Blaise Pascal, Florin Périer, and the Puy de Dôme experiment

The peak of the Puy de Dôme in central France (wikipedia.fr)

November days can be depressing, when thick fog is hanging in town for days and there is no real daylight. Here in Frankfurt, one can at least try to escape, and with some luck, the top of the Feldberg is above the mist, and in the sun. November 1647 in Paris may have been similar gloomy, and made Blaise Pascal daydream of the mountain peaks around Clermont-Ferrand, a town in provincial Auvergne where he had been born in 1623 and grown up, and where his sister and her husband Florin Périer still were living.

What is known for sure, however, is that in that fall, Pascal had the idea for an experiment that, simple as it may be, nevertheless revolutionised our knowledge about the atmosphere and atmospheric pressure. So, on November 15, he sat down and wrote a long letter to his brother-in-law to persuade him to conduct this experiment. It was nothing for an armchair scientist, however, since it involved ascending the more than 1000 meters to the top of the Puy de Dôme, the highest mountain in the vicinity of Clermont-Ferrand.

From Rome ...

Gaspare Berti's experiment in Rome (via the Institute and Museum of History of Science, Florence).

A few years before, engineers in Italy had begun seriously to wonder why they could not succeed in building suction pumps that would be able lift water for the usage in fountains or for the supply of buildings to a height of more than about 10 metres. They even had asked Galileo about this, but he could merely confirm the fact, and not provide a clear answer. In Rome, an amateur scientist named Gasparo Berti set up a series of experiments to study this curious phenomenon in detail. He used a long tube, sealed at one end, filled it completely with water, immersed the open end in a tub also filled with water, and then brought the tube in a vertical position. As he noted, some of the water poured in the tub, but not all, leaving a vertical column of water with a height of about 10 metres in the pipe - this was, obviously, exactly the limiting height of the pumps that had puzzled the engineers.

At the time, in the early 1640s, these experiments stirred quite an excitement. The big question was, is the space at the top of the pipe really empty? And what inhibits the water to pour out completely into the tub? According to the still prevailing physical theories going back to Aristotle, nature abhors the vacuum - there cannot be an empty space, thus, the space above the water line in the pipe has to be filled with some substance (not quite wrong, as we know today, since this space is filled with water vapour with the small vapour pressure at room temperature, but this was mostly not thought of to be the explanation). And maybe, nature does not like to fill space with this substance, and that stops the water from pouring out? However, some people also argued that perhaps the atmospheric pressure - the weight of the air in the atmosphere, which was known since a few years to have a measurable density - may be the culprit instead. There was a definite need for more experiments.

Torricelli-type experiments with pipes filled with mercury - the height of the mercury level is independent of the shape of the vessel, or the inclination angle (via the Institute and Museum of History of Science, Florence).

... via Florence

In Florence, Evangelista Torricelli, a student of Galileo, had the ingenious idea to replace the water in Berti's experiment by mercury - a fluid about 13 times denser than water. By this procedure, Berti's setup became much more easy to handle - what was necessary now were vessels just a tenth in size then those used by Berti. When Torricelli repeated the Berti-type experiment, he found that the column of mercury dropped to a height of about 760 millimetre - that was the origin of the instrument we now call a barometer. But what the most interesting result: the height of the mercury level in the pipe above the level in the tub does not depend of the inclination of the pipe, and it does not depend either of the apparently empty volume above the mercury level in the pipe. This was easy to see if the experiment was repeated with vessels with all kind of sizes and shapes. Now, especially this second result was very hard to arrange with any concept of the abhorrence of the vacuum - apparently, the size of this vacuum didn't play a role at all. But was it indeed the atmospheric pressure, the weight of the air around us, which stopped the mercury from completely pouring out of the pipe into the tub? It was a prime candidate, but how could one know that for sure?

and Rouen ...

News about Torricelli's experiments spread quite fast across Europe: travelling scholars told about them to their colleagues, and some scientists maintaining already regular newsletter services sent around descriptions to their interested readers. Blaise Pascal lived at the time in Rouen, some miles west of Paris, where his father worked as a tax collector for the town. Both father and son had a vivid interest in science, and they heard the news from Italy by a friend who visited them and suggested that they join forces to repeat the experiments of both Berti and Torricelli. In fact, Rouen was a well-suited place to do so, because it was the location of the best glass manufacture of France of the time, and high-quality glass jars were essential for successful experiments.

So, in early 1647, the Pascals repeated and refined Berti's experiment with big glass tubes filled with water and with red wine, as well as Torricelli's experiments with mercury. Some of the experiments were done in public, with Rouen citizens as interested witnesses. Especially the experiment with wine rose much interest - even for the science's sake, since some had argued that the empty space at the top of the tubes would be filled with the vapour of the fluid (correctly, in fact), and that this vapour would push the fluid away (not true) - thus, the level of the more volatile vine should be lower that the one of water. In the experiment, the column of wine was higher - because, as Blaise Pascal explained, the density of wine is lower than that of water, and thus a higher column of wine is required to balance the same external atmospheric pressure. Pascal, like many others, was convinced that the pressure of the air kept the fluids from pouring out of the tubes - but he still needed a way to prove this.


