434 Suppofed Height [Book V. 



tcrnatdy, according to the increafe or decreafe of the 

 heat, preffes the vefiels, and eafes them again alter- 

 nately, thus keeping up a continual circulation in the 

 fluids. Even entire columns of marble have been 

 known to cleave, from the increafed elafticity of feme 

 fmall bubbles of air contained in them. 



Putrefaction and fermentation are procefles de- 

 pending entirely on the action of the air ; for we 

 know by numerous experiments, that neither of thefe 

 changes will take place in vacuo, even in fubjects the 

 mod favourably difpofed to them. 



In fpeaking of the terrefhial atmofphere it has been 

 intimated, that it is found to be nearly the fame as to 

 tfompofition in all climates and in all places, as well 

 upon the tops of high mountains as in the vallies be- 

 low, but that it is confiderably lei's denfe in proportion 

 to the height. The whole globe of the earth is entirely 

 enveloped with it ; the whole atmofphere is carried 

 along with the terreftrial orb, both in its diurnal and 

 annual motion, and is a principal operator in the me- 

 chanifm of nature. 



Various means have been devifed for afcertaining 

 the height of the atmofphere. c Thefe attempts,' 

 ftys Mr. Adams, c commenced foon after it was dil- 

 covered, by means of the Torricellian tube, that air 

 is a gravitating fubftance. Thus it alfo became known 

 that a column of air, whofe bafe is a fquare inch, and 

 the height that of the whole atmofphere, weighs fif- 

 teen pounds j and that the weight of air is to that of 

 mercury, as I to 10,800 : whence it follows, that if the 

 weight of the atmofphere is fufBcient to raife a column 

 of mercury to the height of thirty inches, the height 

 of the aerial column muft be ten thoufand eight hun- 

 dred rimes as much, and confequently a little more 

 than five miles high. 



Jt 



