26 THE ATMOSPHERE. 



LESSON VI. 



THE ATMOSPHERE THE WINDS DEW 



FOGS CLOUDS. 



THE earth is surrounded on all sides by air. 

 The air, with the vapours that it contains, is called 

 the atmosphere. The higher we ascend into this 

 atmosphere, for instance, upon high mountains, the 

 more does the air become rarefied, and the less 

 does it press upon the body. 



The pressure of the atmosphere upon the human 

 body is equal to fifteen pounds upon every square 

 inch ; and, as a man's body contains, upon an 

 average, fifteen square feet of surface, he must 

 sustain a weight of 32,400 pounds, or sixteen tons, 

 for his usual load. By this enormous pressure 

 we should undoubtedly be crushed in a moment, 

 if every part of our body were not filled either 

 with air, or with some elastic fluid, the spring 

 of which is just sufficient to counteract the weight 

 of the atmosphere. On the tops of high moun- 

 tains, the pressure of the atmosphere is considera- 

 bly less than what it usually is on the plain. The 

 height to which the atmosphere extends is gene- 

 rally supposed to be about sixty miles, above 

 which elevation, there are neither clouds nor 

 wind, and where the lightness of the air would 

 render it impossible for any animal to breathe. 



The vapours, which are continually rising from 



