IV. 



OF WATER-MILLS. 8? 



will require about 32 hogsheads every minute to turn LECI 

 the wheel with a third part of the velocity with which , 

 the water falls ; and to overcome the resistance arising 

 from the friction of the geers and attrition of the stones 

 in grinding the corn. 



The greater fall the water has, the less quantity of it 

 will serve to turn the mill. The water is kept up in the 

 mill-dam., and let out by a sluice called the penstock, 

 when the mill is to go. When the penstock is drawn 

 up by means of a lever, it opens a passage through 

 which the water flows to the wheel : and wheit the mill 

 is to be stopped, the penstock is let down, which stops 

 the water from falling upon the wheel. 



A less quantity of water will turn an overshot-mill, 

 (where the wheel has buckets instead of float-boards) 

 than a breast-mill, where the fall of the water seldom 

 exceeds half the height A b of the wheel. So that, 

 where 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 take place. Where the water 

 runs only upon a little declivity, it can act but slowly 

 upon the under part of the wheel at b; in which case, 

 the motion of the wheel will be very slow : and there- 

 fore 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 velocity 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 millstone a sufficient degree of velocity. 



They who have read what is said in the first Lecture, 

 concerning the acceleration of bodies falling freely by 

 the power of gravity acting constantly and uniformly 

 upon them, may perhaps ask, Why should the motion 

 of the wheel be equable, and not accelerated, since tne 



