Chap. XVII.

PUMPS.

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hand more water enters the barrel A C, occupying the vacuum left by the piston, to beforced in its turn as before; it will thus alternately pass through the holes G and H, andiiscend without interruption into the reservoir : as it continually passes through the holeP, the valve O is not necessary, but is convenient; for ifthe play of one piston should be interrupted, the otherwould raise the water, as in ordinary forcing-pumps.

Pig. 1781. raises water continuously, but in a moresimple manner than the preceding : the pump-barrel D Bis united to a recipient X Y Z, of a cylindrical figure,covered by a half sphere Y; these two pieces commu-nicate hy a bole G, opening and shutting with a copperclack valve H :* the suction pipe AD joins the pump-barrel, and the ascending branch ZW the recipient ; bothhave their valves F and V as usual; the piston C is solid,and worked by a frame.

When, after several strokes of the piston, the water risesin the suction pipe above the valve F, it passes thence intothe barrel, and is forced upwards; it then enters the re-cipient, and reaches nearly the same level in the branch1 T above the hole I; the air in the recipient not beingable to escape, and the piston continuing to suck and forcemore water, a part rushes into the branch and part remainsin the recipient, which increases the elasticity of the air, asit is compressed to a less spaee ; the hole G, by which thewater enters, being greater than I, by which it escapes, thepiston always forces more than can at the same time ascendthe branch : as the valve H closes every time the pistondescends, when the air in the recipient acquires an elasticitysuperior to the weight of a column of water having forits base the circle of the recipient, and for its height thatof the ascending branch, the air would exert a pressureon the surface of the water, and oblige it to descend ;

Fig. 1781. CONTINUOUS SlilEAM.

and the diameter of the recipient being much greater than that of the ascending branch, ifthe water descended only a few inches, it would furnish enough to pass into the reservoirduring the time of suction. It would in this manner ascend without interruption, sincethe piston, at each stroke, drives twice as muchwater as can in the same time escape through thehole I.

In order that the air may always have theproper elasticity, and not acquire more than ne-cessary, it is convenient to have a small tube intlie recipient, fitted with a valve, which, beingloaded with a weight proportioned to the elas-ticity of the air, maintains the equilibrium.

Hedderwick's Double-Piston Pump differs fromthose which have rods attached to one side of thepistons, and which produces an unequal pullupon them : this inconvenience is remedied here,by attaching the piston-rod to the centre of thepiston, the lower passing through the centre ofthe upper.

When the long handle of this pump is de-pressed, and the other raised, the two pistonsfollow the same motion; which being reciprocal,gives a double advantage in lifting the water;for, when the one handle is pulled down, thepiston attached to that handle is raised ; andwhen the same handle is raised up, the otherpiston is also raised, which belongs to theopposite handle: such a contrivance wouldraise twice as much water in the same time astwo others with pump-barrels of equal borewith one piston each. It has been proved thata pump with two pistons will actually raise Fig. 178*2. iilulekwick’s double-piston pump.twice and a half as much water as another pump

with one piston only. It must be recollected also, that in the pump with a singlepiston the water, after being put in motion, is suddenly stopped, (.luring the descent of the

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