294 THE FORCING-PUMP 



higher and higher, until after a few strokes of th. 

 piston the water rises above the valve i\ ; after the next 

 downward stroke of the piston it will also rise above 

 the piston itself. No further rarefaction of air can now 

 ( take place, for when the piston rises, the space below 

 it is immediately filled with water by the external 

 atmospheric pressure ; after that, every time the 

 piston is lowered, water from below it passes 

 through the valve in the piston, it is raised when the 

 piston ascends, and flows out through the discharge- 

 tube a. 



The length of the suction-pipe cannot exceed a de 

 finite limit, for, as has been shown previously, th( 

 column of water which can be supported by the pressur< 

 of the atmosphere is not more than about 10 m , am 

 water can thus evidently not be raised in the suction 

 pipe to a height greater than 10 m . The valve v 1 mus 

 hence be somewhat less than 10 m above the surface c 

 the water in the well, if the water is to be raised abov 

 it. In ordinary pumps the suction-pipe must even b 

 shorter, because the piston in such pumps does not wor 

 sufficiently air-tight, the rarefaction of the air in tli 

 suction-pipe is hence rather imperfect, and the colum 

 of water capable of being raised is usually not moi 

 than 7 or 8 m . 



In ordinary pumps the piston is usually renderc 

 more air-tight by a collar of leather which surrounds tl| 

 piston like a tube, and is fixed to it below, being somj 

 what wider at the top. When the piston is raised, tl 

 pressure above is greater than that below it, and tl 

 collar is pressed closely against the sides of the pipe. 

 The forcing-pump, fig. 196, differs from the suctio 



