38 



FLUID PRESSURE 



raised (by pushing down the handle A), air pushing 

 upon v from above closes the valve; thus no air can 

 get into s and a vacuum is formed there. To nil this 

 vacuum, atmospheric pressure upon the water in the 

 well pushes it into the pipe. A few strokes of the 

 piston removes the air entirely from pipe and pump, 

 bringing the water up to the piston, as in Fig. 23. 



Fig. 23 shows the piston on its upstroke. The valve 

 v is closed by the water above it, which is lifted to the 

 spout by lowering the handle. The space below p 

 tends to become a vacuum, but is kept full of water 

 by atmospheric pressure in the well, as explained. 



48. Force Pump The lifting pump can raise water 



only as high as the air can hold it. To send it on to any 

 distance, force has to be exerted 

 upon the water by the pump. A 

 device for doing this is called a 

 force pump; a diagram is shown 

 (Fig. 24) to explain its operation. 

 In Fig. 24, p is a solid piston 

 called a plunger; it has no valve. 

 A valve a opens into, and a valve 

 c out from, the barrel b. As the 

 plunger is raised, a opens, letting 

 water into b. Now when p is 

 pushed downward, force is ex- 

 erted upon the water in 6, which 



causes a to close and c to open. Thus the water is sent 

 to the pipe e under whatever pressure is given it by 

 the plunger. 



