FORCING PUMP. 



pipe G returns by the pipe P to the reservoir K, from which it is 

 again raised by the pump. 



Fig. 1C. 

 H' 



17. In the force-pump, where the water acts upon the piston 

 with a great pressure, it is very important that the piston should 

 move in complete water-tight contact with the pump-barrel. This 

 is best accomplished by an accurately formed metallic plunger, P, 

 fig. 17, working through a collar of leather, A B, which is exactly 

 fitted to it, and with which it is made air-tight and water-tight, 

 by behig lubricated with oil or tallow. When this plunger is raised, 

 the space it deserts is filled by the water which rises through the 

 valve v, and when it descends, the water which filled the space 

 into which it advances, is driven before it through the valve v 

 into the force-pipe. 



18. If the forcing-pump, represented in fig. 16, be attentively 

 considered, it will be perceived that the principles on which the 

 piston acts, in its ascent and descent, are perfectly distinct. In 



189 



