132 THE ATMOSPHERE AND ITS SERVICE TO MAN 



The atmosphere, of course, must be densest at its lowest 

 level since the pressure due to the weight of the air is greatest 

 there. The farther we ascend the less dense the air becomes. 

 This is the chief reason why people from a lower altitude 

 " get out of breath " easily when they go to a higher alti- 

 tude. It is also the reason why balloons and airplanes 



OBSERVATION WAR BALLOONS 



can ascend only to a limited distance. Since the gas in the 

 balloon is less dense than the lower atmosphere, it rises to 

 a point where the density of the air just balances the aver- 

 age density of the balloon and its burden. 



SUMMARY 



The gaseous envelope of the earth is called its atmosphere. 

 The chief gases of the atmosphere are oxygen, which is 



