CHAPTER XX 

 THE ATMOSPHERE 



The Sun the Source of the Earth's Heat. The sun is 



the great source of heat for the earth. Its heat comes to 

 us through the atmosphere, and if it were not for the 

 effects which the atmosphere produces upon this heat, the 

 earth would be very different from what it is; indeed, if 

 it were not for these atmospheric effects, life as it now 

 exists would perish. It could not endure the great heat 

 of day and the intense cold of night, which we should 

 have were it not for the protecting and modifying effects 

 of the atmosphere. Evidently, then, to understand the 

 relations between life and heat, and all the other effects 

 which the sun has upon the earth, it is quite necessary to 

 have in mind a picture of the atmosphere. 



Already you are familiar with a number of important 

 facts about the atmosphere. You know that atmosphere 

 and air are words which refer to the same thing. The 

 principal difference in these words is that atmosphere is 

 commonly used to indicate all of the air, while air is com- 

 monly used to indicate only part of the atmosphere. Thus 

 we say that we breathe air. To say that we "breathe at- 

 mosphere" would express the same fact, but would express 

 it awkwardly. 



What the Atmosphere Is. You nave learned that the 

 atmosphere is a mixture of gases. You have learned that 

 it has weight, exerting a pressure at sea-level of about 



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