HYDEAULIC PRIME MOVERS 428 



excessive loss in gearing, should be capable of close speed regulation and 

 of taking advantage of higher falls and of large quantities of water. For 

 such purposes the water-wheel was. almost entirely superseded by one or 

 other type of turbine. 



The introduction of electric driving with its large and almost instan- 

 taneous changes of load, while giving an additional fillip to the manufacture 

 of high-speed turbines, had its greatest effect in modifying and perfecting 

 the methods of speed regulation, and in increasing the size of the unit, 

 while at the same time rendering it imperative to design a motor which 

 should be highly efficient under a wide range of loads. 



The success which has attended the attempt to satisfy these onerous 

 conditions may be inferred when it is remembered that many manufacturers 

 will now guarantee to construct a turbine which shall give an efficiency of 

 over 80 per cent, over a range of loads of 50 per cent., and which shall 

 respond to an increased demand for power of 33 per cent, with less than 

 3'5 per cent, variation in speed. With smaller load variations the speed 

 variation is almost infinitesimal, and it becomes easy to run a series 

 of alternating current machines in step with such motors. 



The design of hydraulic motors has thus proceeded by well-defined 

 stages, the size and efficiency steadily increasing until at the present time 

 a single unit developing 15,000 H.P. and giving an efficiency of 85 per 

 cent, is not at all uncommon, while further development promises to 

 proceed in the direction of still larger units. So far, indeed, as mechanical 

 difficulties affect the question, there appears to be no reason why units 

 developing up to at least 25,000 H.P. should not be constructed directly 

 the demand arises. 



Wherever a continuous supply of water at a sufficient elevation, or in 

 motion as in a stream, is available, the potential or kinetic energy which 

 this possesses may be turned into useful work. 



Before embarking on any power scheme for utilizing such energy, it is 

 however of the highest importance that the true possibilities of the scheme 

 should be ascertained, for as the usefulness of the supply depends in most 

 cases on its uniformity over long periods of time, the maximum available 

 power is strictly regulated by the least power which is available after the 

 longest probable period of drought. 



This minimum supply can only be satisfactorily ascertained by investi- 

 gation of past records extending over many years. Where such records 

 are not available, every attempt should be made by a close investigation 

 of the rainfall records for the particular districts over a long period of 

 years, and of the character, condition, and area of the gathering ground, 



