PRESSURE OF THE AIR. 123 



* f 



the elevation at which it was observed. The experiment was 

 tried by M. Perier, at the suggestion of his brother-in-law the 

 celebrated Pascal, who himself living at Paris, was not so con- 

 veniently situated as M. Perier, who lived at Clermont, in Au- 

 vergne, in the immediate vicinity of several mountains, which 

 modern geology proves to have once been active volcanoes. Early 

 in the morning of the 19th of September 1648, (two hundred 

 years ago) Perier with a few friends, assembled in the garden of 

 a Monastery situated near the lowest part of the city of Clermont, 

 where he had brought a quantity of mercury, and two glass tubes 

 closely sealed at the top. He filled and watched them as usual, 

 and found the mercury to stand in both tubes at the same height, 

 namely 28 English inches; leaving one behind in the custody of 

 the sub-prior, he proceeded with the other to the summit of the 

 mountain, the Puy de Dome, and repeated the experiment. Here 

 the party were delighted, perhaps we may say surprised, although 

 they expected it, to see the mercury sink more than three inches 

 under the former mark, and remain suspended at a height of 

 only 24.7 inches. In his descent from the mountain he ob- 

 served at two several stations, that the mercury successively rose, 

 and returning to the monastery it stood at exactly the savoe point 

 as at first, thus incontrovertably proving that it was the pressure 

 of the atmosphere which balanced the suspended column. 



We will close this chapter with a description of a barometer or 

 baroscope, which any one who has access to a tin-shop can 



construct, it is an instrument of great delicacy in its indications. 

 Let A B C D, be a vessel partly filled with water, in which the 



