THE SUCTION-PUMP 



57 



slowly and with great difficulty. For this we should be 

 truly grateful. We should accept our wonderful "heritage 

 of knowledge" with an understanding of all the toil and 

 all the time it took to build it up. For what we know 

 there is little credit to ourselves. About all we have to do 

 is to open a book and read. 



The most important thing about a pump is the piston 

 (see Fig. 23). It works up and down in a cylinder. When 

 you push the handle of the pump down, 

 the piston comes up in the cylinder, and 

 does the same thing that you do when 

 you suck a straw. // removes the pressure 

 on the water just below it. The pressure 

 of air on the water in the well forces the 

 water in the pipe to follow the piston up 

 the cylinder. Of course the piston must 

 be air-tight, and there must be a valve hi 

 it which will let the water come through 

 but will not let it get back. So when 

 you force the piston down again, the 

 water which has entered the cylinder 

 will pass through it (the piston) and be in a position to 

 run out of the spout. Also there must be a valve at the 

 point where the pipe joins the cylinder. Why? All this 

 you can understand better by studying the figure and ex- 

 plaining it. When a pump has a leaky valve, you can 

 sometimes make it work by priming it, that is, by pouring 

 in water at the top. Can you explain this? 



If the distance between the surface of the water in the 

 well and the piston in such a pump as we have been describ- 

 ing is greater than about thirty-two feet, the pump will 

 not work. This should not surprise you when you remem- 



Fic. 23. The suc- 

 tion-pump; try to 

 explain the differ- 

 ent parts. 



