PART III 

 THE ATMOSPHERE 



CHAPTER XI 

 GENERAL CONCEPTION OF THE ATMOSPHERE 



Substantiality. When the atmosphere is still we are hardly 

 conscious of its existence, but many familiar phenomena show that 

 the air is very substantial. Thus wind, which is only air in motion, 

 may be so strong that trees and buildings are blown down by it. 

 The substantiality of the air may be shown in another way. If 

 the air is pumped out of a cylinder whose top is covered by a thin 

 piece of rubber, the rubber cover is pressed down into the cylinder, 

 The force which presses it down is the weight of the air above. 

 This shows that the air is something real, has weight, and exerts 

 pressure. The amount of its pressure, that is, its weight, is nearly 

 15 pounds (14.7) on every square inch of surface at sea-level. 



Relation to the rest of the earth. The atmosphere is often 

 called an envelope of the earth. It is, however, a part of the earth. 

 It goes with the rest of the earth through space, and it is essential 

 to the life of the earth and to most of the processes in operation on 

 the earth's surface. It helps to distribute moisture, and it makes 

 the extremes of heat and cold less than they would be if it did not 

 exist. Without it the conditions on the earth would be very differ- 

 ent from what they now are. Furthermore, the atmosphere is not 

 merely an envelope of the rest of the earth, for it goes down into 

 the soil and rocks as far as there are holes and cracks, and its con- 

 stituents are dissolved in the waters of sea and land. 



Density. The atmosphere is made up of gases, and from the laws 

 which govern the distribution of gases it is known that the air must 

 be densest at its bottom and less dense above. This is the same 



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