty-four feet, and that siphons would not work over hills of that same height. Baliani described this effect in a letter to Galileo Galilei, and the latter responded with an explanation of the phenomena: he proposed that it was the power of a vacuum which held the water up, and at a certain height (in this case, thirty-four feet) the amount of water simply became too much and the force could not hold any more, like a rope that can only withstand so much weight hanging from it (Middleton 8). While his explanation may not be correct, the fact that Galileo was willing to invoke the existence of vacuum to explain the observations shows that he was very much a mechanical philosopher, for an Aristotelian would never stand for such a thing. The traditional scholastic Aristotelians has always held that a vacuum could not exist. They had various reasons for believing that, most of which came from arguments put forth by Aristotle, who had defined space as an extension of the substance filling it, which meant that it was logically impossible for there to be a space which contained nothing. The Aristotelian philosophy was dominant historically, but in the seventeenth century new mechanical philosophers like Galileo were developing a new way of looking at the world, and that included considering the possibility of a vacuum.
Galileo's ideas reached Rome in 1638 in his Discorsi. Rafael Magiotti and Gasparo Berti were excited by these ideas, and decided to seek a better way to attempt to produce a vacuum than with a siphon. Magiotti devised such an experiment, and in 1641 Berti (with Magiotti, Athanasius Kircher and Nicolo Zucchi present) carried it out. The experiment consisted of filling with water a long tube that had both ends plugged up, then placing the tube into a basin already full of water. The bottom end of the tube was opened, and the water that had begun inside of it poured out of the bottom hole into the basic. However, only part of the water in the tube flowed out, and the level of the water inside the tube stayed at a precise level, which happened to be thirty-four feet, the exact height Baliani and Galileo had observed limited a siphon (Middleton 12). What was most important about this experiment was that the lowering water had left a space above it in the tube which had had no intermediate contact with air to fill it up. This seemed to suggest the possibility of a vacuum existing in the space above the water.
The space above the water had to be explained, but the Aristotelians wouldn't stand for the existence of a vacuum. They sought to reject the vacuum and present alternate explanations. It was obvious that light was transmitted through the space in the tube (since objects behind the tube could be seen through it), and it was commonly believed that light could not travel through a vacuum, so the space could not contain a vacuum. In addition, Kircher (one of the original observers of Berti's experiment) devised the idea of using a magnet outside the tube to drop a hammer inside the tube onto a bell inside the tube. Since it was agreed by all that sound could not travel through a vacuum, then if the bell's ring was heard, the space could not be a vacuum. Sure enough, the experiment was later done with a bell, and a sound was heard. This was sufficient for the Aristotelians to show that there was not a vacuum there, but then they still needed to explain what exactly was in the space above the water in the tube.
The Aristotelians proposed two possible explanations. One was that the water gave off 'spirits' (a sort of tiny vapor) which filled the space and drove the water down. The other, and most widely accepted argument (offered most strongly by Descartes) was that the space was filled with a substance called aether, which was extremely tenuous and able to flow through tiny pores in the glass tube to fill in the space left by the receding water. The aether, which had been used by Aristotelians to explain how light from far-away stars reaches us through the heavens, was a substance, and so it was able to conduct light and sound. With this, the Aristotelians seemed to have disproven the vacuum and provided their own explanation for the observed phenomena. Things at this stage looked gloomy for the mechanical philosophers who thought there existed a vacuum in the space above the water.
That all began to change when a man by the name of Evangelista Torricelli dared to look at the entire problem from a different angle. In a letter to Michelangelo Ricci, concerning the experiments with the water barometer, he wrote:
Many have said that a vacuum does not exist, others that it does exist in spite of the repugnance of nature and with difficulty; I know of no one who has said that it exists without difficulty and without a resistance from nature. I argued thus: If there can be found a manifest cause from which the resistance can be derived which is felt if we try to make a vacuum, it seems to me foolish to try to attribute to vacuum those operations which follow evidently from some other cause; and so by making some very easy calculations, I found that the cause assigned by me (that is, the weight of the atmosphere) ought by itself alone to offer a greater resistance than it does when we try to produce a vacuum. (Torricelli)
It was traditionally thought (especially by the Aristotelians) that the air did not have lateral weight - that is, that the miles of air above us don't weigh down on the air at our level. Even Galileo had accepted the weightlessness of air as a simple truth. Torricelli questioned that assumption, and instead proposed that the air had weight, and that it was the weight of the air (not the attractive force of the vacuum) which held (or rather, pushed) up the column of water. He thought that the level the water stayed at (thirty-four feet) 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 use the term now).
