220 



GENERAL SCIENCE 



because this is not so expensive as digging. In sandy 

 places where the water is known to be near the surface, 

 sharp-pointed pipes with a number of 

 small holes near the pointed end are 

 driven down until they reach the water. 

 A common pump is then put in the pipe 

 and water can be obtained. 



In all wells the piston of the pump must 

 be within 34 feet of the water, or the air 

 pressure will not raise the water to the 

 first valve. Since the valves cannot be 

 made perfectly air-tight, the pump will 

 work better if the piston is not more than 

 20 or 25 feet from the surface of the water. 

 Force pumps are used in deep wells be- 

 cause the piston can be placed low and 

 the water forced from the piston to the 



top. Even in a common lifting pump 

 FORCE PUMP WITH , * . , . , , .. 



PISTON IN WATER the P lst n 1S P laced several ieet d wn in 

 the well in order to make it easier for the 

 air to force the water up to it, and also to prevent 

 freezing in winter. Freezing is prevented by opening a 

 valve just above the piston, which permits the water 

 above the piston to flow out of the tube. This valve 

 must be closed again when one wants to pump water. 



QUESTIONS AND EXERCISES 



1. How does the air aid one in drinking a liquid through a straw ? 



2. Make a siphon of a rubber hose and explain the cause of 

 the flow of the liquid. 



3. Explain the difference between a common lifting pump and 

 a force pump. 



4. Why do high-pressure force pumps have air domes ? 



5. When ought you to install a centrifugal pump rather than 

 a force pump ? 



