ON HYDRAULIC MACHINES. 253 



out of repair, I thought it more eligible to substitute for it a common 

 'forcing pump, than to attempt to make any further improvement in it, 

 under circumstances so unfavourable. But if the wheel with its pipes were 

 entirely made of wood, it might in many cases succeed better : or the pipes 

 might be made of tinned copper, or even of earthenware, which might be 

 cheaper and lighter than lead. (Plate XXII. Fig. 303.) 



The centrifugal force, which is an impediment to the operation of 

 Wirtz's machines, has sometimes been employed, together with the pressure 

 of the atmosphere, as an immediate agent in raising water, by means of the 

 rotatory pump. This machine consists of a vertical pipe, caused to revolve 

 round its axis, and connected above with a horizontal pipe, which is open 

 at one or at both ends, the whole being furnished with proper valves to pre- 

 vent the 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 

 horizontal pipe causes it to be discharged at the end, its place being sup- 

 plied 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 de- 

 nomination has not uncommonly been extended to hydraulic machines of 

 all kinds ; but the term, in its strictest sense, is to be understood of those 

 machines 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 constitute 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, when 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 

 unnecessarily 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 ap- 

 paratus of this kind, described by Professor Robison,tan active man, loaded 

 with a weight of thirty pounds, has been able to raise 580 pounds of water 



* West in Tilloch's Ph. Mag. vol.xi. 



f Mechanical Philosophy, art. Pump, ii. 671. 



