HYDRAULIC PRIME MOVERS 425 



head or tail race levels. It is of great size and weight in proportion to 

 the power developed, and has a low rotative speed. On the other hand, its 

 construction is simple, its repair inexpensive and easy, and the construction 

 of the supply channel, tail-race, and housing in general inexpensive, while 

 for heads of less than 1 foot it forms the only suitable type of motor. 



For all heads above 1 foot, where large power is desired, one or other 

 type of turbine becomes suitable, while in certain cases for comparatively 

 small powers, and where a high head is available and a slow rate of 

 rotation is required, the piston engine is most satisfactory. 



In passing through a prime mover, water may do work either by 

 changing potential energy or kinetic energy or pressure energy into 

 work ; or by a combination of these processes. 



In the overshot water-wheel, for example (Art. 120), rotation is pro- 

 duced almost entirely by the weight of the water ; in impulse wheels 

 deriving their motive force from the impact of a high velocity jet of 

 water, work is done solely in virtue of the kinetic energy of the jet ; in 

 turbines of the reaction type the pressure energy of the water is partly 

 changed into kinetic energy in the wheel itself, this being absorbed in 

 producing rotation of the wheel ; while in a piston engine the water does 

 work in virtue of its pressure, its velocity being so small as to be negligible. 



In designing any type of hydraulic prime mover, certain general 

 principles should be borne in mind. 



(a) All shock, whether of water on moving or stationary surfaces, or on 

 water moving with a low velocity, should be avoided as being productive 

 of loss of energy in eddy formation. 



This may be prevented by arranging that as far as possible any stream 

 of water on meeting a solid surface is moving tangentially to the surface, 

 and that passages conveying the working fluid are not subject to abrupt 

 changes of sectional area or of form. 



(b) Abrupt changes in the direction of motion are productive of eddy 

 formation and should be avoided by designing passages and channels with 

 as far as possible an uniform or gradually changing curvature. 



(c) Frictional losses should be reduced to a minimum by reducing the 

 area of the wetted surfaces to a minimum compatible with easy curves 

 of flow, and by reducing the relative velocity of flow over such surfaces 

 to a minimum. 



(d) As far as possible, the motive fluid should be rejected devoid of 

 energy, and therefore moving with as low an absolute velocity as will 

 suffice to carry it out of the motor. 



The possibility of conforming to these general principles varies with the 



