Pascal’s treatise, while still retaining so much of the old traditional physics, was made an object of lively attack by the Jesuit Rector of the College of Paris, Stephen Noël. Pascal replied to him at first directly; and then in answer to a second attack—and so far also in answer to a treatise by the Jesuit, entitled “Le Plein du Vide,” published in 1648—he made a more elaborate statement in a letter addressed to M. le Pailleur, and in a further letter addressed to Father Noël in the same year. There can hardly be any doubt that this was the commencement of Pascal’s hostile relations with the Jesuits. On their part, they failed not to remember in after years, and in a more serious struggle, that he was an old enemy; whilst he on his part probably drew something of the contemptuous scorn which he poured upon them from the recollection of their obstinate ignorance in matters of science. Meanwhile, in defending himself from the attacks of ignorance, Pascal did not fail to open his own mind to fuller scientific light. As soon as the explanation of Torricelli was communicated to him, he accepted it without hesitation, and resolved to carry out a further series of experiments with the view of verifying this explanation, and of banishing for ever the scholastic nonsense of Nature’s abhorrence of a vacuum. If the weight of the air was really the cause which sustained the height of the mercury in the Torricellian tube, he saw at once that this height would vary at different elevations, according to the varying degree of atmospheric pressure at these elevations. He proceeded accordingly to test the result; but the higher levels around Rouen were too insignificant to enable him to draw any decisive inference. Accordingly, he communicated with his brother-in-law in Auvergne with the view of having an adequate experiment made during an ascent of the Puy de Dôme, which rises in the neighbourhood of Clermont to a height of about 3000 feet. The state of his own health prevented him from conducting the experiment personally, and M. Périer was detained by professional avocations from undertaking it immediately. But at length, in September 1648, the experiment was carried out successfully, and the results communicated to Pascal. I cannot do better than quote the account of this important event as rendered by an eminent scientific authority, {33} from M. Périer’s own recital of the facts in his letter to Pascal:—