[ "I ] 



of light float be allowed to pass along this distance of 20 yards. 

 This gives the velocity of the water at the middle of the 

 channel. At the sides and at the bottom, the velocity is less. 

 If the bottom and the sides of the channel are made of bricks, 

 17 per cent, of the velocity ascertained in the above manner, 

 should be taken off; if the sides and bottom are of earth 

 29 per cent, should be taken off, and if they are stony, 

 irregular and rough, 36 per cent, should be taken off, in 

 estimating the average velocity of a stream. Then by multi- 

 plying the section of the stream utilised by the reduced 

 velocity one gets the quantity of water expressed in the terms 

 of so many cubic feet per minute. 



157. Efficiency. The effective horse-power of the princi- 

 pal forms of water-motors are : 



Of the theoretical horse-power. 

 For ordinary Undershot wheels ... ... 35 percent. 



For ordinary Breast-wheels ... ... 55 



For ordinary Overshot wheels ... ... 68 



For Turbines ... ... 70 ,, 



For new fashioned wheels and turbines ... 75 to 80 per cent. 



The effective power varies according to the ingenuity 

 of construction and erection, which minimises friction. 

 Properly constructed a brea:>t and overshot wheel and a 

 turbine may have a 75 per cent, efficiency. On the 75 per cent. 

 basis, it has been ascertained that the height of the fall in 

 feet multiplied by the number of cubic feet of water per 

 minute, and divided by 706, gives the actual horse power. So, 



x 706 __ No. of cubic ft. of water required 



_ 

 Height of the tall in feet 



horse-power x 706 



quantity of water in cubic feet per minute " 

 height of fall in feet required to produce the horsepower. 

 The undershot wheel though less efficient is more suitable 

 for level countries like Lower Bengal, If the floats are 

 P 



