182 



WELLSS NATURAL PHILOSOPHY. 



pipe, descends into the water -vrhich it is designed to FiG. 173. 



raise. In tlie barrel works a piston containing a valve, 



p, opening upward. A similar valve, g, is fixed in the 



bony of the pump, at the top of the suction-pipe. S is 



a spout from which the water raised by the working of 



the piston is discharged. 



The operation of the pump in raising water is as fol- 

 lows ; — when the piston is raised from the bottom of the 

 cjiinder, the air above it is drawn up, leaving a vacuum 

 below the piston ; the water in the well then rushes up 

 through the valve g, and fills the cylinder ; the piston ia 

 then forced down, shutting the valve, g, and causing the 

 water to rise through the piston-valve, p ; the piston is 

 then raised, closing its valve, and raising the water 

 above it, which flows out of the spout, S. 



,^ , 391. Water rises in a pump 



Whv does . . ^ 



water rise in a siinplv and entirely by the 



common pump f '■ " n i ' ^ 



pressure of the atmosphere 

 (15 pounds on every square inch), which 

 pushes it up into the void, or vacuum left by the up- 

 drawn piston. 



To what height 

 will water rise 

 in the common 



392. The common, or suction pump, can 

 will water rise j^^^ j,g^jgQ 'watcr bcvond the point of height at 



pump? which the column of water in the pump tube 



is exactly balanced by the weight of the atmosphere. The 

 utmost limit of this does not exceed 34 feet. 



The height to which water is thus forced up in a pump is simply a question 

 of balance ; 15 pounds' pressure of the atmosphere can support onlj- 15 pounds' 

 weight of water ; and a column of water, one inch square and 34 feet high, 

 will weigh 15 pounds. As the pressure of the atmosphere is subject to va- 

 riations, and as the mechanism of the pump is never absolutely perfect, the 

 length of the pipe through which water is to be elevated ought never to 

 exceed in practice 30 feet above the level of the water in the well, or reser- 

 voir. 



What is 

 Valve? 



393. A valve, in general, is a contrivance by 

 which water or other fluid, flowing through a 

 tube or aperture, is allowed free passage in one direction, 

 but is stopped in the other. Its structure is such, that, 

 while the pressure of fluid on one side has a tendency to 

 close it, the pressure on the other side has a tendency to 

 open it. 



