The action of this pump may be more shortly described by saying that the piston withdraws the air from the barrel, and produces is vacuum, into which the water rushes through the suction-pipe, impelled by the pressure of the atmosphere on its surface. This atmospheric pressure balances a column of water of about 33 ft. in height; so that if the barrel be placed at a greater height than this from the surface of the water in the well, the water will not rise into it, and the pump will not draw.
'With regard to its efficiency—that is to say, the relation between the power expended and the work produced, as measured by the water raised—we may remark, that the power is expended-1st, in raising the water through the required height; 2d, in over coming the friction of the moving parts of the pump; 3d, in the friction and fluid resist ance of the water in passing through the valves and pipes; 4th, in the losses arising from the want of proper proportion between the various parts of the pump. The losses arising from these last sources are very great, and vary so much according to the construction of each particular pump, that no useful estimate can be formed of the efficiency. We may say, however, that x pump of this description, to yield 50 per cent of the applied power, must be well proportioned and carefully constructed, 2. The Lift and ordinary forms arc very similar to the suction pump before described, with this exception, that the valve represented by E in Fig. 1, instead of being fixed on the piston, is placed in the discharge-pipe, the piston itself being solid. The water is drawn up into the barrel by suction in the manner just described in the suction-pipe, and then the pressure of the piston in its downward stroke forces it through the outlet valve to any height that may be required. Another form is provided with a different description of piston, called the plunger-pole. Its action is precisely the same as that of the other, with this exception, that the plunger-pole, instead of emptying the barrel at every stroke, merely drives out that quantity which it displaces by its volume. It is simply a solid rod of metal moving through a water-tight stuffing-box. This stuffing-box is made by placing, on a circular flange of metal, rings of india-rubber or other packing, the inner diameter of which is slightly less than that of the plunger pole. On these is placed a ring of metal, and through the whole are passed bolts, which,
on being screwed tight, force the packing tightly against the plunger-pole. It posseSses many advantages, for the packing can be tightened and repaired without removal of the piston or stoppage of the pump; also, the cylinder is not worn by its action, nor does it require to be accurately bored out., as in the other form of pump.
In these pumps, it will ha oliservA that the water is forced into the ascending pipe or column only on the downward stroke; it will thus be discharged in a series of rushes or jerks. As it is a great object to procure a continuous discharge, both for its conveni ence, and for the saving of the power wasted by the continual acceleration and retarda tion of the ascending column, various methods have been used for that. purpose. 'The most common is the reservoir of air, which is an air-tight receptacle fixed vertically on the discharge-pipe; the water forced into the pipe by the down-stroke compresses this air, which, acting as a spring, returns this force to the ascending column during the period'of the up-stroke, and so, by taking the blow of the water, and returning it gradually, equalizes the pressure, and renders the discharge uniform. Another method is the double-action force-pump, by which equal volumes of water are forced into the ascending column by both up and down strokes. A solid piston is worked by a rod of half the section of t-he piston itself. During the up-stroke, the upper surface forces a volume of water into the ascending column, and the lower surface draws in twice that volume. In the down-stroke, these two volumes are sent through a pipe into a recepta cle, communicating with the upper face of the piston. One of the volumes tills the space between the piston rod and the cylinder which would otherwise be left empty by the descent of the piston; the other volume is sent into the ascending column; so that a vol ume of water equal to half the content of the barrel is sent into Se ascending column by both the up and the down strokes.