270 TENSION OF THE PISTON HOD. 



than 33 feet; otherwise the quantity of water lifted by {lie 

 piston at each stroke will be small. 



Sen. 2. It is not essential to the construction that there 

 should be two cylinders ; a single cylinder, with a valve 

 somewhere below the lowest point of the piston-range will 

 be sufficient, provided the lowest point of the range be less 

 than 33 feet above the surface in the reservoir. 



It is not necessary to the working of a pump that the 

 suction pipe should be straight; it may be of any shape, 

 and may enter the reservoir at any horizontal distance be- 

 low the barrel of the pump. 



144. Tension of the Piston Rod. (1) If the water in 

 BC (Fig. 73) has risen to P when the piston is at M, let TT' 

 be the pressure of the air in MP ; then we have TT' 

 pressure of water at P = pressure of water at C gpPG ; 

 hence 



(1) 



But the tension on the rod is the difference between the 

 atmospheric pressure above the piston and the pressure of 

 the air in MP ; hence calling A the area of the piston and 

 T the tension of the rod, we have from (1) 



T = (n TT') A = ffpPG-A. (2) 



If one inch be taken as the unit of length, and li be the 

 height in inches of the water barometer, we have gph = 15 

 Ibs., nearly, which in (2) gives 



T = 15^. (3) 



(2) When the pump is in full action. Let AH be 

 the range of the piston, and let CD = 7j, then at each 

 stroke, the volume DH of water is lifted, and therefore the 

 tension of the rod when the piston is ascending will be 

 gpA (h + HD) until the water begins to flow through the 

 spout, 



