Water-Pumps. 409 



es the water into the part IVYX* When the piston is raised to 

 its highest position, the valve E is made to close by the super- 

 incumbent water, and retains the fluid there until, by a fresh 

 stroke of the piston, more water is forced upwards through the 

 valve EJ that which was before in the upper part of the pump 

 being expelled through a proper orifice or spout in the neigh- 

 bourhood of Jf, in order to make way for a new supply. By 

 continuing the operation, water is delivered at every stroke of 

 the piston. 



512. "The forcing pump unites in some measure the properties 

 of the other two. The piston ABCD, which here has no valve, 

 being elevated, rarefies the air in the space DGHVOC, and the 

 water rises towards K-, the subsequent descent of the piston 

 forces some of the air in this space through the valve L ; the 

 next ascent of the piston closes the valve L, and raises the water 

 in GK ; and so on till the water passes through the valve E and 

 enters the space DIOC. Then the piston being pushed down, 

 closes the valve E, and some of the condensed air is forced 

 through the valve L. A further stroke raises more water into 

 the space 7)O/C, and expels more air through L. At length the 

 water reaches L, and the subsequent strokes raise it into the 

 tube MO m w, whence it is carried off by a spout, as in /the 

 other pumps. Or, if this pump be closed at mn, excepting a 

 narrow pipe P5, when the water is raised by the process 

 just described to o r, above the bottom S of the tube, the elastic 

 force of the compressed air in the space nrom will compel the 

 water to issue from the aperture P in a continued stream or jet, 

 thus forming a fire engine or artificial fountain. 



513. Let us now enquire into the fundamental properties of 

 these machines. By means of the lifting pump, water may be 

 elevated to any height we please, provided we employ a suffi- 

 cient force. But the estimate of this force requires various con- 

 siderations. We must have regard to the dimensions of the pis- 

 ton, the barrel of the pump, the height to which the water is to 

 be raised, and the velocity with which the water is to be moved, 

 beside the effects of friction, &c. At present, however, we shall 

 not examine these particulars in all their extent; but shall 

 confine ourselves to one of them. Now it is certain that the 



Mech. 52 



