Chap. XVII.

PUMPS.

1171

to pump, the water will rise to the barrel and ascend inthe pipe G.

The pumps of the third kind have sometimes two pistons,one of which sucks while the other forces the water; thoseof Notre Dame at Paris were of this description (fig. 1776.);they consisted of a frame, C D, attached to the rods Mand N, which when raised, the water from the river passedinto the suction pipe, E F, by the pressure of the externalair, and raising ’the valve Y mounted in the barrel A B,which the piston I left void ; when the frame descended,the valve X opened, the other Y closed, and all the waterfrom the pump-barrel, passing through the piston, dischargeditself into the cistern H G- On the other hand, the pistonO descending left a vacuum in the pump barrel P Q; theair which pressed on the surface H \V of the water in thecistern raised the valve T, and the barrel was filled : onthe piston ascending, thevalve T closed, forced thewater to open the other,

V, and passed into thepipe R S, which closed assoon as the piston de-scended. Thus the cis-ternalwaysremained full,the piston I sucking asmuch as theother forced;the diameter of the lowerbarrel should be rathermore than that of theupper, in order thatthere may always bemore water in the cisternthan can be raised, inorder to supply the losssustained.

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Fig. 177. r ).

PUMP USED AT MARLY.

Fig. 1776. PUMPS AT NOTREDAME BRIDGE.

Fig. 1775. is another pump of the same kind belonging to the Machine of Marly,has a communication pipe, H LM K I F D C E G, in one piece, one end of which, G H,joined to the suction pipe N O, which dips in thewater, the other, LMK, bent at a right angle, uniteswith the branch KSM, which conducts the water tothe first reservoir. In the middle is a branch E C D Fjoined to the pump-barrel ABCD, in which the pistonQ. works, perfectly cylindrical and solid, traversed bya rod YV, suspended as will be hereafter described.

When the piston ascends to I, the air in the part PXdilates in the space Y Z; that in the suction pipe N Oopens the valve P, mixes with the preceding, and thevalve R remains closed by the atmospheric weight:but when the piston is lowered, the valve P closes,the other R opens, and the air is forced in the pipe S:when, after a certain number of strokes, the water ar-rives in the barrel, it is forced into the ascendingbranch S.

Fig. 1777. B is asuction and forcing-pump once usedat York Buildings, on the Thames. The suction pipeA B is united to the barrel C E F D, having a valve Mat the junction ; the ascending branch FGLK has alsoa valve N to close the aperture I H of the bent partG I. The piston O P Q is a hollow cylinder of copperfilled with lead to give it sufficient weight to forcethe water into the branch ; and as the height of thewater might be so great that the weight of the pistonwould not effect this, it is also loaded with plates oflead, T, placed on the rod V; the head of the piston,therefore, which does not enter the pump-barrel, hasa square form, sufficiently large to serve as a base tothe weight T. Fig. 1777. pump used at york buildings.

Fig. 1778

4 f 2