HYDRAULIC MACHINERY, FOR MINES 849 



than the inner circumference ; describe from the centre another circle, ef, cutting the 

 line a b in b ; this point b will be the centre from which the curve of the bucket should 

 bo drawn. 



Breast Wheel. The water acts on the periphery of this wheel both by its weight 

 nnd impulse, one foot of fall affording as much power as two feet of head. The cir- 

 cumferential velocity is commonly about 4 feet per second, and the speed of the water 

 entering the buckets 6 feet per second. 



The slower the wheel revolves, and the higher it receives the water, the more the 

 buckets may be sloped, so as to retain the water to the proper point of discharge. 



For the purpose of expelling the air from the buckets, the stream should be of less 

 width than that of the buckets, or otherwise openings should be formed in the backing 

 5 of an inch wide, or the bucket constructed as shown in fig. 1177. A good example 

 of the form of iron buckets employed in the breast wheel, as well as the arrangement 

 for supplying and taking off the water, is shown in fig. 1178: a, shrouding; b, the 

 buckets ; c, stays between the buckets ; d, sole plate or backing ; e, breast work ; /, 

 cistern or mill-race ; g, sluice-race ; k, sluice-cover ; Jc, tail-race. The distinguishing 

 feature of- the arrangement shown consists in the wheel working against a circular 

 breast of masonry, fitted close to the outer circumference of the shrouding and ends of 

 the buckets. The cistern leading into the wheel is likewise fitted close to its peri- 

 phery. The loaded portion of the wheel is therefore enclosed, and hence, in forming 

 the buckets, it is not absolutely necessary to contract the mouth for the purpose of 

 retaining the water. 



Overshot Wheel. The overshot wheel receives its water a little distance to the right 

 or left of the vertical centre line. Smeaton's experiments showed that the best effect 

 was obtained when the circumferential speed was limited to 3^ feet per second, or 210 

 feet per minute. "With, however, the increase of diameter in wheels, it is found that 

 the speed may be advantageously augmented. A wheel 20 feet in diameter may 

 move six feet per second, and where economy in the propelling power is not a special 

 object, the circumference of the wheel may run at 8 feet per second. This greater 

 speed will be found in some cases desirable ; the wheel may be lighter and cheaper 

 than a slow-running wheel, and less gear will be required to bring the speed up to the 



1179 





required rate for driving machinery. In practice it is found that a fall of 7 inches 

 will bring water upon a wheel full 3 feet per second, and a fall of 19 inches will give 

 a velocity to the stream of quite 6 feet per second. In reference to wheels of this 

 class it may be observed : (1) when the volume of water is large and variable, and the 

 fall of an intermediate height, it is found advantageous not to lay the water upon the 

 top of the wheel, so that it may work overshot, but to make the diameter of the wheel 

 greater than the mean height of the fall, and to lay on the water at about 45 from 

 VOL. II. 3 I 



