272

Plimger Pump,

[Book III.

closed, and water will ascend through B A and enter the cylinder at A •then if the piston be raised it will force all the water above it through thevalve at C, the only passage for it, while at the same time a fresh portionwill enter the cylinder throngh the valve at B. Thus at every stroke ofthe piston, whether up or down, the Contents of the cylinder are forcedout at one end, and it is replenished at the same time through the other •this pump therefore discharges double the quantity of water that an ordi-nary one of the same dimensions does. The piston rod may be insertedthrough either end of the cylinder, as circumstances may require. Thesepumps are frequently used in a horizontal position.

Another Variation of the forcing pump consists in making the piston ofthe same length as the cylinder but rather less in diameter, so that it maybe moved freely in the former without touching the sides. These pistonsare made wholly of metal and turned smooth and cylindrical, so as toWork through a stuffing box or cupped leathers. The quantity of waterraised at each stroke has therefore no reference to the capacity of thecylinder, however large that part of one of these pumps may be, for theliquid displaced byt the piston can only be equal to that part of the latterthat enters the cylinder. Switzer has given a figure and description of anold engine composed of three of these pumps “ that has been some yearserected in the county of Surrey.” Newton has figured the piston bellowsdescribed by Yitruvius as furnishing wind to hydraulic Organs in a similarway. ln Commandine’s translation of Heron’s Spiritalia, page 159, thesame kind of plunger is figured in a pump belonging to a water organ;and at p. 71, a fire-engine, with two working cylinders, has pistons of thesame kind. These pistons were formerly named plungers, and the pumpsplunger-pumps. Their construction and action will be understood by thefigure, which represents one of a nurnber that were em-ployed in the water-works, York Buildings, London , inthe last Century. The piston was of brass, cast hollowand filled with lead, the outside being “ turned trueand smooth.” A short rod attached to the upper endof the piston was connected by a chain to the archedend of a vibrating beam, that Was moved by one ofNewcomen’s engines. The piston was therefore mere-ly raised by the engine, while its own weight carriedit down: to render it sufficiently heavy for this pur-pose, a number of leaden disks (or cheeses, as theywere named from their form) having holes in theircentres, were slipped over the rod and rested upon thepiston, as in the figure. These were increased untilthey were found sufficient to press down the pistonand force the water up the ascending pipe. The cup-ped leathers through which the piston worked, weresimilar to those now used in the hydrostatic press. Asmall cistern was sometimes formed on the top of thepump, that the water it contained might prevent airfrom entering through the stüffing box or between thecupped leathers : it served also tö Charge the pumpthrough a small pipe or cock. A valve opening up-wards was sometimes placed just above the plug^ of„ _ the cock, and the latter left open when the machine

was started, that the air within the cylmder mignr.escape ; and as soon as the water rose and filled the pump, the cock wasshut. It is immaterial at what part of the cylinder the forcing or ascend-