CHAPTER II 



THE ATMOSPHERE 



THERE are many facts connected with the atmosphere, 

 a knowledge of which are essential to the proper under- 

 standing of some of the more interesting portions of 

 our subject, and for that reason we shall now proceed 

 to study certain aspects of meteorology. 



Surrounding the solid Earth is an envelope of gas 

 which we call the atmosphere, and which is variously 

 estimated at from 150 to 200 miles in thickness. It 

 is very difficult to measure the thickness or depth of 

 the atmosphere since we live at the bottom of it, and 

 it is not nearly so easy to take soundings upwards in 

 a sea of gas, as it is to take them in the case of oceans 

 of water from the top downwards. 



For a similar reason it is not easy to determine the 

 conditions in the upper regions of the atmosphere, and 

 we must rely on such observations as can be made from 

 the tops of high mountains and from balloons and 

 aeroplanes, coupled with the application of known 

 physical and mathematical laws, for information con- 

 cerning them. 



The atmosphere is made up of a mixture of gases 

 which we call air, consisting of oxygen 20*8 parts by 

 volume, nitrogen 79*2 parts by volume, and variable 



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