... to the Puy the Dôme

There is a debate among historians who had suggested first to conduct the Torricelli experiment on a mountaintop. Some argue that it was Descartes' idea, some assign priority to Pascal. Anyway, while writing up a report on the Rouen experiments and discussing their results with other scholars some time in late 1647, Pascal understood that if the weight of the air is indeed to driving force in all these experiments, it should be lower the higher the place where experiment is done, since then the layer of air above is thinner. Thus, the column of mercury in Torricelli's experiment should be the lower the higher the place where the experiment is performed. Pascal had no clue how big the effect might be, but he thought, and hoped for, that the about 1000 metre of difference in altitude between his hometown of Clermont-Ferrand and the peak of nearby Puy de Dôme might be enough. Thus, the letter to his brother-in-law, Florin Périer.

I am not sure if it took Périer some time to get warm to the idea to carry some pounds of mercury and fragile glasswork on top of Puy de Dôme just because his brother-in-law, sitting comfortably at home in Paris, had some new ideas about esoteric topics such as the weight of the air and empty space. The experiment was performed only about one year after Pascals letter - but for sure, after it was done, Périer like everyone else was excited about its outcome.

On September 19, 1648, Florin Périer and some friends perform the Torricelli experiment on top of Puy de Dôme in central France. The height of the mercury column is 85 mm less than in Clermont-Ferrand at the base of the mountain, about 1000 metre below. (From Louis Figuier, Les merveilles de la science, Vol. 1, 1867. According to this illustration, Périer and company not only climbed up the mountain, they also travelled in time, since their clothes follow the latest fashion of the 18th century.)

Finally, on Saturday, September 19, 1648, Florin Périer and some of his friends from Clermont-Ferrand embarked on the experiment. Early in the morning, they measured the height of the mercury column in two Torricelli experiments at a low-lying place in town, the Jardin des Minimes, the garden of a monastery - it was 711 mm. While one of the instruments was left behind there and observed during the day by a monk, the other was carried on top of the Puy the Dôme. To the big surprise of all, there, about 1000 metre higher than where they had started, the height of the column was only 627 mm! Florin and his friends repeated the measurement several times, and took several measurements on their way back. It was all consistent: while they climbed down the mountain again, the column of mercury climbed up in the glass tube, and back to the monastery, it was again at 711 mm, the height the stationary reference instrument had held during the whole day.

Florin Périer was so surprised and amazed by this big effect that he repeated the experiment the next day. This time, less arduously and fitting to a Sunday, he carried the instrument only the 50 metres on top of the tower of the cathedral of Clermont-Ferrand. This difference in height was enough to be clearly measurable, about 4 mm. Blaise Pascal, when hearing of the result, immediately set out to reproduce the experiment at the Tour Saint-Jacques in Paris, where a statue now pays tribute to Pascal and the experiment.

The results of the Puy de Dôme provided very strong evidence that it is indeed the weight of the air, thus the atmospheric pressure, which balances the weight of the mercury column in Torricelli's experiment. Hence, Torricelli's instrument measures this pressure - it is a barometer. And since the change in pressure with height is very well detectable, the barometer serves as an altimeter at the same time - as it is still used in aviation today.

In appreciation of the contributions of Torricelli and Pascal, two units of pressure have been named after them: one Torr, now officially out of use, is the equivalent of one "mm Hg", the pressure a mercury column with a height of one millimetre. And the derived SI unit for pressure is the Pascal, where 1 Pa is the pressure of a force of one Newton exerted on an area of one square metre.

As for the nature of the empty space above the level of the fluid in the barometer, the situation was not immediately settled after the Puy de Dôme experiment. Descartes, and later his students, insisted that it was not empty, but filled with some aether, which was just everywhere. Now we know that up to the vapour pressure, which can be minimised by reducing the temperature, it is indeed a vacuum - but the vacuum is complicated anyway.

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• The website "Horror Vacui" of the Institute and Museum of History of Science, Florence, Italy, has more information about the discovery of the weight of air, the existence of the vacuum, and the contributions of Evangelista Torricelli.
• Besides for his experiments on pressure in fluids and the vacuum, Blaise Pascal is known for his work in probability theory and geometry, for his mechanical calculator, and as a christian writer and philosopher. You can find more about Pascal online at the Encylopedia Britannica, the MacTutor History of Mathematics and the Stanford Encyclopedia of Philosophy.
• The Rouen experiment is outlined in much more detail in "Writing and Sentiment: Blaise Pascal, the Vacuum, and the Pensées" by Matthew L. Jones, Stud. Hist. Phil. Sci. 32 (2001) 139–181, doi:10.1016/S0039-3681(00)00024-8 (substription required) and in Simone Mazauric: "Gassendi, Pascal, et la Querelle du vide", which have been my sources.
• The wikipedia entry on Pascal cites an extract form Périer's report about the Puy-de-Dôme experiment. You can check out Pascals letter to his brother-in-law asking him to do the experiment and the full text of the report at gallica.bnf.fr, in the first pages of the "Récit de la grande expérience de l'équilibre des liqueurs".

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TAGS: Physics, Vacuum, Puy de Dome