ON MODES OF CHANGING THE FORMS OF BODIES. 233 



bushels of corn in a day. It is commonly reckoned the work of a labourer to 

 thresh about six bushels in a day. (Plate XVIII. Fig. 237.) 



Some kinds of grain are occasionally ground in mills of iron or steel, which 

 consist of a solid cylinder or cone turning within a hollow one, both the 

 surfaces being cut obliquely into teeth. But the common mill for grinding 

 corn is composed of two circular stones of silicious grit, placed horizontally ; 

 the upper one revolves with considerable velocity, and is supported by an axis 

 passing through the lower one, at a distance variable at pleasure: When the 

 diameter is five feet, the stone usually makes about 90 revolutions in a minute; 

 if the velocity were greater, the flour would be too much heated. The corn 

 is shaken out of a funnel, or hopper, by means of projections from the revolv- 

 ing axis, which strike against the orifice ; it passes through the middle of the 

 upper millstone, and is readily admitted between the stones; the lower stone 

 is slightly convex, and the upper one somewhat more concave, so that the 

 corn passes over more than half the radius of the stone before it begins to be 

 ground: after being reduced to powder, it is discharged at the circum- 

 ference, its escape being favoured by the convexity of the lower stone, as well 

 as by the centrifugal force. The surface of the stones is cut into grooves, 

 in order to make them act more readily and eft'ectually on the corn. The re- 

 sistance, in grinding wheat, has been estimated at about a thirty fifth of the 

 weight of the millstone. The stones have sometimes been placed vertically, 

 and the axis supported on friction wheels : but the common position appears 

 to be more eligible for mills on a large scale. It is said that a man and a 

 boy can grind by a hand mill a bushel of wheat in an hour; in a watermill, 

 the grinding and dressing of a bushel of wheat is equivalent to the effect of 

 20160 pounds of water falling through a height of 10 feet, which is 

 about as much as the work of a labourer for a little more than half an hour. 

 In a windmill, when the velocity is increased by the irregular action of the 

 wind, tlie corn is sometimes forced rapidly through the mill, without being 

 sufficiently ground. There is an elegant method of preventing this, by means 

 of the centrifugal force of two balls, which fly out as soon as the velocity is 

 augmented, and as they rise in the arc of a circle, allow the end of a lever to 

 rise with them, while the opposite end of the lever descends with the upper 

 millstone, and brings it a little nearer to the lower one. The bran or husk 

 is separated from the flour, by sifting it in the bolting mill, which consists of a 



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