CHAPTER X. 

 The Laws of Heat with respect to Air. 



WE have seen in the preceding chapter how 

 many and how important are the offices dis- 

 charged by the aqueous part of the atmosphere. 

 The aqueous part is, however, a very small part 

 only : it may vary, perhaps, from less than 

 l-100dth to nearly as much as l-20th in weight, 

 of the whole aerial ocean. We have to offer 

 some considerations with regard to the remainder 

 of the mass. 



1. In the first place we may observe that the 

 aerial atmosphere is necessary as a vehicle for 

 the aqueous vapour. Salutary as is the opera- 

 tion of this last element to the whole organized 

 creation, it is a substance which would not have 

 answered its purposes if it had been administered 

 pure. It requires to be diluted and associated 

 with dry air, to make it serviceable. A little 

 consideration will show this. 



We can suppose the earth with no atmosphere 

 except the vapour which arises from its watery 

 parts : and if we suppose also the equatorial 

 parts of the globe to be hot, and the polar parts 

 cold, we may easily see what would be the 

 consequence. The waters at the equator, and 



