408 Hydrodynamics. 



ton leaves void ; it therefore forces up the valve E, and enters into 

 the body of the pump, its elasticity diminishing in proportion as 

 it fills a greater space. Hence it will exert on the surface GH of 

 the water a less effort than is made by the exterior air in its 

 natural state upon the surrounding parts of the same surface #G, 

 HS] and the excess of pressure on the part of the exterior air 

 will cause the water to rise in the upper pipe GK to a certain 

 height //JV, such that the weight of the column GJV, together with 

 the spring of the superincumbent air shall just be a counterpoise 

 to the pressure of the exterior air. At this time the valve E 

 closes of itself; and if the piston be lowered, the air contained 

 between the piston and the base IV of the body of the pump, 

 having its density augmented as the piston is lowered, will at 

 length have its density, and consequently its elasticity, greater 

 than that of the exterior air ; this difference of elasticity will 

 constitute a force sufficient to push the valve L of the piston 

 upwards, and some air will escape till the exterior and interior 

 air are reduced to the same density. The valve L then falls 

 again ; and if we again elevate the piston, the water will be 

 raised higher in FGHK, for the same reason as before. Thus, 

 after a certain number of strokes of the piston, the water will 

 reach the body of the pump ; where being once entered, it will 

 be forced at each stroke of the piston through the spout X ; for 

 the water above the piston will then press upon the valve and 

 keep it shut while the piston is rising ; so that a cylinder of wa- 

 ter whose height is equal to the stroke OT of the piston (or the 

 vertical distance through which it passes) will be raised by each 

 upward motion and forced through the aperture X, provided it is 

 of an adequate magnitude. 



511. The lifting pump is represented in figure 246. Its man- 

 ner of operation is this ; the piston PCD is here placed below 

 the horizontal surface RS of the water, and when it is made to 

 descend, it produces a vacuum between the valve E (which is 

 pushed down by the exterior air) and the base CD of the piston. 

 The weight of the water, together with that of the exterior air 

 about R and 5, presses up the valve L, and the water passes into 

 the body of the pump ; and when the water ceases to enter, the 

 weight of the valve L closes it. Then, if the piston be raised, it 

 raises all the water above it, forces up the valve , and introduc- 



