74 OF MILLS. 



A less quantity of water will turn an overshot- 

 mill (in which the wheel has buckets instead of float- 

 boards) than a breast-mill, where the fall of water 

 seldom exceeds half the height of the wheel j so 

 that, when there is but a small quantity of water, 

 and a fall great enough for the wheel to lie under it, 

 the bucket, or overshot- wheel, is always used: but 

 where there is a large body of water with a little 

 fall, the breast or float-board wheel must be used. 

 Where the water runs only upon a small declivity, 

 it can act but slowly upon the under part of the 

 wheel; in which case, the motion of the wheel will 

 be slow; and therefore the floats ought to be very 

 long, though not high, that a large body of water 

 may act upon them ; so that what is wanting in ve- 

 locity may be made up in power; and then the 

 cog-wheel may have a greater number of cogs, 

 in proportion to the rounds in the trundle, in 

 order to give the mill-stone a sufficient degree of 

 velocity. 



It was the opinion of Smeaton, that the powers 

 necessary to produce the same effect on an under- 

 shot-wheel, a breast-wheel, and an overshot-wheel, 

 must be to each other as the numbers 2.4, 1.7-5, 

 and 1. 



Practical Rales for the Construction of Mills. 



1. Measure the perpendicular height of the fall 

 of water, in feet, above that part of the wheel on 

 which the water begins to act, and call that the 

 height of the fall. 



2. Multiply this constant number 64.2882 by 

 the height of the fall in feet, and the square root 

 of the product will be the velocity of the water at 



