i;o HISTORY OF SCIENCE. 



to undertake further researches on this subject, and he made known 

 his results in a treatise entitled "New Experiments on the Vacuum." 

 One of his experiments consisted in using wine, a liquid lighter than 

 water, instead of mercury, the result being that the liquid was main- 

 tained at a height proportionately greater than in the case of water. 

 But Pascal caused a crucial experiment to be performed, in order to 

 set at rest the question of the validity of Torricelli's explanation. If, 

 Pascal reasoned, it is the pressure of the air which balances the 

 column of mercury in the tube, the height of the column would be 

 less were the experiment performed on the summit of a mountain. 

 Pascal induced his brother-in-law, Perier, who possessed some ac- 

 quaintance with scientific experiments, to undertake the application 

 of this test. The mountain selected was the Pay-de-D6me in Au- 

 vergne, and Perier accordingly proceeded to Clermont, and there 

 remained for some months waiting for weather favourable to his object 

 On the morning of the 2oth September, 1648, Perier, accompanied 

 by several other persons, set out on his expedition, taking with him 

 two similar glass tubes, about 4 feet long, and closed at one end, filled 

 with mercury. With these the Torricellian experiment was performed, 

 and the levels of the mercury within and without the tubes having been 

 marked on them with a diamond, the differences of level were measured 

 several times. These differences were found in both tubes to be 26 Paris 

 inches, 31 lines. One of the tubes was then fixed, and left in the con- 

 dition of the experiment, and one of the Fathers of the convent was 

 appointed to take care of it, and notice any changes that might occur 

 in the height of the mercury during the day. 



The party of observers, carrying with them the second tube, now 

 empty, and a supply of mercury, etc., then set out for the summit of 

 the mountain. Here the Torricellian experiment was repeated exactly 

 as before, and the height of the column of mercury was again care- 

 fully measured. It was now only 23 inches, 2 lines. There was thus 

 a difference between the two measurements of 3 inches, i 1 lines (about 

 31 English inches). The measurement of the column of "mercury was 

 several times repeated, under certain variations of the surrounding cir- 

 cumstances, in order to ascertain whether its height was affected by any 

 of these. 1 1 was measured in the open air ; it was measured under cover 

 of a little chapel ; it was measured in a shower ; it was measured in, a 

 mist; but the result remained unchanged. The party then began to de- 

 scend, and when they had come about half-way down, it was thought 

 advisable to repeat the experiment, in order to ascertain whether the 

 decrease of the height of the mercurial column was proportional to 

 the elevation at which the experiment was tried. The mercury was 

 found now to have a height of 25 inches; and several other like 

 determinations showed that the mercury stood higher as the level 

 of the place was lower. Towards the close of the day, the party ar- 

 rived at the convent at Clermont, and there found Father Chastin 



