id="page34"/>de Dôme began to appear through the clouds, and Périer resolved to proceed with the experiment.  The leading characters in Clermont, whether ecclesiastics or laymen, had taken a deep interest in the subject, and had requested Périer to give them notice of his plans.  He accordingly summoned his friends, and at eight in the morning there assembled in the garden of the Pères Minimes, about a league below the town, M. Bannier, of the Pères Minimes; M. Mosnier, canon of the cathedral church; along with MM. la Ville and Begon, counsellors of the Court of Aides, and M. la Porte, doctor and professor of medicine in Clermont.  These five individuals were not only distinguished in their respective professions, but also by their scientific acquirements; and M. Périer expresses his delight at having been on this occasion associated with them.  M. Périer began the experiment by pouring into a vessel 16 lb. of quicksilver, which he had rectified during the three preceding days.  He then took two glass tubes, four feet long, of the same bore, and hermetically sealed at one end and open at the other; and making the ordinary experiment of a vacuum with both, he found that the mercury stood in each of them at the same level and at the height of 26 inches 3½ lines.  This experiment was repeated twice, with the same result.  One of these glass tubes, with the mercury standing in it, was left under the care of M. Chastin, one of the Religious of the House, who undertook to observe and mark any changes in it that might take place during the day; and the party already named set out with the other tube for the summit of the Puy de Dôme, about 500 toises (a toise is about six feet in length) above their first station.  Before arriving there, they found that the mercury stood at the height of 23 inches and 2 lines—no less than 3 inches and 1½ line lower than it stood at the Minimes.  The party were ‘struck with admiration and astonishment

at this result;’ and ‘so great was their surprise that they resolved to repeat the experiment under various forms.’  The glass tube, or the barometer, as we may call it, was placed in various positions on the summit of ‘the mountain’—sometimes in the small chapel which is there; sometimes in an exposed and sometimes in a sheltered position; sometimes when the wind blew, and sometimes when it was calm; sometimes in rain, and sometimes in a fog: and under all these various influences, which fortunately took place during the same day, the quicksilver stood at the same height of 23 inches 2 lines.  During their descent of the mountain they repeated the experiment at Lafon-de-l’Arbre, an intermediate station, nearer the Minimes than the summit of the Puy, ‘and they found the mercury to stand at the height of 25 inches—a result with which the party was greatly pleased,’ as indicating the relation between the height of the mercury and the height of the station.  Upon reaching the Minimes, they found that the mercury had not changed its height, notwithstanding the inconstancy of the weather, which had been alternately clear, windy, rainy, and foggy.  M. Périer repeated the experiments with both the glass tubes, and found the height of the mercury to be still 26 inches 3½ lines.  On the following morning M. de la Marc, priest of the Oratory, to whom M. Périer had mentioned the preceding results, proposed to have the experiment repeated at the top and bottom of the towers of Notre Dame in Clermont.  He accordingly yielded to his request, and found the difference to be 2 lines.  Upon comparing these observations, M. Périer obtained the following results, showing the changes in the altitude of the mercurial column corresponding to certain differences of altitude of position:—

When Pascal received these results, all the difficulties were removed; and perceiving from the two last observations in the preceding table that 20 toises, or about 120 feet, produce a change of 2 lines, and 7 toises, or 42 feet, a change of ½ a line, he made the observation at the top and bottom of the tower of St Jacques de la Boucherie, which was about 24 or 25 toises, or about 150 feet high, and he found a difference of more than 2 lines in the mercurial column; and in a private house 90 steps high he found a difference of ½ a line. . . .  After this important experiment was made, Pascal intimated to M. Périer that different states of the weather would occasion differences in the barometer, according as it was cold, hot, dry, or moist; and in order to put this opinion to the test of experiment, M. Périer instituted a series of observations, which he continued from the beginning of 1649 till March 1651.  Corresponding observations were made at the same time at Paris and at Stockholm by the French ambassador, M. Chanut, and Descartes; and from these it appeared that the mercury rises in weather which is cold, cloudy, and damp, and falls when the weather is hot and dry, and during rain and snow, but still with such irregularities that no general rule could be established.  At Clermont the difference between the highest and the lowest state of the mercury was 1 inch 3½ lines; at Paris the same; and at Stockholm 2 inches 2½ lines.”

From the account here presented of these researches, there is no difficulty in determining the exact credit due to Pascal on the one hand, and his Italian predecessors on the other.  He completed what they had begun, and verified what they had indicated.  As the Abbé Bossut has expressed it, Galileo proved that air was a heavy fluid; Torricelli conceived that its weight was the cause of the suspension of the water in a pump and the mercury in a tube.  Pascal demonstrated that this was the fact.  No one was more anxious than Pascal himself

that Torricelli should be acknowledged as the real discoverer of the principle which it was left to him to establish by the test of experiment.  He claimed, however, his own definite share in the discovery, both as having carried on a series of independent experiments, and as having converted what he himself calls the “conjecture” of Torricelli into an established fact.  It was painful to him, therefore, to have this share denied, and even open accusations made against him that he had appropriated, without acknowledgment, the results of Torricelli’s researches.  This accusation was made in certain theses of philosophy maintained in the Jesuit College of Montferrand in 1651, and dedicated to Pascal’s own friend, M. de Ribeyre, first president at the Court of Aides at Clermont.  Pascal’s name was not indeed mentioned in these theses; but there could be no doubt of the allusion made to “certain persons loving novelty” who claimed to be the inventors of a definite experiment of which Torricelli was the real author.  It was this accusation which drew from Pascal his letter to M. Ribeyre, bearing the date of 12th July of the same year, in which he has described, with admirable lucidity and temper, his relations to the whole subject.  In this letter he distinctly says that the Italian experiments were known in France from the year 1644; that they were repeated in France by several persons in several places during 1646; that he himself had made, as we have already seen,