64? Experimental Philosophy. [Lecture 6. 



of five hundred miles, it has been computed that 

 one cubic inch, such as the air we breathe, would 

 be so much rarefied at that height, that it might 

 fill a hollow sphere equal in dimensions to the 

 orbit of Saturn. 



We need not, however, have recourse to cal- 

 culations to prove a fact so generally understood. 

 All persons who have visited the tops of high 

 mountains know by experience that the air is 

 thinner or rarer at those altitudes than below. 

 -As they ascend their breathing becomes quicker, 

 the atmosphere is clearer, neither clouds nor va- 

 pours can rise to such heights ; and it is common 

 in these situations to see the lightning below pass 

 from one cloud to another, while all above is clear 

 and serene. Ulloa, who went to take the mea- 

 sure of a degree upon the earth's surface, informs 

 us, that while he stood on the top of one of the 

 Andes in Peru, the clouds, which were gathered 

 below the mountain's brow, seemed like a tem- 

 pestuous ocean, all dashing and foaming, with 

 lightnings breaking through the waves, and some- 

 times two or three suns were reflected from its 

 bosom. " In the mean time he enjoyed a cloud- 

 less sky, and left the war of the elements to the 

 unphilosophical mortals on the plain below 

 him." 



The reason of all this must be evident. The 

 clouds are vapour, that is, water rarefied by heat ; 

 vapour is lighter than air near the surface of the 

 earth, but in the higher regions the air is thinner 