If Torricelli was right about the weight of the air being the cause of the observed effects in the barometer, then a substance heavier than water should fall further in the tube than water did (that is, it should stand lower than thirty-four feet). The idea of using a heavier liquid was probably not Torricelli's own: Magiotti (who had witnessed Berti's original experiment) later on claimed to have suggested in a letter to Torricelli that if sea water had been used, it would have stood lower in the tube; and Galileo (of whom Torricelli was briefly a student) himself may have suggested that other substances (including wine and mercury) would stop at a higher or lower level in a pump or siphon than water did (Middleton 22). Regardless of who came up with the idea, Torricelli was the first to seriously take it up as an experiment, and he shared his predictions of what would happen if mercury were used in a barometer with his close friend Vincenzio Viviani. Early in 1644, Viviana carried out Torricelli's experiment, and it was seen that the mercury (which was known to be fourteen times heavier than water) stopped flowing into the basin when the level in the tube reached a height fourteen times smaller than that at which water stopped (Middleton 23-30). Thus, Torricelli's ideas that the weight of the air from the atmosphere caused the liquid to stop falling seemed to be confirmed.
However, obstacles still remained. While Torricelli had denied that it was the attractive force of a vacuum which held up the liquid, he still believed that the space above the liquid was indeed a vacuum. The Aristotelians' arguments against this conclusion still stood unanswered, and the Aristotelian philosophers continued to modify their explanations for the space above the liquid in order to account for the new observations made with mercury (for as well as denying a vacuum, they denied that air had weight). For example, it was suggested that mercury gave off 'spirits' in the same way water was supposed to (by some Aristotelians), and that the vapors of mercury were stronger than the vapors of water, and so pushed the column of liquid down further. The original Aristotelian arguments against the space in a barometer being a vacuum (that light and sound were transmitted through the space, but that those things can't be transmitted through a vacuum) were answered by Emmanuel Maignan in a second hand account of the Berti experiment given a few years after the experiment took place. In his account, he noted that the production of sound by a bell inside the space above the liquid does not disprove the vacuum as Kircher had thought, since the bell was still physically attached to the instrument and so conducted vibrations through that connection, and from this sound would be transmitted even if the space were a vacuum. Maignan responded to the problem of light being transmitted through the space by suggesting that light itself is actually a substance (though an extremely tenuous one), and so could travel through a vacuum (Middleton 14). However, his conception of light was ahead of its time and not generally accepted, so the Aristotelians did not accept it (and thus even after their other arguments were ruined by experiments later on, the light problem remained their strongest argument for retaining the aether theory into the next century). And the Aristotelian philosophers' account of why mercury fell shorter than water still appeared tenable.
It took another great mind to put that Aristotelian account on trial, and ultimately render the verdict that it failed. That mind belonged to Blaise Pascal. He, 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 (Middleton 37). 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 Aristotelian's expected the wine to stand lower (since more vapors would mean more pushing against 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. The mechanical explanation, however, could account for this, since wine weighs less than water and so is pushed in further by the same amount of force from the air (Westfall 45). The mechanical explanation that the force of external air pushing on the basin is what causes the liquid to stop falling where it does seemed to be confirmed once again, and the Aristotelians' explanation seemed to falter once again.
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 in higher altitudes. Therefore, Pascal wrote to his brother-in-law, Florin Perier, living near the mountain called the Puy de Dome, requesting that the latter perform a crucial experiment. Perier was instructed to take a barometer up the Puy de Dom and make measurements along the way of how high the column of mercury stood. 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 of 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 (Spiers 101-104).
The Puy de Dome experiment struck the Aristotelian philosophers hard. None of their explanations, which assumed something other than the weight of the air to be the cause of the barometric observations, could account for why the height of the mercury would vary according to altitude. They were forced to admit defeat, and concede that it was the weight of the air which caused the liquid in a barometer to stop at a certain level rather than drain out completely. However, they continued to insist that the space above the liquid column was not a vacuum (based mainly on the argument that light cannot traverse a vacuum); instead, they fell back once again on the notion of an aether, which transmitted light, but did not affect or cause the failure of the liquid to drain completely from the tube. After years of moving from ad hoc modification to ad hoc modification, the Aristotelians had finally lost the explanatory battle to account for the observations made with an instrument which had begun as a simple water siphon and become a precise measuring tool. The mechanical philosophy had won out through persistent quantitative experiment.
Bishop, Morris. Pascal: The Life of a Genius. New York: Reynal & Hitchcock, 1936.
Middleton, W. E. Knowles. The History of the Barometer. Baltimore: Johns Hopkins Press, 1964.
Spiers, I.H.B., and A.G.H. Spiers. The Physical Treatises of Pascal. New York: Columbia University Press, 1937.
Torricelli, Evangelista. "Letter to Michelangelo Ricci Concerning the Barometer." Classic Chemistry Archive. Ed. Carmen Giunta. Le Moyne College. 21 Jan. 2002 http://web.lemoyne.edu/faculty/giunta/torr.html.
Westfall, Richard S. The Construction of Modern Science. Cambridge: Cambridge University Press, 1977.