Chap. XVII.

PUMPS.

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hole II closed by a second valve G, and cut in a copperplate which is united to the suctionpipe A B, and cast at the same time. The diameter E F exceeding the suction pipe formsa flange, the breadth of which is the interval E G and I F between two concentric circles ; onthis a leather roundle, N K L, is fitted, hollowed from N to Lto admit the tail of the valve, whose diameter is less thanG I, and greater than the hole H, in order that it may closeit more exactly. The figure shows that when the flangesare fitted together, the tail of the valve N and the leatherroundle, OQP, will be between them, which are fastenedtogether by nuts and screws. The iron or copper plate Rwith which the valve is loaded, that it may close morequickly, is also circular, and its diameter should be rathergreater than that of the hole H, especially in forcing*pumps, that the pressure which it must sustain may notbend it

When the piston is raised, a vacuum is produced in thespace I S T G, or the air is more highly rarefied, and thatin the suction pipe being no longer in equilibrio withthat in the pump-barrel, forces open the valve G, whichcloses the communication between the two pipes, expandsin. the space ISTG, and becomes equally rarefied fromthe surface of the water to the base ST of the piston.

The weight of the atmospheric air then pressing on thesurface of the water, Y Z, forces it into the suction pipe toa certain height, and having reduced the air containedthere to its natural density occupies its place. Thus thewater will ascend more rapidly at first, because as it drivesforward the air, it condenses it, and becomes its own obstacleto further progress: for example, if it stop at 3 feetabove the water, and the weight of the atmosphere isequivalent to a column of water 31 feet high, the elasticityof the air remaining in the pump is capable of sustaininga column of water 28 feet high, after the condensationcaused by the water which has ascended.

If the piston be lowered, the valve G will close, the aircontained in the space ISTG, being more and more com-pressed as the piston descends, acquiring an elasticity su-perior to the atmospheric pressure, and will raise the valveX, and escape by the hole K LM. If the piston be againraised, the valve N will close, and the air in the pipe A B,

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Fig. 1771. SUCTION PUMP.

dilating in the space IT, the atmospheric weight will raise the water higher than at first;continuing to work the piston, the water will rise in the pump-barrel to a certain height, 5 6,and the space ISTG will be filled partly by water and partly by air, which will be com-pressed into the space 5ST6. On lowering the piston, the valve C will close; the airremaining in the pump-barrel will be forced through the piston with a part of the water,which having once risen above the valve N, there will be no more air below, and the waterwill accompany it to the height ST: on lowering the piston, the water will pass aboveit, and when it is again raised will run into the cistern VX; thus the whole play of thepump consists in the action of the external air, and the play of the two valves N and G,which open and shut alternately.

Forcing Pumps have the same parts as suction pumps, but are differently placed, as shownin the figure: the pump-barrel ABCD dips in the water to be raised, and is united to anascending pipe, ECHF, by flanges and screws; this pipe is composed of two pieces, thefirst, CH E F, is bent in such a manner as to offer no obstacle to the motion of the frameTX\ V, which carries the rod N of the piston M; and the second EGH F, the diameterof which is uniform, conducts the water to the height required. The piston of this pumpdiffers a little from the suction, being pierced with a hole L, covered by a valve K, whichopens from above downwards, its rod N O being attached to the cross piece It S and T V ofthe frame, which is suspended to a piece, Z, attached to a crank or lever. The barrel issometimes in two pieces, to allow the water to ascend more freely : the upper part ispierced with a hole covered with a valve I.

The piston being at the base of the pump-barrel, it will leave a vacuum or highly rarefiedair, arising from that between the valves I and K when first lowered. The water will thenbe pressed upwards by collateral columns, aided by the atmospheric pressure; the valve Kwill open; the water will pass through the piston, mount in the pump-barrel, and compressthe air remaining there, which will be reduced to its natural state ; but as soon as thepiston remounts the valve K will close, and the water above !>t*ing driven upwards, will

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