ON HYDRAULIC MACHINES,

331

escape of the water when the machine is at rest. As soon as the rotationbecomes sufficiently rapid, the centrifugal force of the water in the horizon-tal pipe causes it to be discharged at the end, its place being supplied by-means of the pressure of the atmosphere on the reservoir below, which forcesthe water to ascend through the vertical pipe. It has also been proposed toturn a machine of this kind by the counterpressure of another portion-ofwater, in the manner of Parent’s mill, where there is fall enough to carry itoff. This machine may be so arranged that, according to theory, little of theforce applied may be lost; but it has failed of producing in practice a veryadvantageous effect. (Plate XXIII. Fig. 304.)

A pump is a machine so well known, and so generally used, that the deno-mination has not uncommonly been extended to hydraulic machines of allkinds; but the term, in its strictest sense, is to be understood of those ma-chines, in which the water is raised by the motion of one solid within another,and this motion is usually alternate, but sometimes continued so as to consti-,tute a rotation. In all the pumps most commonly used, a cavity is enlargedand contracted by turns, the water being admitted into it through one valve,and discharged through another.

•

One of the simplest pumps, for raising a large quantity of water to a smallheight, is made by fitting two upright beams or plungers, of equal thicknessthroughout, into cavities nearly of the same size, allowing them only roomto move without friction, and connecting the plungers by a horizontal beammoving on a pivot. The water being admitted, during the ascent of eachplunger, by a large valve in the bottom of the cavity, it is forced, when theplunger descends, to escape through a second valve in the side of the cavity,and to ascend by a wide pipe to the level of the beam. The plungers oughtnot to be in any degree tapered, because of the great force which would be un-necessarily consumed, in continually throwing out the water, with great velocity,as they descend, from the interstice formed by their elevation. This pump maybe worked by a labourer, walking backwards and forwards, either on the beam oron a board suspended below it. Jly means of an apparatus of this kind, describ-ed by Professor liobison,an active man, loaded with a weight of thirty pounds,has been able to raise 580 pounds of water every minute, to a height of 1 14 .feet, for ten hours a day, without fatigue ; this is the greatest effect producedby a labourer that has ever been correctly stated by any author ; it is equi-