24

WATERWHEELS.

Wheel.” In this case the supply is regulated by an adjustablesluice, over which the water ilows to vanes on the wheel, whiehare substituted for buckets. A channel of brickwork or stonesurrounds the wheel, and in this the vanes move. The water,filling this channel, acts by its weight in turning the wheel.The efficiency of an overshot or breast wheel sometimes reaches75 per cent., but a lower efficiency of about 65 per Cent, isa safer estimate, unless the wheel is very well designed andconstructed.

In the ordinary overshot wheel, the open buckets of wood oriron are shaped so as to prevent the water shooting over thewheel. This is also obviated by making the capacity of thebücket sufficient to hold about three times the volume ofwater discharging into it. The buckets are placed betweentwo shrouds, and the power exerted by the wheel is measuredby the volume of water, and the height through which it falls.If H is the net fall in feet, Q the weight of water per secondin pounds, then the gross horse-power exerted by a wheel willbe—

Q.H.

33,000

Besides the power due to head, there is the additional powerdue to velocity,

\2g/

when the water is delivered to the wheel with a previouslyacquired velocity.

If V be the velocity which the water has acquired before it isdischarged on the wheel, v the diminished velocity after it hasbeen acting on the wheel, and Q the weight in pounds of waterwhich has been discharged per second, then the theoreticalpower exerted on the wheel will be—

Q(V-v)*

2y