50 SCIENCE OF HOME AND COMMUNITY 



lifted, the valve V in the piston opens and the water passes 

 through while the piston is being pushed down to the bottom 

 of the cylinder. When the handle is pressed down, the 

 piston P is lifted, the valve V in the piston closes and the 

 water above is lifted and flows out through the spout. At 

 the same time the lower valve 5 is opened upwards and the 

 water enters and fills the cylinder ; and then the operation 

 is repeated as before. 



It is easy to understand how the piston and its valve works, 

 but what causes the lower valve to open and the water to 

 rise ? To push water up in this way requires some force and 

 we are curious to know what this force is. It is the weight 

 of the air pressing down on the surface of the well which 

 forces the water up the pipe. 



Weight of air. Perhaps you do not realize that the air 

 has weight, but it has been proved by experiment. By 

 means of an air pump the air has been pumped out of air- 

 tight vessels and the vessel weighed before and after removing 

 the air. The second weight is found to be less than the 

 first, the difference being the weight of the air. Air is very 

 light but it extends up so high, perhaps twenty-five to fifty 

 miles or more, that the total weight of air bearing down on 

 any surface is very great. 



Experiments have shown that the air bearing down on 

 each square inch of surface weighs fifteen pounds, so that the 

 air bearing down on your hand weighs about three hundred 

 pounds. How, then, can you hold your arm out straight 

 with all this weight on it ? It is because the air that touches 

 the back of your hand pushes up with the same force that the 

 air on the palm of your hand pushes down, so that the two 

 balance and you do not feel the weight. Likewise the air 

 exerts an equal pressure sidewise, thus keeping its weight 

 balanced in every direction. It is only when by a machine, 

 such as an air pump, the air is removed from some space, 

 that the unbalanced pressure shows its tremendous force. 



