Chap. XVII.

PUMPS.

1177

bulk by forcing in the water ; if an additional quantity bo thrown up, so as to compressthis half to one-third its original bulk, there will be the pressure of three atmospheresinternally to one externally, which will support a column of water 66 feet high.

Fig. 1791. section or fire-engine.

Fig. 1790.

's.

Attached to the water-mains in many towns is a fire-plug of ingenious contrivance, which,when the water is supplied from a high service, answers all the purposes of an engine, andcan be made available at a moment’s notice.

The fire-engine now preferred has twocylinders of 7 inches in diameter, and a strokeof 8 inches : the whole weight of such anengine, when strongly made and renderedefficient for use, is about 17.^ cwt., withouttaking into consideration that of the hoseand tools required, which may make anaddition of 4 or 5 cwt.; this, however, can beeasily drawn by two horses with five firemenand a driver any short distance. There aretwo pumps, both sucking and forcing, thepistons of which are moved up and down bya double lever: when one piston is up, thewater flows from the reservoir into the barrel,and when the same piston is depressed, a valveshuts, and the water of the barrel is forced intoan air-vessel into which is inserted a pipewhich reaches nearly to the bottom of thevessel. The same effect is produced by theother piston, only at a different moment, forwhen one is rising and sucking water fromthe reservoir, the other is forcing into theair-vessel the water which it had raised intothe barrel by its previous ascent: when thewater has risen in the air-vessel to a certainheight, the air becomes more and more com-pressed by every new quantity of waterforced into it, until the elasticity of the air,acting like a spring on the surface of thewater, compels it to mount the hose whichis attached to the engine. The usual rate ofworking such an engine is 40 strokes of eachcylinder per minute, which gives 88 gallons,and it requires 26 men to keep it at work steadily for 3 or 4 hours. The lever is in theproportion of 4J to 1, and with 40 feet of leather hose and a | inch jet, the pressure is30 pounds on the square inch: this gives 10-4 pounds for each man to move 226 feetper minute. The friction must be added, which is equivalent to 2^ per cent, for every ad-ditional 40 feet of hose.

A variety of forms are adopted for the jet, but that generally used has the curve of

Fig. 1792.