Water-Pumps. 407 



Thus, if D' be equal lo TTT V4 5? an d tue number of strokes 

 n =. 11, we shall find 



so that r : 6 + r : : 1 : 3, and 6 : r : : 2 : 1 



Pumps for Raising Water. 



509. THE term pump is generally applied to a machine for 

 raising water by means of the pressure of the atmosphere. Of 

 pumps there are a great many different sorts ; we shall speak 

 only of those in most common use, and shall give merely such 

 a general description of their construction as will enable the 

 student to understand the principles on which their operation 

 depends. 



The pumps most generally used are, the sucking pump, the 

 lifting pump, and the forcing pump ; these have some parts in 

 common, and particularly the pistons and valves ; they will 

 therefore be treated in a connected way. 



The piston is a body ABCD of circular base, which may be Fi s- 245 - 

 moved through the interior part of the tube or body of the pump, 

 filling it exactly as it moves along. The sucker or valve E is 

 moveable about a joint in such a manner as either to permit 

 or to prevent the passage of the water, according as it presses 

 upwards or downwards. In figures 245, 246, there are likewise 

 valves in the pistons. FGHK is another tube joined to the body 

 of the pump, and is generally called the pipe or sucking pipe : 

 its lower extremity is immersed in the water, of which we sup- 

 pose RS to be the horizontal surface. 



510. The sucking pump is represented in figure 245. In this 

 pump if we suppose a power P applied to the hendle of the piston 

 so as to raise it from / to C. the air contained in the space 

 DVKHGFC tends by its spring to occupy the space that the pis- 



