Let this apparatus be now immersed in water, whose surface is WW, and let the piston be drawn up to CD. When the piston is permitted to descend by its own weight, the valve v will open, and the water will rise till it fills the barrel. If the piston is now drawn up by a power at a, the valve v will shut, and the piston c d will push the water upward through the valve x, into the rising pipe or main CDEF. By again letting down the piston, the valve x will shut by the weight of the superincumbent water, and the water below will again rise into the barrel through the valve v.
3. Description of the Forcing Pump.
The forcing pump, represented in Plate CCCCLXX. Fig. 5. consists of a working barrel ABCD, a suction pipe CHET, and a main or rising pipe DGH. The piston c d, fixed to the spear or rod a b, is solid, and forms a kind of double cone, which is widest at the middle, being covered with two hoops of strong leather. At CD there is a valve v rising up.wards, and another x of the same kind, in the rising pipe at e f.
When the piston abcdis pushed downwards, it drives the air before it, closing the valve v, and opening the valve x. Upon drawing up the piston, the air in the space Die now expands in the barrel, and the valve x is shut by the superior pressure of the atmosphere. From the rarefac tion of the air in the barrel the equilibrium of the air above and below the valve v is destroyed, and consequently the predominating pressure of the air in the suction pipe CDEF lifts the valve v, and expands into the barrel. The whole air in the suction pipe being thus more rare than that of the atmosphere, the pressure of the latter upon the surface of the water will force it up a short way in the suction pipe, till the equilibrium is restored. By a second stroke of the pump the same effect is produced, and the water rises'a little higher in the suction pipe, till it gets into the barrel, when it will be forced up the main or rising pipe DHG by the descent of the piston c d.
In examining the operation of this pump, it must be obvious that it begins its action as a sucking pump, and finishes by being a lifting pump. On this account the piston is made with a double cone, as the air and water _ would pass by the sides of the lower cone when the piston is drawing up ; but this is prevented by the leather of the upper cone applying itself to the surface of the barrel.
As the forcing pump• works by starts, it must obviously futihish an intermitting stream of water. There are many cases, particularly in fire engines and watering engines, where it is desirable to have a constant current ; and this effect may be obtained very simply, by the application of an air vessel MN, Plate CCCCLXX. Fig. 6 and 7. which is fitted upon the main or rising pipe of the forcing pump. In both these figures, the parts of the forcing pump to tho left of, and below the valve ef, arc exactly the same as that in Fig. 5.
In Fig. 6. the air vessel MN is joined laterally to the rising main pipe; and in Fig. 7. it surrounds an interrup tion of the main GH. In order to explain the action of these air vessels, let us suppose that the pump has re ceived water above the valve ef, a part of that water will /get into the vessel MN, and compress the air within it with a force proportional to the height of the column in the main GH. The next stroke of the piston draws up more water, and raising it higher in the main, the air in the air vessel is more powerfully compressed. When the water is at last raised to the place where it is to be deliver ed, or to the end of the main from which it is to issue, the air in the air vessel is so much compressed, that it balances the whole height of the column above it. Now, if the aperture at the top of the main, from which the water flows, were large enough to allow the water to issue with the same velocity as that of the piston, it would flow peace ably over, rising no higher by each successive stroke, and occasioning no additional compression of the air in the air vessel. But if the aperture of the main is diminished to half its size, the water forced up by the piston has not time to issue during the stroke, and consequently a part of it must go into the air vessel, and increase the compres sion of the included air. If the piston has now ended its stroke, and raises no more water, the compression of the air in the air vessel exceeding the pressure of the water in the main, the air will press upon the surface sv w of the water in the vessel, and force it out at the aperture of the main, in an uniform current, while the piston is returning to make another stroke.