'1 his explanation, however natural and obvious it may now seem, was by no means so readily admitted as might have been expected from its extreme plausi bility. The principle of the horror of a vacuum was too firmly believed to yield at once to the sim plicity of truth. Attempts were accordingly made to reconcile the experiments of the 'pumps and the tube of Torricelli, with that absurd opinion. It was maintained, that a subtile fluid or aerial spirit, was eva porated from the surface of the water and the mer cury, which filled the upper part of the tube, and left only as much activity to the horror of a vacuum as was sufficient to sustain the column of those fluids.
When Pascal, who was then at Rouen, was inform ed of the experiments of the Italian philosophers, he was anxious to repeat them ; and soon after obtained the same results. It does not appear, however, that he was aware of the conclusions which Torricelli had drawn ; but, by reflecting on the nature of the expe riment, he was soon convinced that the principle of the horror of a vacuum was altogether gratuitous_and improbable, and that the suspension of the mercury was owing to some other cause. To place the mat ter beyond all dispute, he employed tubes of glass 40 feet long, and having- filled one of them with water, and another with wine, he inverted them respectively in basons of these fluids, after the manner of the ex periment of Torricelli. The water remained suspend ed at the'height of 31 feet 1 inch and 4 lines ; and the wine, at the height of 33 feet 3 inches. These experiments were performed at Rouen in' 1646, in presence of several men of science, all of whom were attached to the old opinions. The conviction which they produced on their minds was complete, and they immediately embraced the new doctrines. Pascal published an account of the experiments the follow ing year, in a work entitled Experiences nouvelles touchant le vide. This work was severely attacked, particularly by P. Noel, a Jesuit, who was then rec tor of the college of Paris. All the prejudices of a bad philosophy, and all the virulence of error, were summoned to the attack ; and Pascal had the morti fication to find, that many were still disposed to ques tion the conclusions which be had drawn from his ex periments.
At length an experiment occurred to him which lie saw would for ever silence the objections of his op ponents, and establish his opinion beyond the possi bility of controversy. If the mercury in the Torricel lian tube, said he, is supported by the pressure of the air, it ought to stand higher or lower according to the length of the columns of the atmosphere at the place of observation ; on the contrary, if the weight of the air has no connection with the height of the mercury, the mercury ought to stand at the same ele vation, at all heights in the atmosphere. He there
fore prepared to make the experiment on a large scale; and in order that the difference between the heights of the mercury at the places of observation might be an appreciable quantity, he pitched upon the moun tain Puy-de-dome, in the neighbourhood of Clermont, as well adapted to his purpose. Being at that time in Paris, lie wrote to his brother-in-law Perrier, a man of distinguished talents, who was then going to Clermont, requesting him to perform the experiment on his arrival. Various circumstances prevented the experiment being tried till the 19th of Sept. 1648, when it was performed accuracy and skill.
The result coincided with the expeZtations of Pascal : as they ascended the side of the mountain, the mer cury gradually subsided in the Torricellian tube; and when they reached the summit, it stood 3 inches A lines lower than at the bottom. The experiment was re peated on different sides of the mountain, and always with a similar result.
Pascal no sooner was informed of the details of these experiments, than he repeated them on a small scale at the top and bottom of the steeple of St Jacques-la-Boucherie ; and he observed a correspond. ing difference between the heights of the mercurial columns. There now remained no longer any pretext of ascribing the elevation of the mercury in the tube to the horror of a vacuum ; for, it would have been absurd to pretend that nature had a greater abhor rence of a vacuum in a valley, than on the top of a mountain ; and accordingly all those who were sin cerely desirous of discovering the real cause of the phenomenon in question, adminted the conclusions of Pascal concerning the weight of the air, and applaud ed the simple and decisive method which he had taken to demonstrate its influence. • On the whole, the his tory of this research affords a signal instance of the slow and gradual progress of human knowledge : Ga lileo proved that the air was possessed of weight ; Torricelli conjectured that this fluid caused the ascent of water in pumps, as well as the suspension of the mercury in the tube which bears his name ; and Pas cal converted this conjecture into a demonstration.