78 



GENERAL SCIENCE 



causes it to flow through the spout. Notice the location 

 and working of the two valves in such a pump. When 

 the piston is raised and a partial vacuum is created in 

 the chamber of the pump, the lower valve opens and 

 water is forced into the chamber. When the piston is 

 lowered, the lower valve is closed by the weight of the 

 water on it and the valve in the piston 

 opens and allows the water to pass 

 through into the chamber above. As 

 the piston is again raised the piston valve 

 closes and the water above it is raised to 

 the level of the spout, where it escapes, 

 while the lower valve is again opened 

 and more water enters the chamber as 

 before. .V 



In the force pump (Figure 78) the piston 

 has no valve. When the piston is raised, 

 a partial vacuum is created in the cham- 

 ber and the water rises in the chamber 

 as it does in the case of the common 

 pump. When the piston is lowered, the 

 valve at the bottom of the chamber 

 closes and the water is forced out through 



FIG. 77. Common the discharge valve into the discharge 

 suction Pump. p i pe ^ n d into the air chamber.' The 

 pressure of the water in the discharge pipe compresses 

 the air in the air chamber and this pressure in turn causes 

 the water to flow from the discharge pipe in a continuous 

 stream. On such a pump the discharge nozzle must be 

 smaller than the supply tube in order to insure sufficient 

 pressure to keep the flow of water continuous. 



The centrifugal pump is a valveless pump which is 

 especially valuable where a pump is needed which will 



