| Difference of altitude. | Changes in the height of the mercury. |
| Toises. | Lines. |
| 500 | 37½ |
| 150 | 15½ |
| 27 | 2½ |
| 7 | ½ |
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, definite experiments in 1647, and published the results in the same year; and that he had then not mentioned the name of Torricelli, because, while he knew that the experiments were made in Italy four years before, he did not then know that the experimenter was Torricelli; but
that so soon as he learned this fact—which he and his friends were so eager to know, that they sent a special letter of inquiry to Rome—he was “ravished with the idea that the experimenter was so illustrious a genius, whose mathematical writings, already well known, surpassed those of all antiquity.” He says, in conclusion, that it was only in the same year (1647), after the publication of his own researches, that he learned “the very fine thought” of Torricelli concerning the cause of all the effects which had been attributed to the horror of a vacuum. But “as this was only a conjecture as yet unverified,” he then, with the view of ascertaining the truth or falsehood of it, conceived the plan of the experiments carried out by M. Périer at the top and the foot of the Puy de Dôme. “It is true, sir,” he adds, “and I say it boldly, that this series of experiments was my own invention; and therefore I may say that the new knowledge thus acquired is entirely due to me.”
To this letter M. Ribeyre made a satisfactory and touching reply. He expresses disapproval of the allusion of the Jesuit father, but as the discourse was otherwise free from offence, he was willing to attribute it to a “pardonable emulation among savants,” rather than to any intention of assailing Pascal. He makes, in short, the best excuse he can for the Jesuit, and hastens to assure Pascal that his reputation needed no justification:—
“Your candour and your sincerity are too well known to admit any belief that you could do anything inconsistent with the virtuous profession apparent in all your actions and manner. I honour and revere your virtue more than your science; and as in both the one and the other you equal the most famous of the age, do not think it strange if, adding to the common esteem which all have of you, a friendship contracted many years ago with your father, I subscribe myself yours,” etc.
But Pascal had to sustain suspicion and attack in a quarter more formidable than that of the Jesuit fathers at Montferrand. We have already spoken of the rather unhappy commencement of relations between him and Descartes. Farther on we get a more pleasant glimpse of these relations, in a letter from Jacqueline Pascal to Madame Périer, dated 25th September 1647, and apparently shortly after Pascal had retired to Paris, along with his younger sister, leaving their father for some time still at Rouen. This letter is so interesting, both in its bearing on the question which arose between Descartes and Pascal, and in itself, as giving the only account we have of personal intercourse between these two illustrious men, that we present it almost entire:—
“I have delayed writing to you,” Jacqueline says, addressing her sister, [39a] “because I wished to tell to you at length of the interview of M. Descartes and my brother, and I had no leisure yesterday to say that on the evening of Sunday last M. Habert [39b] came, accompanied by M. de Montigny, a gentleman of Brittany, with the view of letting me know, in the absence of my brother, who was at church, that M. Descartes, his compatriot and good friend, had expressed a strong desire to see my brother, for the sake of the great esteem in which both he and my father were everywhere held, and that he begged to be allowed to wait upon him next day at nine o’clock in the morning, if this would not inconvenience him, whom he knew to be an invalid. When M. de Montigny proposed this, I felt hindered from giving a definite answer, because I knew that my brother was reluctant to force himself to conversation, especially in the morning. Nevertheless, I did not think it right to refuse, so we arranged that he should come at half-past ten next day. Along with M. Habert and M. de Montigny there were also a young man in the dress of a priest, whom I did not know, M. de Montigny’s son, and two or three other young people. M. de Roberval, whom my brother had informed of the intended visit, was also present. After some civilities, talk fell upon the instrument [probably that which Pascal had used in the experiments], which was very much admired, while M. de Roberval showed it. Then they spoke of the idea of a vacuum; and M. Descartes, on hearing of the experiments, and being asked what he thought was within the tube (dans la seringue), said with great seriousness that it was some subtle matter, to which my brother replied what he could. M. Roberval, believing that my brother had difficulty in speaking, took up the reply to M. Descartes with some heat, yet with perfect civility. M. Descartes answered with some harshness that he would talk to my brother as much as he wished, because he spoke with reason, but not to any one who spoke with prejudice. Thereupon, finding from his watch it was mid-day, he rose, being engaged to dine at the Faubourg Saint Germain. M. Roberval also rose, in such a way that M. Descartes conducted him to a carriage, where the two were alone, and battled at one another more strongly than playfully, as M. Roberval, who returned here after dinner, told us. . . . I have forgotten to tell you that M. Descartes, annoyed at seeing so little of my brother, promised to return next day at eight o’clock. . . . He desired this, partly to consult regarding my brother’s illness, as to which, however, he did not communicate anything of importance, only he counselled him to remain in bed every day as long as he could till he was tired, and to take plenty of soup. They spoke of many other things, for he was here till eleven o’clock, but I cannot tell you more particularly what they said, as I was not present on this occasion. We were prevented during the whole day from making him take his early bath. He had found it give him a little headache, but that was because he had taken it too late; and I believe the bleeding at the foot on Sunday had done him good, for on Monday he conversed freely and strongly all day—in the morning with M. Descartes, and after dinner with M. de Roberval, with whom he argued for a long time on many things, both belonging to theology and physics, and yet he took no further harm than perspiring much, and slept rather sound during the night.”
The revelations of this letter are very curious. The respectful desire of Descartes, already so distinguished, to make Pascal’s acquaintance, and to enter into conversation with him; his resentment of Roberval’s interference, and their earnest altercation, prolonged in the carriage after leaving Pascal’s house; the evidently serious character of Pascal’s maladies, and the watchful attention of his sister. It is clear through all that Descartes had been busily occupied with the same physical problems as Pascal, and that he was somewhat jealous of the results towards which Pascal and his friends were tending. Evidently there was a certain measure of unfriendliness between Roberval and Descartes. I am unable, however, to see any traces of a coterie surrounding Pascal and inimical to Descartes, as M. Cousin suggests. [41] If such a coterie existed at this time in Paris, of which the “hasty and jealous Roberval” was the centre, and which delighted in “abusing Descartes, and attacking him on all sides,” Jacqueline’s frank and lively letter seems enough to show that while Roberval was Pascal’s friend and Descartes’s disputant, there was nothing in the