424 HYDRAULICS AND ITS APPLICATIONS 



to estimate the minimum supply likely to be available under the worst 

 probable combination of circumstances, and the scheme may then, and 

 not until then, be developed. 



The method of utilizing the supply depends largely on its magnitude, 

 form, and locality. Where, though comparatively small, it is continuous, 

 the available horse-power may be largely increased by the formation of a 

 storage reservoir capable of impounding at least a 24 hours' inflow. By 

 this means energy may be utilized for the eight hours or so comprising a 

 working day at a- rate greatly in excess of the mean rate of inflow. 



Where the natural configuration of the country necessitates the power 

 plant being placed at some distance from the storage reservoir, the supply 

 is usually led through an open canal or ditch having a slight gradient, 

 into a smaller storage reservoir termed the forebay, which is placed as near 

 to the power plant as possible. From the forebay the supply is then taken 

 to the prime mover by means of a closed pipe termed the penstock. 



In the case of a water-wheel installation the penstock may consist of an 

 open channel. 



The supply of water to the prime mover is regulated by means of 

 sluices or gates, which may either form an integral part of the machine, 

 as in the case of most turbines, or may be fitted in the supply pipe or 

 channel. 



After doing work, the water is rejected into a discharge channel termed 

 the tail-race. 



The most suitable type of prime mover for any particular case 

 depends on 



(1) The quantity of water available. 



(2) The supply head. 



(3) The regulritay of flow. 



(4) The possibility of floods. 



(5) The purpose for which power is required. 

 Those types in general use consist of 



(1) W T ater- wheels. 



(2) Turbines. 



(3) Piston engines. 



Each has its own sphere of usefulness, and in determining the type to 

 be adopted each installation demands special consideration, guided by the 

 circumstances peculiar to the case. 



In general, the water-wheel is only suitable for small powers and for 

 comparatively low heads and where close speed regulation is not essential. 

 Its efficiency is greatly affected by a variation in the supply and in the 



