CHAPTER XXIX 
WATER WHEELS AND TURBINES 
_ 420. Water Wheels and Turbines are prime movers, which utilise 
the potential and kinetic energy of water. In one class of water wheels 
the wheel acts by the direct weight of the water delivered to it. In a 
second class the wheel acts partly by the weight of the water and partly 
by the impulse due to the weight and velocity of the water striking the 
wheel. In a third class the action is entirely by impulse. In a fourth 
class the action is entirely due to the reaction of the moving water on 
the wheel. 
In a water wheel there are usually a considerable number of buckets 
or vanes placed round the periphery, and the water is delivered to the 
wheel on a part of its circumference, filling or striking one or a few 
buckets only at one time. 
In a turbine the revolving wheel has numerous buckets or vanes, 
which are all supplied with water simultaneously. Turbines have 
almost entirely superseded the slow-moving and cumbrous vertical water 
wheels. Turbines occupy less space, and are cheaper to construct than 
the older vertical wheels of the same power; they are also highly 
_ efficient, and suitable for large or small falls. 
421. Overshot Wheels.—An overshot water wheel is shown in Fig. 
‘781. The water is led to the wheel by a head race, and the quantity 
entering the buckets is regulated by a 
sluice A, which is operated by hand, or -gay-pae 
controlled by a governor driven by the S=s4-32h., 
wheel. The water enters the buckets at 
or near the top of the wheel} and acts 
almost entirely by its weight, descending 
in the buckets on about one half of the z 
wheel. The buckets empty themselves, ' = 
_ when near their lowest position, into the h © 
| 
tail race. A small part of the effort on i 
the wheel is due to the impulse of the 
_ water as it enters the buckets. 
| If h is the total fall in feet, and Q the 
_ number of cubic feet of water delivered i 
_ to the wheel per second, and w the weight 
_ of 1 cubic foot of water, then the avail- 
able horse-power is ~ : Fia. 781. 
To utilise as much as possible of the available power an overshot 
wheel must have a diameter nearly equal to the fall 2, but to obtain 
485 
* 
