CHAPTER XVIII 



ATMOSPHERE 



Introduction. Fish live in an ocean of Avater, but 

 just as truly do we live in an ocean of gas, which we 

 call the atmosphere. This atmosphere which envelops 

 us at all times extends more than two hundred miles 

 above us. 



No part of our environment is of more immediate im- 

 portance to us than the air we breathe. If it is pure 

 we are strong. If we are deprived of air for but a 

 single hour we die. 



Perhaps no other part of our environment has had so 

 great an influence in our development. How we dress, 

 Avhat we grow, and what we eat are chiefly determined 

 by tfee conditions of the atmosphere, with reference to 

 its temperature, moisture, and winds. 



We are not always conscious of the air around us. 

 We know roughly that on certain days the air seems 

 heavy, while at other times we feel its bracing effect. 

 Generally it is quiet and we are utterly unconscious of 

 it ; at other times we are very much aware of it, because 

 of its heavy winds. 



Our atmosphere is thus at one time heavy ; at another 

 time hot and oppressive, or cold and invigorating. At 

 one time it is almost quiet, and at another traveling at 



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