BAROMETER. 



287 



Experi- 

 ments of 

 Pascal. 



Barometer, bility. The principle of the horror of a vacuum 

 ^ "V >* 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 1C4G, 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 nouvellcs 

 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 he 

 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 ot 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, he 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. 1048, 

 it v;a3 performed with equal accuracy and skill. 



Experi- 

 ment 

 I'uy-de- 

 dome. 



equal accuracy aad skill. 



The result coincided with the expectations 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 3inches i^ 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 

 Jacqucs-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 cf the 

 phenomenon in question, admitted 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. 



But the triumph of Pascal was too complete not to 

 excite afresh the malignity of his enemies : Descartes, 

 among others, attempted to deprive him of the honour 

 of the discovery of the pressure of the air; and in a 

 letter which he wrote to M. Carcavi, dated June 11. 

 1649] accuses Pascal of a want of candour, and asserts 

 that he first suggested to him the experiment of Puy- 

 de-dome. Pascal, on the other hand, maintains, in 

 the most solemn manner, that the experiment was en- 

 tirely his own, and that he never received the smallest 

 hint of it from any person. It would be improper 

 to enter here into the merits- of the dispute ; but it 

 appears to us, that the pretensions of Descartes are al- 

 together groundless, and that the discovery of atmo- 

 spherical pressure is due to Pascal alone. 



It is obvious, from the experiment of Puy-de- 

 dome, that the Torricellian tube, if properly gra- 

 duated, may be employed to measure the heights of 

 mountains; a purpose, we already mentioned, to which 

 it is now frequently applied. Boyle suggested this 

 application of it in 1665. 



After it was ascertained, that the weight of the 

 air was the true cause of the suspension of the mer- 

 cury in the tube of Torricelli, Perrier continued to 

 make daily observations with it, from the beginning 

 of 1619 to the end of March 16.51. One of his 

 friends at Paris, and Descartes, who was then at 

 Stockholm, made similar observations during the 

 same period, and they found, that the column of 

 mercury varied in length, according to the tempera- 

 ture, the wind"., the moisture, and other circumstan. 

 ccs connected with the state of the atmosphere. 

 Thus tir.,' tube of Torricelli became an instrument, 

 not only for shewing the weight of the air, but for 

 pointing out the changes of weather which liappeu 



Barometer. 



Pressure of 

 the air 

 proved by 

 Pascal. 



Applica- 

 tion of the 

 barometer 

 to the mea- 

 surement 

 of heights, 

 and mete- 

 orological 

 purposes. 



