HYDRAULICS. 



than water will of course occupy the 

 upper part of this vessel, and as soon 

 as the action of the pump has filled it 

 with water up to the line o o, or just 

 above the lower end of the open pipe 

 pp, all air that is above the water will 

 be confined and unable to escape. If 

 now the working of the piston be sup- 

 posed to throw water more rapidly into 

 the air-vessel than it can escape by the 

 pipe p p, it is evident that such confined 

 air will be condensed into less compass 

 than it naturally occupies, in order to 

 make room for the water ; and as the 

 elasticity of air is constant and increases 

 in power with its degree of condensation 

 without limitation, so the spring of the 

 air in the air-vessel will become a coun- 

 terpoise, or equivalent for any height to 

 which the pipe pp may be carried ; and 

 although the water in the pump ex- 

 plained at page 12, (fig. 8.) would not 

 admit of condensation, so as to permit a 

 fresh quantity of water to enter the as- 

 cending pipe without putting all its 

 contents into motion, yet the introduc- 

 tion of the air-vessel obviates this diffi- 

 culty, for now the new quantity of 

 water is not delivered into a former 

 quantity of inelastic water, but into a 

 vessel filled with air which readily al- 

 lows a change of dimensions ; and while 

 the piston i (fig* 1 0.) is rising and pro- 

 jecting no water, the previously con- 

 densed air in n has time to re-expand 

 into its former volume, by expelling an 

 equivalent quantity of water up the pipe 

 pp, and thus, if the air-vessel is large 

 enough, a constant and equable current 

 may be maintained. 



Fig. 1 1 shows the other form of the 

 forcing-pump, though this construction 

 is generally called The Lift and Force- 

 pump. Its formation is the same as the 

 last-described figure, except that the 

 piston is not solid, but is perforated, 

 and covered by a valve opening upwards, 

 as in the common lifting-pump : the pis- 

 ton-rod q likewise moves in an air-tight 

 manner, through a stuffing-box, or collar 

 of leather, on the top of the working- 

 barrel, which in this case is closed ; and 

 the lateral delivering-pipe with its air- 

 vessel proceeds from the upper, instead 

 of the lower, part of the working-barrel. 

 This pump not only has the stuffing- 

 box, but three valves, instead of two as 

 in the last example. It is consequently 

 rather more intricate and expensive in 

 its construction, with no other advan- 

 tage than that it is rather more cleanly 

 in its working ; for if the piston of the 



former pump is not quite water-tight, a 

 quantity of water may flow over the 



fig.ll. 



open top of its working-barrel, which 

 cannot be the case in this pump if well 

 made. Their action is very nearly alike, 

 for this last pump raises water through 

 the suction-pipe h by the elevation of the 

 piston i : on depressing the piston, that 

 water passes through it by its valve, 

 and gets above it to fill the upper part 

 of the working-barrel ; on the re-ascent 

 of the piston, the water, being unable 

 to escape at the top of the barrel on 

 account of the cover and stuffing-box cc, 

 is forced up the lateral pipe / into the 

 air-vessel, and from thence passes away 

 by the ascending pipe p as before. The 

 first pump raises water by the down and 

 this by the up stroke ; but this is easily 

 changed if required, by adopting a lever 

 of the first, instead of one of the second 

 k'nd, as shown in the figures. 



The air-vessel, shown in this figure, 

 likewise differs from that in fig. 10, 

 because the delivering pipe p of the first 

 passes through the top, and the latter 

 through the bottom of the air-vessel ; 

 but they both proceed from near the 

 bottom, and in either case, when the 

 water has risen to the dotted line o o, so 

 as to cover the lower end of this pipe, 

 the air will be confined, and their opera- 

 tions must be alike. The air-vessel 

 must be suited in its capacity to the 

 magnitude of the pump or pumps that 



