PUMP. A machine used in the arts, and in the ordinary business of life, for the purpose of raising water from a lower to a higher level ; or for the purpose of forcing it through pipes to be distributed as may be required. Pumps are occasionally used in similar manner, in dealing with other aqueous fluids, with trifling modifications in their details, according to the nature of the fluids ; and a peculiar class of pump is employed for the purpose of either removing, or of distri buting air or other gaseous fluids. The general principles of the pumps applicable to the respective kinds of ileitis, arc however so closely allied, that it may suffice here to notice only the more common descriptions of water and of air-pumps, referring occasionally to some of the most important modifications to which they are subjected in order to adapt them to particular circumstances.
Water pumps are divided into two general classes, called 1st, suction pumps, or 2nd, forcing pumps, according to their mode of action. The suction pumps are those in which the atmospheric pressure is made use of to force a column of water to rise in a tube, from which the air has been withdrawn in such a manner an to leave a vacuum in its interior. The forcing pumps are those in which a mechanical effort is applied to the water to raise it to the required level. Evidently the power of the suction pump is limited in this respect ; namely, that the column of water cannot rise beyond the level at which the weight of the column of water balances the weight of the atmosphere ; whereas the power of the forcing pump is only limited by the dynam ical effort employed, and by the strength of the machinery itself. Suction pampa are therefore only used in simple cases, and where the water has not to be raised to any great height ; at times, combined suction and forcing pumps are used, but wherever large volumes of water have to be moved, forcing pumps are the only ones which are habitually employed. The common lifting pumps are, it may be added, simply those modifications of the forcing pumps, which are used in eases where the quantity of water to be raised is not very great, or where the height above the limit of action of the suction pump is not very considerable.
The component parts of all kinds of pumps are, 1°, the cylinder in which works, T, the piston ; 3°, the suction, or feed-pipe ; 4°, the various clacks or valves; and 5', the delivery pipe. In the largo pumping works for town supplies, there are placed upon the delivery pipes either some air vessels, or some stand pipes or water towers, for the purpose of equalising the pressure on the distributing pipes, and of obviating the hydraulic jar produced by the alternate motion of the piston in Its up and down strokes. The motive power used in working the pumps, and the various details of the pipes, pistons, piston-rods, valves, fie, aro no numerous that it will only be possible hero to describe some of the most important modifications of them.
The cylinder is usually circular in its transverse section, although in some cases it is made quadrangular, or even polygonal. For small lifts it may beemade either of wood or of metal ; but the former material wears away too rapidly for mirks destined to be used for any length of time The use of wooden pumps is, therefore, almost exclusively confined to agricultural purposes, or to ships, principally on account of their economy in the first instance ; whilst east-iron or brass, is almost exclusively used for the cylinders of pumps employes] in industrial operations. Of whatever material the cylinder may be made, it is, however, essential that it should be perfectly straight and true in its bore; even when the piston does not work in close contact with it. The power of a pump, of whatsoever description it may be, depends upon the inner diameter of the cylinder. It is considered to be small, when the diameter is less than from 5 to 6 inches ; it is considered to be large NA heu it exceeds 1 foot in diameter ; and it is only in the great mining operations, or in municipal water supplies that the diameter of pump cylinders exceeds 2 feet. The length of the cylinder only exceeds that of the stroke of the piston by a small quantity.