84 FERGUSON'S LECTURES. 



a little lower upon one side of the spindle than on the 

 other ; and this will cause one edge of the upper stone 

 to drag all around upon the other, whilst the opposite 

 edge will not touch. But this is easily set to rights, by 

 raising the stone a little with a lever, and putting bits 

 of paper, cards, or thin chips, between the rynd and the 

 stone. 



The diameter of the upper stone is generally about 

 six. feet, the lower stone about an inch more : and the 

 upper stone, when new, contains about 22j cubic feet, 

 which weighs somewhat more than 1900 pounds. A 

 stone of this diameter ought never to go more than 60 

 times round in a minute ; for if it turns faster, it will 

 heat the meal. 



The grinding surface of the under stone is a little 

 convex from the edge to the center, and that of the upper 

 stone a little more concave : so that they are farthest 

 from one another in the middle, and come gradually 

 nearer towards the edges. By this means, the corn at 

 its first entrance between the stones is only bruised ; but 

 as it goes farther on towards the circumference or 

 edge, it is cut smaller and smaller ; and at last finely 

 ground just before it comes out from between them. 



The water-wheel must not be too large, for if it be, 

 its motion will be two slow ; nor too little, for then it 

 will want power. And for a mill to be in perfection, 

 the floats of the wheel ought to move with a third part 

 of the velocity of the water, and the stone to run round 

 once in a second of time. 



In order to construct a mill in this perfect manner, 

 observe the following rules : 



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 



