THE COMMON PUMP. 269 



pheric air, and the valves V and V closed by their own 

 weight; the water will stand at the same level C both 

 within and without the suction pipe. Now raise the piston, 

 the air in BC will tend by its elastic force to occupy the 

 space which the piston leaves void; it will therefore open 

 the valve V, and will pass from the pipe to the barrel, its 

 elasticity diminishing in proportion as it fills a larger space. 

 It will, therefore, exert less pressure on the water at C 

 than the atmosphere does at C outside the pump ; hence 

 the atmospheric pressure on the surface of the water outside 

 will force water up the pipe BC, until the pressure at C is 

 equal to the atmospheric pressure. As the piston rises the 

 water will rise in BC, the pressure of the air above M keep- 

 ing the valve V closed. When the piston descends, the 

 valve V closes, and the air in MB, becoming compressed as 

 the piston descends, will at length have its elastic force 

 greater than that of the exterior air above the piston, and 

 will open the valve V, and will escape through it. 



This process being repeated a few time5, the water at 

 length ascends through the valve V into the barrel, and at 

 the next descent of the piston, will be forced through the 

 valve V and be then lifted to the spout S, through which 

 it will flow. While this water is being lifted, the atmos- 

 pheric pressure on the surface of the water outside the pipe 

 forces more water into the pump, so that, on the next 

 descent, the piston gets more water to lift ; and thus the 

 process continues, the suction pipe and barrel remaining 

 full, so that a cylinder of water equal to that through which 

 the piston is raised will be poured out at each upward mo- 

 tion, provided the spout S is large enough. 



SCH. 1. The height BC must be less than the height of 

 the water barometer, or else the water will never rise to the 

 valve V. Although the height of the water barometer is 

 about 33 feet, yet in consequence of unavoidable imperfec- 

 tions in construction, the height of the valve V above the 

 surface of the water in the well should be considerably less 



