Barometer - History

History

Although Evangelista Torricelli is universally credited with inventing the barometer in 1643, historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, unintentionally built a water barometer sometime between 1640 and 1643. French scientist and philosopher René Descartes described the design of an experiment to determine atmospheric pressure as early as 1631, but there is no evidence that he built a working barometer at that time.

On July 27, 1630, Giovanni Battista Baliani wrote a letter to Galileo Galilei explaining an experiment he had made in which a siphon, led over a hill about twenty-one meters high, failed to work. Galileo responded with an explanation of the phenomenon: he proposed that it was the power of a vacuum that held the water up, and at a certain height the amount of water simply became too much and the force could not hold any more, like a cord that can support only so much weight. This was a restatement of the theory of horror vacui ("nature abhors a vacuum"), which dates to Aristotle, and which Galileo restated as resistenza del vacuo.

Galileo's ideas reached Rome in December 1638 in his Discorsi. Raffaele 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 sometime between 1639 and 1641, Berti (with Magiotti, Athanasius Kircher and Niccolò Zucchi present) carried it out.

Four accounts of Berti's experiment exist, but a simple model of his experiment consisted of filling with water a long tube that had both ends plugged, then standing the tube in a basin already full of water. The bottom end of the tube was opened, and water that had been inside of it poured out into the basin. However, only part of the water in the tube flowed out, and the level of the water inside the tube stayed at an exact level, which happened to be 10.3 m, the same height Baliani and Galileo had observed that was limited by the siphon. 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.

Torricelli, a friend and student of Galileo, dared to look at the entire problem from a different angle. In a letter to Michelangelo Ricci in 1644 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.

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 above bodies. 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 use the term now).

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, then called "quicksilver", which is about 14 times heavier than water, a tube only 80 cm was now needed, not 10.5 m.

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.

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.