244 PHYSICAL SCIENCE BK. vi 



part has been eaten away by torrents, while parts 

 have been exposed by the disruptive action of great 

 tides. Through the interstices thus produced air 

 enters. If it so happen now that the sea has 

 shut it in and driven it deeper, and the waves 

 prevent its escape by the same road, egress and 

 regress being alike closed, the air rolls about 

 within the earth. Its natural tendency is to hurry 

 straight forward, but as that path is closed, it 

 presses upward and lashes the earth, whose weight 

 lies heavy upon it. 



XVI 



1 I MUST further mention a view held by the majority 

 of writers, which probably I shall myself support. 

 The earth does not lack air within ; that everybody 

 knows. I do not mean merely the air which holds 

 it together and unites its parts, which exists even in 

 stones and dead bodies ; but I mean that fresh vital 

 air which supports all life. Unless the earth pos 

 sessed this store of air, how could she infuse it into 

 so many trees and crops, which derive their life from 



2 this and no other source ? How could she nourish 

 all the different roots that sink into the soil in one 

 place and another, some merely attached to the sur 

 face, others sunk deeper, had she not an abundant 

 supply of the breath of life, which produces so many 

 varied growths and rears them with its nourishing 

 draught ? These are the slighter arguments that I 

 hitherto urge. Why, all the heaven we see, which 

 is shut in by fiery ether, the highest portion of the 

 universe, all these stars, whose number cannot be 

 conceived, all this concourse of heavenly bodies, 

 and, to mention only one more, this sun, that urges 



