With reference to the efficiency of these pumps, it is impossible to give any accurate estimate, since as high as 70 per cent of the applied power is claimed to be returned by forms of the pump shown in figs. 7 and 8, while some other descriptions experimented on in 1851 gave only 18 per cent of useful effect.
It will be evident, from the above description of the pump, that the height to which the water will be raised depends entirely upon the speed of revolution of the wheel; and it is by this that the application of centrifugal pumps is limited to comparatively low lifts of say less than 20 ft., as•the speed for high lifts requires to be greater than can be conveniently and usefully attained in practice. They are best applied when raising large quantities of water through low lifts. It will also be observed, that on account of the simplicity of their parts, and the absence of valves, they are much less liable than other pumps to be choked by the entrance of solid materials. In some descriptions of this pump, the oxterior whirlpool chamber is dispensed with; and to the vanes of the wheel is given such a curvature backward from the direction of motion, that the water leaving the circumference of the wheel is spouted backward from the vane-passages with a speed equal to that of the wheel in the opposite direction, so that it has only a radial motion with reference to a fixed object ; in other words, that the force is acquired from the radial component of the pressure of the vanes, instead of the cen trifugal force of the revolving water. Those pumps, how ever, give the best results which, as the one above described, combine both actions. In all cases, curved vanes are much superior to straight ones. • 5. The Jet-pump.—This pump is worked by water-power. and is worthy of notice on account of the extreme simplicity of its parts, and of not requiring the care of an attendant while in operation.
Fig. 3 is a representation of this pump, 0 is the water which it is required to raise to the level of the water D, and B is the water in the stream available for working the pump.
The water B passes down the pipe A, and is discharged from the jet or nozzle, E, into the conical pipe F. Bound the nozzle is the vacuum-chamber G, at the bottom of which is attached the conical pipe F, and into the side of which the suction-pipe H enters from the water to be pumped. The water, in passing from the nozzle into the conical pipe, car ries air with it, and so gradually forms a vacuum in the chamber G, when the water rises into it from the level C, through the pipe H; and it is in turn carried with the jet down the conical pipe into the D. The velocity of the water coming from the jet is gradually retarded by the action of the conical pipe, the speed decreas ing as the area of suction increases; and the vis viva of its motion is by this retardation converted into a sucking force, drawing the water from the suction-pipe through the vacuum chamber into the conical pipe. The water issuing from the jet will have a speed equal to that produced by a column of the height BC, or the sum of the fall and lift. This pump may be viewed, for purposes of explanation, as a syphon, into the shorter leg of which a jet of water is injected, which overcomes the pressure due to the difference of levels, and reverses the ordinary motion of the water in a syphon. An efficiency of 18 per cent has been obtained from this pump, which is low, as compared with that obtained from other descriptions of pump; yet in cases where waste of water power is not so much to be -avoided as expense in erecting. working, and maintenance, these pumps possess decided advantages. The case to which they are peculiarly applicable is the drainage of marshes, which have streams of water adjacent to them descending from a higher level.