AIR 



sea. 



This atmosphere varies from place to place ; at the sea 

 level it is heavy, on the mountain 

 top less heavy, and far above the 

 earth it is so light that it does not 

 contain enough oxygen to permit man 

 to live. Figure 40 illustrates by a pile 

 of pillows how the pressure of the air 

 varies from level to level. 



Sea level is a low portion of the 

 earth's surface, hence at sea level there 

 is a high column of air, and a heavv 



FIG. 40. To illustrate the de- . 



crease in pressure with height, air pressure. As one passes from sea 

 level to mountain top a gradual but 



steady decrease in the height of the air column occurs, 

 and hence a gradual but definite lessening of the air pressure. 

 77. Air Pressure. If an empty tube (Fig. 41) is placed 

 upright in water, the water will not rise in the tube, but if the 

 tube is put in water and the air is then 

 drawn out of the tube by the mouth, the 

 water will rise in the tube (Fig. 42). This 

 is what happens when we take lemonade 

 through a straw. When the air is with- 

 drawn from the straw by the mouth, the 

 pressure within the straw is reduced, and 

 the liquid is forced up the straw by the air 

 pressure on the surface of the liquid in 

 the glass. Even the ancient Greeks and 

 Romans knew that water would rise in a FIG. 41. The water in 

 tube when the pressure within the tube was 

 reduced, and hence they tried to obtain 

 water from wells in this fashion, but the water could never 

 be raised higher than 32 feet. Let us see why water could 

 rise 32 feet and no more. If an empty pipe is placed in a 



