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 x, is forced up the lateral pipe l into the air-vessel, and from thence passes away by the ascending-pipe k as before. When the water has risen in the air-vessel to the dotted line p o, so as to cover the lower end of this pipe, the air will be confined, and their operations must be alike. The air-vessel must be suited in its capacity to the magnitude of the pump or pumps that deliver water into it (for several pumps are frequently made to open into one common air-vessel), and ought, in all cases, to contain at least six or eight volumes of the pump, in order that the increasing expansive force of the air may not influence the motion of the piston during a single stroke; but for this no precise rule can be given, as the relative dimensions may vary to suit the circumstances of the case.
These forcing pumps with air-vessels are now very generally adopted in water-works for supplying cities or towns; and the height at which the water is at any time delivering, may be very nearly estimated if the air-vessel is large, and the supply equable, by examining the degree of condensation of the air within it. This is very conveniently done by a gauge, consisting of a glass tube with a closed top, applied by a stop-cock to the lower part of the air-vessel, or that which is always filled with water; at r s such a gauge is represented; and as it has an open communication with the air-vessel when the cock * is open, the air in the top of the tube will suffer the same condensation as that within the vessel. The height of the spaces occupied by air within the tube must be measured; and as the air at its ordinary density will balance a column of water thirty-three feet, high, so if confined air is loaded with the weight of such a column, it will shrink, or be condensed into half its former bulk; whenever, therefore, the air contained in the tube r is diminished to half its original length, the condensation within the air-vessel must be equal to two atmospheres: or, what is the same thing, the water in the pipe p must stand at the elevation of 33 feet.
If the water in p is raised to twice 33 feet, or 66 feet, then the condensation within the air-vessel must be equal to three atmospheres; and the air within it, as well as within the tube, will be diminished to one-third of its original bulk; one-fourth of the bulk will indicate four atmospheres of condensation, and be equal to the elevation of the water column to 132 feet, and so on, more or less, as the barometer may vary.
That useful machine, the fire-engine, or engine for extinguishing fires, is nothing more than two forcing-pumps, of the construction shown at page 358. working into one common air-vessel placed between them, and from which the spouting-pipe for directing the water upon the fire proceeds. The handles are so disposed, that while the piston of one pump is up, the other is down; and they are elongated for the purpose of enabling a great number of men to work them at the same time, for the purpose of throwing a very large quantity of water, which is rendered a continuous stream by the action of the air-vessel. See Fire-Engine.
It is curious that the most ancient pump we are acquainted with, namely, that of Ctesibius, at least, as it is handed down to us, very closely resembles the present fire-engine, for it consists of two forcing-pumps, disposed as just described; but instead of discharging their contents into an air-vessel, they merely deliver them into an intermediate close cistern, from which the water ascends by a perpendicular pipe, and in which nothing is wanting but the condensation of air. It must, however, be observed, that both the pumps last described would be forcing pumps, without their respective air-vessels; and though they act much more advantageously with, they are sometimes constructed without those appendages.
We now proceed to describe a pump with a double action, producing the same effect in its up as in its down-stroke; the water being alternately raised and forced on the opposite sides of the piston; that is to say, by the up-stroke of the piston, the water above it is forced out of it into an air-vessel, and, at the same time, the cylinder is re-charged by the water following the piston underneath; then by the return or down-stroke, the water underneath is forced out, and it flows in above, ready for the repetition of the operation, and so on continuously. In the diagram on the following page, a represents a solid piston, its rod working air-tight in a stuffing-box b, fixed at the top of the pump-barrel c c. The water from the well, supposed to be not more than thirty feet deep, ascends into the vacuum of the pump by the pipe d, and is conducted by a branched pipe ef to the top and bottom of the barrel alternately, through valves g and h, which open inwards. On the opposite side of the working-barrel are two corresponding apertures, furnished with valves i and j, opening outwards, and conducting the water by a branch-pipe k l into a single tube m, leading into an air-vessel o, whence it is discharged by the tube p.
In the figure, the piston is shown as having nearly reached the bottom of the cylinder; by the force thus exerted it has shut the valve h, and impelled the water through the valve j, the branch l, and pipe m, into the air-ressel o, where the elasticity of the air, pressing upon the surface of the water, has forced it up the pipe p. During this operation there has been a tendency to the production of a vacuum above the piston; consequently, the pressure of the air, acting upon the surface of the water in the well, has compelled it to follow the piston in its descent, and to fill the chamber above; afterwards, upon the ascent of the piston, the upward force shuts the valve g, and opens the valve i, through which the water is propelled along the branch k, pipe m, air-vessel o, and out of the pipe p. During this ascending stroke, the water from the well passing along d andf, has opened the valve h, shut the valve j, and re-filled the barrel under the piston; and thus the process is continued as long as the pump is worked.