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pestle to be lifted once at every revolution of the fly; we must then consider what weight, passing through 22 feet in a second, will be equivalent to 30 pounds moving through one foot in a second. This will be 30 divided by 22, or 1 pounds. Were a fly of this kind to be applied, therefore, and the machine set a-going, the fly would just be able to lift the pestle once, after the moving power was withdrawn: but by increasing the weight of the fly to 10, 12, or 20 pounds, the machine, when left to itself, would make a considerable number of strokes, and be worked with much less labour than if no fly had been used, though, no doubt, at the first, it would be found a considerable incumbrance to the motion.

This is equally applicable to the action of pumps; but the weight which can be most advantageously given to a fly, has never yet been determined by mechanics. It is certain, however, that the fly does not communicate any absolute increase of power to the machine; for if a man, or other animal, is not able to set any engine in motion without a fly, he will not be able to do it though a fly be applied, nor will be able to keep it in motion, though set a-going with a fly, by means of a greater

power.

This may seem to be contradicted by the example of a common clock; for if the pendulum be once stopped, the weight is not able to set it in motion again, though it will keep it going when once put in motion by an external power. This, however, depends not upon the insufficiency of the weight, but on the particular mechanism of the pallet-wheel, which is such, that when once the pendulum is stopped, it would require a much greater weight than that commonly applied to set

it in motion; and if the usual weight were to act fairly, it would be more than sufficient to move all the machinery, and make the pendulum vibrate also with much greater force than it does.

OF MILLS.

A mill, in the strict sense of the word, signifies a machine for grinding corn, though the term millwork is frequently applied to all kinds of machinery where large wheels are used.

Mills are distinguished into various kinds, either according to the powers by which they are moved, or the uses to which they are applied. Thus, there are water-mills, horse-mills, and windmills; corn-mills, fulling-mills, powder-mills, boringmills, &c.

The limits of this work not permitting us to enter into the detail of all these different sorts of mills, we shall confine ourselves to that most useful machine, the corn-mill.

In ancient times, corn was ground only by handmills consisting of two stones similar to those used in water-mills, but much smaller, the upper one having a piece of wood fixed into it to move it by. They are still used in some parts of Scotland, and are called querns.

These, however, have given place to water-mills and wind-mills, which are now commonly used. Water-mills are of three kinds :-breast-mills, undershot-mills, and overshot-mills, according to the manner in which the water is applied to the great wheel. In the first, the water falls down upon the wheel at right angles to the float-boards, or bucket, placed all round the wheel to receive it. In the second, which is used where there is no fall of

water, the stream strikes the float-boards at the lower part of the wheel. In the third, the water is poured over the top, and is received in buckets formed all round the wheel.

The following is a description of a corn-mill of the most common sort:

AB (Plate 3. fig. 12.) is the water wheel, which is generally from 18 to 24 feet in diameter, reckoned from the outermost edge of any float-board at A, to that of the opposite one at B. The water striking on the floats of this wheel, drives it round, and gives motion to the mill. The wheel is fixed upon a very strong axis, or shaft C, one end of which rests on D, and the other on E, within the mill-house.

On this shaft, or axis, and within the mill-house, is a wheel F, about eight or nine feet in diameter, having cogs all round, which work in the upright staves, or rounds, of a trundle G. This trundle is fixed upon a strong iron axis, called the spindle, the lower end of which turns in a brass foot fixed at H, in a horizontal beam H, called the bridgetree; and the upper end of the spindle turns in a wooden bush fixed into the nether mill-stone, which lies upon beams in the floor I. The top of the spindle above the bush is square, and goes into a square hole in a strong iron cross, a b c d (Fig. 13.), called the rynd; under which, and close to the bush, is a round piece of thick leather upon the spindle, which it turns round at the same time as it does the rynd.

The rynd is let into grooves in the under surface of the running mill-stone K, and so turns it round in the same time that the trundle G is turned round by the cog-wheel F. This mill-stone has a large hole quite through its middle, called

the eye of the stone, through which the middle part of the rynd and upper end of the spindle may be seen; whilst the four ends of the rynd lie below the stone in their grooves.

One end of the bridge-tree which supports the spindle rests upon the wall, whilst the other is let into a beam called the brayer, LM.

The brayer rests in a mortice at L; and the other end M hangs by a strong iron rod N, which goes through the floor I, and has a screw-nut on its top at O; by the turning of which nut, the end M of the brayer is raised or depressed at pleasure, and consequently the bridge-tree and the upper mill-stone. By this means, the upper mill-stone may be set as close to the under one, or raised as high from it, as the miller pleases.

The nearer the mill-stones are to each other, the finer the corn is ground; and the more remote from one another, the coarser.

The upper mill-stone is inclosed in a round box, which does not touch it any where, and is about an inch distant from its edge all round. On the top of this box stands a frame for holding the hopper P, to which is hung the shoe Q, by two lines fastened to the hinder part of it fixed upon hooks in the hopper, and by one end of the string R fastened to the fore part of it; the other end being twisted round the pin S. As the pin is

turned one way, the string draws up the shoe closer to the hopper, and so lessens the aperture between them; and as the pin is turned the other way, it lets down the shoe, and enlarges the aperture.

If the shoe be drawn up quite to the hopper, no corn can fall from the hopper into the mill; if it be let down a little, some will fall; and the quan

tity will be more or less, according as the shoe is more or less let down; for the hopper is open at bottom, and there is a hole in the bottom of the shoe, not directly under the bottom of the hopper, but nearer to the lowest end of the shoe, over the middle eye of the mill-stone.

There is a square hole in the top of the spindle, in which is put the feeder E (Fig. 13.); this feeder, as the spindle turns rounds, jogs the shoes three times in each revolution, and so causes the corn to run constantly down from the hopper through the shoe, into the eye of the mill-stone, where it falls upon the top of the rynd; and is, by the motion of the rynd, and the leather under it thrown below the upper stone, and ground between it and the lower one. The violent motion of the stone creates a centrifugal force in the corn going round with it; by which means it gets farther and farther from the centre, as in a spiral, in every revolution, until it be quite thrown out; and being then ground, it falls through a spout, called the milleye, into a trough placed to receive it.

When the mill is fed too fast, the corn bears up the stone, and is ground too coarse; and, besides, it clogs the mill, so as to make it go too slow. When the mill is too slowly fed, it goes too fast, and the stones, by their attrition, are apt to strike fire. Both which inconveniences are avoided by turning the pin S backward or forward, which draws up or lets down the shoe, and thus regúlates the feeding, as the miller sees convenient.

The heavier the running mill-stone is, and the greater the quantity of water that falls upon the wheel, the faster will the mill bear to be fed; and consequently it will grind the more. And, on the contrary, the lighter the stone, and the less the