178 LECTURE XIX. 



making butter. The purpose of agitation is perhaps more effectually 

 answered by an alternate motion, which has sometimes been produced in a 

 barrel churn, by means of a cord attached to a heavy pendulum. 



Threshing machines are of two kinds ; the one consists of a number of 

 flails, beating the corn nearly in the same manner as they are used by 

 labourers: in the other, which is more commonly employed in this 

 country, the corn is drawn along by two revolving rollers, and caused to 

 pass between a cylinder and its concave cover, while a number of blocks, 

 projecting from the surface of the cylinder, beat or rub out the grains 

 very effectually from the ears ; the corn falls out at the lower part, and is 

 winnowed by a fan which the machine turns at the same time. In this 

 manner it is said that a horse will thresh about 100 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 revolving axis which strike against the ori- 

 fice ; 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 re- 

 duced to powder, it is discharged at the circumference, 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 effectually on the corn. The resistance, in grinding 

 wheat, has been estimated at about a thirty-fifth of the weight of the mill- 

 stone. 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 

 20,160 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, the 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 a 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, 



