Chap. 9.} PafM's and Perrier* s Experiments. 419 



inches long, was reduced to the length of about twenty- 

 eight inches. Now, if we compare the experiment of 

 Galileo with that of Torricelli, we {hall find that fluids 

 a<5t in counterpoife to each other, exactly in proportion 

 to their refpective denfities ;' and that the fame caufe 

 (the prefiure of the air) Which elevates water to the 

 height of thirty- two feet, cannot fuftain a column of 

 mercury above the height of twenty-eight or thirty 

 Inches. 



Pafchal added confiderably to the proofs of this 

 doctrine which Torricelli had afforded, and he rea- 

 foned in this manner: If, faid he, the air is the caufe 

 of this phenomenon, it is becaufe it has ponderance 

 and fluidity ; it muft prefs, therefore, in the fame man- 

 ner as liquids, and its preflure mud be greater or lefs 

 according to its heights and every column , of what- 

 ever fluid is placed in counterpoife with it, will always 

 be longer or fhorter in proportion to its denfity. 

 Hence he proceeded to prove, that a column of air 

 muft produce a preflure greater or lefs, and was ca- 

 pable of fuftaining a column of any fluid higher or 

 lower in proportion to its own height, and confequently 

 that a column of v/ater or mercury, at the bottom of 

 a mountain,. would rife higher in the Torricellian va- 

 cuum than at the fummit. M. Pafchal next pre- 

 vailed upon his brother-in-law, M. Perrier, who was 

 at Clermont in Auvergne, to make the following ex- 

 periment at the bafe and fummit of the mountain 

 known by the name of Puy de Dome. 



M. Perrier fixed a tube of Torricelli's upon a perpen- 

 dicular plank (fee Plate XXIX. Fig. 5.) graduated into 

 inches and lines ; and having obferved te what height the 

 mercury was raifed in the tube at the foot of the moun- 

 tain, he found that it fell gradually in proportion as he 

 E e 2 afcended 



