146 GENERAL SCIENCE 



99. Movements of the Air. Air, like the flowing brook 

 and the waves of the sea, is never quiet or at perfect rest. 

 If it moves less than 3 feet per second its movement is 

 not perceptible to the body. Its speed varies from this 

 unnoticeable movement to that of the raging tornado. 

 These movements are caused by unequal heating in 

 different localities. Air, when heated, expands and be- 

 comes less dense and is then forced away by the cooler 

 air. The air at high altitudes moves more freely and 

 faster than the air near the earth's surface. The move- 

 ments of the upper air can be detected by observing the 

 movements of clouds, which sometimes travel in a direction 

 opposite to the way the wind is blowing near the ground. 



100. Density of the Air. The air surrounds the entire 

 earth and when it is spoken of as a whole it is usually 

 called the atmosphere. It extends from 50 to 100 miles 

 above us. It is most dense on the earth's surface at 

 sea level and rapidly decreases in density as one ascends. 

 The air is so rare on the tops of the highest moun- 

 tains that man or other animals cannot go there. It 

 is even difficult for some people to stand the change in 

 the density of the air while crossing the Rocky Moun- 

 tains. About one-half of the earth's atmosphere by 

 weight is within three miles of the earth's surface. 

 Some air is in water, some in the soil, and some in the 

 deepest openings in the earth. 



101. The Weight of Air. --To the ordinary observer 

 air does not seem to have any weight or to offer any 

 resistance to objects passing through it. Recall some 

 experiences that you have had and some unintentional 

 observations that you have made and see if you think 

 that air has no weight. A person running or riding on 

 a bicycle can feel the air pressing against him with con- 



