plain, that, within this distance from the 

 highest and lowest points, a full bucket 

 occasions a positive loss of power, and 

 even beyond this limit, but near it, very 

 little advantage can be gained. 



There is a certain velocity with which 

 an over-shot wheel should move, in 

 order to produce the greatest effect. 

 This will be evident from considering 

 two extreme cases. If the wheel be 

 so loaded as to render the weight of 

 water insufficient to move it, the velo- 

 city becomes nothing ; and it is evident 

 that the effect becomes nothing. If, on 

 the other hand, the wheel be supposed 

 to turn as rapidly as the water would 

 fall freely, it is evident that, the effect of 

 the weight of the water in the buckets 

 will be nothing, since they descend as 

 fast as the water itself would. Between 

 these limiting cases there is an interme- 

 diate velocity, which will give the best 

 possible effect. 



Mr. Smeaton concludes from expe- 

 rience, that the best general rule for the 

 velocity of the circumference of an over- 

 shot wheel is three feet per second ; 

 and he considers that this equally ap- 

 plies to large and small wheels. In 

 deviating, however, from this rule, he 

 considers that high wheels lose less of 

 their effect, in proportion to their whole 

 power, than smaller ones. 



(48.) In cases where the height of the 

 fall is considerable, and the supply of 

 water very limited, a contrivance of the 

 kind represented in fig. 21 is frequently 

 used. 



An endless chain, carrying a series of 

 buckets C F E D, is made to revolve on 

 two wheels A B, called rug-wheels. 

 The water flows into the highest bucket 

 at X ; and when it descends, the next 

 bucket D takes its place, and is likewise 

 filled, and thus every bucket on the 

 side C is filled, while those on the side 



fig. 22. 



MECHANICS.; ai 



E, being inverted, are empty ; the chain 

 of buckets is therefore constantly car- 

 ds'- 21. 



ried round in the direction C F E D by 

 the weight of the water. 



(49.) The breast-wheel is another 

 means by which the weight of w r ater is; 

 applied as a mechanical agent. This 

 wheel is furnished at its edge or rim with 

 flat boards, c&\\edjloat- boards, the planes 

 of which are at right angles to the plane 

 of the wheel, and in the direction of the 

 radii. The water is delivered at some 

 point near the end of the horizontal dia- 

 meter. The float-boards are fitted to 

 the mill-course, as represented mfig. 22, 

 so as to leave only as much play as is 

 absolutely necessary for the free motion 



