Hydraulics. 57 



water would rise in it no higher than thirty-two 

 feet. We must recollect what was said respect- 

 ing the cause of the water's rising in the body of 

 the pump. We know it was the pressure of the 

 atmosphere on the surface of the exterior water 

 that forced it to rise. From this circumstance it 

 is evident that the air has weight. But again, 

 as the atmosphere, or that mass of air which 

 surrounds the globe, is only of a limited height 

 (supposed about forty -five miles) and that of 

 gradually diminishing density, it follows that its 

 weight or pressure must be limited also ; and it 

 is found that a column of water of thirty- two or 

 thirty-three feet high is, at a medium, equal in 

 weight to a column of air of the same diameter 

 or thickness the whole height of the atmosphere. 

 Consequently the pressure of the atmosphere can 

 never force water through any vacant space 

 higher than about thirty-three feet. By the ac- 

 tion of a common pump of four inches bore and 

 thirty feet high, a single man can discharge 

 twenty-seven gallons and a half of water in a 

 minute ; if the pump is only ten feet above the 

 surface of the well, the quantity discharged in 

 that time may be eighty-one gallons six pints. 



The forcing pump is upon a different plan. 

 Here the piston is without a valve, and the water 

 which rises through the valve in the box is 

 forced out by the depression of the solid piston. 

 Thus, in fig. 29, when the piston or plunger g* is 

 lifted up by the rod D, the water beneath forces 



D 5 



