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PASCAL AND THE GREAT EXPERIMENT J. G. ANDISON IN France, the year 1948 has already marked the hundredth anniversary of the Revolution of February, and in November the University of Bordeaux will be celebrating the two-hundredth anniversary of Montesquieu 's Esprit des lois. This year is also the anniversary of another event, intrinsically much less important than the other two, but related to a discussion which was decisive in discrediting the qualitative physics of Aristotle and the School, and in establishing the experimental method of mechanistic physics- the Great Experiment of the Puy-de-Dome. I It was on September 19, 1648, that Florin Perier; upon instructions from Pascal, successfully performed the experiment of Torricelli's barometric tube at the top of the Puy-de-Dome (4,800 feet), and also at a point called Lafon de I'Arbre which is a little more than a third of the way up the mountain. At the summit the column of mercury measured 23 1/ 6 pouces (6 pouces =6 1/2 inches ~pproximately) in height; at Lafon, it measured exactly 25 pouces, in comparison with the 26 7/24 pouces which it measured in the garden of the Minim fathers at Clermont. According to Pascal, this established the fact that atmospheric pressure was the cause of the suspension of the mercury in the tube, and the phenomenon a particular case of the general principle of the equilibrium of liquids. Pascal thereupon composed his Recit de la Grande Experience, a certain number of copies of which were printed without licence some time between October and December. The R ecit is a veritable shout of triumph. "Dear reader," writes Pascal, "the universal consent of men and the throng of philosophers agree in the affirmation of this principle, that Nature would rather allow its own destruction than the smallest vacuum. Certain superior minds have adopted a more moderate principle: for although believing that Nature abhors a vacuum, they have nevertheless concluded that this abhorrence has limits and can be overcome by force; but no one has as yet advanced this third principle: that Nature has no horror of the vacuum, that she makes no effort to avoid it, and in fact admits it without difficulty and without resistance." On the one hand, therefore, we have the universal ignorance of universal mankind, and on the other hand, Pascal alone. . . . And yet we should in all fairness not overlook the fact that four years earlier, on June 11, 1644, someone else had more modestly affirmed: "Many people have said it is impossible to produce a vacuum, others that this is possible, but not without resistance on the part of Nature and without effort. As far as I am aware, no one has ever said that a vacuum can be produced 41 42 THE UNIVERSITY OF TORONTO QUARTERLY without difficulty and without any resistance on Nature's part." These lines were written by Evangelista Torricelli to his pupil Michelangelo Ricci in one of the letters in which he describes and interprets his experiment of the column of mercury. Extracts of these letters were sent by Ricci to Father Mersenne in Paris, and later Mersenne brought back from Rome complete copies which he gave to Roberval, professor of mathematics in the College de France and friend and collaborator of Pascal. The strange similarity of these two texts would in itself be enough to dilute our enthusiasm for Pascal's triumph. But already on December 13, 1647, Descartes, in a letter to Mersenne in which he announces having received Pascal's preliminary outline (the Experiences nouvelles touchant le vide), makes the following statement: "I had suggested to M. Pascal that he investigate whether the quicksilver rose as high on the top of a mountain as it does at the foot; I do not know whether he has done so." Later, upon hearing through Huygens of Perier's successful experiment of September 19 (for Pascal had neglected to send him a copy of the Recit), he wrote to Carcavi for particulars, adding: "I should rightfully expect this from him [Pascal) rather than from you, because it was I who suggested two years ago that he perform this experiment." And...

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