284 WORK OF BREAST WHEELS. 



V = $v, i. e., the wheel works to the best advantage 

 when the velocity of its periphery is one-half that of 

 the stream. 



Sen. If the velocity of the periphery of this wheel is 

 too great, water is thrown out of the buckets before reach- 

 ing the bottom of the fall. In practice, the circumferential 

 velocity of water wheels of this kind is from 4| to 10 feet 

 per second, about 6 feet being the usual velocity of good 

 iron wheels not of very small size. The velocity of the 

 water therefore is limited to about 12 feet per second, and 

 the part of the fall operating by impulse is therefore about 

 2^ feet. The rest of the fall operates by gravitation, but a 

 certain fraction is wasted by spilling from the buckets, and 

 emptying them before reaching the bottom of the fall. The 

 great diameter of wheel required for very high falls is in- 

 convenient, but there are examples of wheels 60 feet in 

 diameter and more. 



The efficiency of these wheels under favorable circum- 

 stances is 0.75, and is generally about 0.65. 



155. Work of Breast Wheels. When the variation 

 of the head-water level exceeds 2 feet, a breast wheel is 

 better than an overshot. In 

 breast wheels the buckets are 

 replaced by vanes which move in 

 a channel of masonry partially 

 surrounding the wheel. The 

 water falls over the top of a slid- 

 ing sluice in the upper part of 

 the channel. The channel is 

 thus filled with water, the weight 

 of which rests on the vanes and 

 furnishes the motive force on the wheel. There is a certain 

 amount of leakage between the vanes and the sides of the 

 channel, but this loss is not so great as that by spilling from 

 the buckets of the overshot wheel, 



