378 



AIR 



FIG. 245. To illustrate the 

 decrease in 

 height. 



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 245 illus- 

 trates by a pile of pillows how the pres- 

 sure 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 heavy 

 air pressure. As one passes from sea 

 level to mountain top a gradual but 

 steady decrease in the height of the air 

 pressure with co lumn occurs, and a gradual but definite 



lessening of the air pressure. 

 Air pressure. When an empty tube (Fig. 246) is placed in 



water, the water does not rise to the top of the tube. But if 



the tube is put in water and the air is then 



withdrawn by suction, the water rises in the 



tube (Fig. 247). This is what happens 



when we take lemonade through a straw. 



When the air is withdrawn from the straw 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 tube when the pressure within the Fl ?- 2 46. The water 



in the tube is at the 

 tube was reduced, and they tried to obtain same level as that in 



water from wells in this fashion. But ^ glass - 

 the water could never be raised higher than 34 feet. Let 

 us see why water could rise 34 feet and no more. If an empty 

 pipe is placed in a cistern of water, the water in the pipe does 



