PUMPS are machines for raising water and other fluids to a higher level. They are divided into several classes according to their mode of action. Of these, as the most important, we shall describe in detail the following; 1. The lift or suction-pump; 2. The lift and force-pump; 3. The chain-pump; 4. The centrifugal pump; 5. The jet-pump, 1. The Lift or AS'udion-pump.—The diagram (Fig. 1.) represents the ordinary suction pump. A is a cylinder, which is called the barrel; with it is connected at the bottom a pipe, B, which communicates with the water to be raised; and at its top is another pipe, C. which receives the water raised. In the barrel are placed two valves, D and E. D is fixed in position at the bottom of the barrel; E is attached to and forms part of the piston F, which moves up and down the barrel when motive-power is applied to the rod G. The piston, or bucket, consists of a cylindrical piece of wood or metal, which fits exactly the barrel in which it moves, so that no water or air can pass between its circumfer ence and the sides of the cylinder. This tight fitting is attained in wooden pistons by surrounding them with a leather ring; and in those of metal, by hemp or other packing, which is wrapped round a groove made in their outer surface. The hollow interior of the piston is closed at the top by the valve E, which is a kind of door opening on a hinge, at one side of it, in an upward direction, on the application of pressure, and shutting on to its seat on the piston when the pressure is removed. When opened, water or air can pass through it to the upper side of the piston; but when shut, none can pass from one side of the piston to the other. The other valve, D, is similar to it in all respects. except that, as before stated, it is fixed in the bottom of the barrel; it also can only open upward.
To describe the action of the pump, we shall suppose the piston to be at the bottom of the barrel, and the pump to contain nothing but air. On moving the piston up the barrel—the valve in it being shut, and kept so by the atmospheric pressure above it—no air can pass from above it into the part of the barrel from which it is moving; the air contained in which becoming rarefied, by having to occupy a greater space, exerts less pressure ou the valve D at the bottom of the barrel than the air in suction-pipe B below it. This valve is thus opened, and
the air from the suction-pipe enters the barrel; so that when the piston has arrived at the top, a volume of air equal to the contents of the barrel has passed from the suction-pipe into the barrel. When the piston descends, it compresses the air in the barrel, which shuts the valve D; and when the density of the compressed air becomes greater than that of the atmosphere, the valve E in the piston is forced open, and the air in the barrel passes to the upper side of the piston. The next upward stroke of the piston again draws a like quantity of air from the suction-pipe into the barrel; and, as none of this air again enters the pipe, but is passed to the upper side of the piston by its downward stroke, the suction-pipe is by degrees emptied of the air it contained. During this process, however, motion has taken place in the water at the foot of the suction-pipe. The surface of the water at His pressed upon by the weight of the atmosphere with a pressure of about 15 lbs. on every square inch; and, by the laws of fluid pressure, if an equal pressure is not exerted on the surface of the water in the suction-pipe, the water will rise in it, until the pressure on the surface, plus the weight of its fluid column, balances the pressure of the atmosphere on the surface EI outside; so that, as the air in the suction-pipe is rarefied, the water rises in it, until, when all the air is extracted from it, the water stands at the level of the valve D. By the next upward stroke of the piston, the barrel being emptied of air, the water follows the piston, and fills the barrel as it filled the suction-pipe. The pressure produced by the downward stroke shuts the valve D, and forces the water in the barrel through the valve E. The succeeding upward stroke carries this water into the pipe above, and again fills the barrel from the suction-pipe. In like manner, every successive upward stroke discharges a body of water equal to the content of the barrel into the pipe above it, and the pump will draw water as long as the action of the piston is continued.