430 



HYDKAULICS AND ITS APPLICATIONS 



the supply by means of a closed guide passage of suitable depth out of the 

 bottom of an open forebay (Fig. 188). 



The velocity of efflux is then increased, and the velocity of rotation may 



FIG. 187. 



be increased proportionately. The diameter of the wheel is reduced, and 

 also its width for a given volume of water. A cheaper wheel, and one 



rotating at a more conve- 

 nient speed for transmission 

 purposes is thus obtained. 

 The efficiency is, however, 

 slightly reduced, since losses 

 due to shock and friction 

 are increased by the higher 

 velocities, as are the losses 

 due to rejection of kinetic 

 energy in the tail water, 

 while in addition, the in- 

 creased centrifugal action 

 tends to empty the buckets 

 sooner. The latter action, 

 however, while reducing the efficiency of the wheel, tends to make it, to a 

 certain extent self-governing under variable loads, since a diminution in 

 load, followed by an increase in speed, tends to empty the buckets. 



Kit;. 188. 



