ON HYDRAULIC MACHINES. 331 



escape of the water when the machine is at rest. As soon as the rotation 

 becomes 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 forces 

 the water to ascend through the vertical pipe. It has also been proposed to 

 turn a machine of this kind by the counterpressure of ^another portion of 

 water, in the manner of Parent's mill, where there is fall enough to carry it 

 off. This machine may be so arranged that, according to theory, little of the 

 force applied may be lost ; but it has failed of producing in practice a very 

 advantageous 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 all 

 kinds ; 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 enlarged 

 and 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 small 

 height, is made by fitting two upright beams or plungers, of equal thickness 

 throughout, into cavities nearly of the same size, allowing them only room 

 to move without friction, and connecting the plungers by a horizontal beam 

 moving on a pivot. The water being admitted, during the ascent of each 

 plunger, by a large valve in the bottom of the cavity, it is forced, wheti the 

 plunger 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 ought 

 not 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 may 

 be worked by a labourer, walking backwards and forwards, either on the beam or 

 on a board suspended below it. By means of an apparatus of this kind, describ- 

 ed by Professor Robison,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 In- 

 fect, for ten hours a day, without fatigue ; this is the greatest effect produced 

 by a labourer that has ever been correctly stated by any author ; it is equi- 



