ON MODES OF CHANGING THE FORMS OF BODIES.

23S

bushels of com in a day. It is commonly reckoned the work of a labourer tothresh about six bushels in a day. (Plate XVIII. Pig. 237.)

Some kinds of grain are occasionally ground in mills of iron or steel, whichc °nsist of a solid cylinder or cone turning within a hollow one, both theEffaces being cut obliquely into teeth. But the common mill for grindingc °rn is composed of two circular stones of silicious grit, placed horizontally;the upper one revolves with considerable velocity, and is supported by an axisPassing through the lower one, at a distance variable at pleasure: When the(b^uieter is live feet, the stone usually makes about 90 revolutions in a minute;

the velocity were greater, the llour would be too much heated. The corn* s shaken out of a funnel, or hopper, by means of projections from the revolv-es axis, which strike against the orifice; it passes through the middle of the('Pper millstone, and is readily admitted between the stones; the lower stonels slightly convex, and the upper one somewhat more concave, so that thec ° 1- n passes over more than half the radius of the stone before it begins to bejpound: after being reduced to powder, it is discharged at the cireum-le nce, its escape being favoured by the convexity of the lower stone, as wellby the centrifugal force. The surface of the stones is cut into grooves,111 order to make them act more readily and effectually on the corn. The re-Sls tance, in grinding wheat, has been estimated at about a thirty fifth of theVe '8'bt of the millstone. The stones have sometimes been placed vertically,* '■be axis supported on friction wheels : but the common position appearsbe more eligible for mills on a large scale. It is said that a man and ath^ Can by a baud mill a bushel of wheat in an hour; in a watermill,

0 grinding and dressing of a bushel of wheat is equivalent to the effect of- Uj ^ P oull ds of water falling through a height of 10 feet, which isI ° ut as much as the work of a labourer for a little more than half an hour.

^ . a windmill, when, the velocity is increased by the irregular action of theig a-' ! *be corn is sometimes forced rapidly through the mill, without being0 j.- lc Wntiy ground. There is an elegant method of preventing this, by meansau ^ le centrifugal force of two balls, which fly out as soon as the velocity isr 'se entei ^’ an das they rise in the arc of a circle, allow tire end of a lever to^ill^^b them, while the opposite end of the lever, descends with the upperi s st '°ne, and brings it a little nearer to the lower one. The bran or busky drate d from the flour, by sifting it in the bolting mill, which consists of a.

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