The upper part of the fall at I is rounded off to a segment of a circle, called the crown of the fall, and the water runs over it. The lower edge of the shuttle when put down is made to fit this curve, so as to make a tight joint; and in consequence, when the shuttle is drawn up, the water will run between its lower edge and the crown in a sheet or stream which strikes upon the first float that presents itself, nearly in a direction perpendicular to the plane of the floatboard, or of a tangent to the wheel. The float-boards of the wheel are directed to the centre, but there are other boards placed obliquely which extend from one float-board to the rim of the wheel, and nearly fill the space between one floatboard and the next. These are called rising-boards, and the use of them is to prevent the water flowing over the floatboard into the interior of the wheel; but the edges of these boards are not continued so far as to join to the back of the next float, because that would make all the boards of the wheel close, and prevent the free escape of the air when the water entered into the space between the floats.

As the water strikes with some force, the rising-boards are very necessary to prevent the water from dashing over the float-boards into the interior of the wheel.

This is the form of breast-wheel employed by Mr. Smeaton in the great number of mills which he constructed; but although he speaks of the impulse of the water striking the wheel, he always endeavoured to make the top of the breasting, or crown of the fall, as high as possible; so as to attain the greatest fall and the least of the impulsive action. All rivers and streams of water are subject to variation in height from floods or dry seasons, and in some this is very considerable: it was therefore necessary to make the crown I of the fall, at such a height as that, in the lowest state of the water R, it would run over the crown in a sheet of three or four inches in thickness, and work the wheel. When the water rose higher in the mill-dam, it would then have a pressure to force it through, and in that case would strike the wheel so as to impel it by the velocity.

Mr. Smeaton was well aware that the power communicated by this impulse was very small. In some cases, where the water was very subject to variation, he used a false or movable crown, that is, a piece of wood which fitted to the crown I, and raised the surface thereof a foot or more, so as to obtain the greatest fall when the water stood at a mean height; but when the water sunk too low to run over this movable crown, it could be drawn up to admit the water beneath it.

This effect has since been produced in a more perfect manner by making the crown of the fall a movable shuttle, to rise and fall according to the height of the water in the mill-dam, by which means the inconvenience before-mentioned is avoided. IMPROVED BREAST-WHEEL, IN WHICH THE WATER RUNS OVER THE SHUTTLE.

FIG.110 is a section of one of this kind. A is the water which is made to flow upon the float-board B, and urges the wheel by its weight only, the water being prevented from escaping or flowing off the float-boards by the breast or sweep DD, and the side-walls which enclose the floats of the wheel. The upper part of the breast DD is made by a cast-iron plate, curved to the proper sweep to line with the stone-work. On the back of the cast-iron plate the moving shuttle e is applied; it fits close to the cast-iron so as to prevent the water from leaking between them, and the water runs over its upper edge. F is an iron groove or channel let into the masonry of the side-walls, and in these, the ends of the sliding shuttle are received; ƒ is an iron rack, which is applied at the back of the shuttle, and ascends above the water-line where the pinion g is applied to it to raise or lower the shuttle. The axis of the pinion is supported in a frame of wood II, bH is a toothed sector and balance-weight, which bears the shuttle upwards, or it might otherwise fall down by its own weight, and put the mill in motion when not intended. G is a strong planking, which is fixed across between the two side-walls, and retains the water when it rises very high, as in time of floods; but in common times the water rises only a few inches above the lower edge of the planking. When the shuttle is drawn up to touch this lower edge, the water cannot escape; but when the shuttle is lowered down, it opens a space e through which the water flows upon the float-boards of the wheel.

Fig. 111 is a section of the most improved form for a breast-wheel, taken from the Royal Armoury Mills at Enfield Lock, erected by Messrs. Lloyd and Ostel. The general description of this, is like the former, but it is constructed in a better manner, and unites strength with durability. The breast of masonry is surmounted by a cast-iron plate A, 24 feet high, which is let into the masonry of the side-walls at each end, and the lower part is formed with a flanch, by which it is bolted to the stone breast at top, This plate is made straight at the back for the shuttle B to lie against, and it slides up and down, The ends of the gate are guided

by iron groove pieces or channels which are let into the stone-work of the side walls, and being made wedge-like, they fix the ends of the cast-iron breast fast in its place. The grooves are not upright, but inclined to the perpendicular so much, that the plane of the gate is at right angles to a radius of the wheel drawn through the point where the water falls upon the wheel. D is a strong plank of wood, extended between the iron grooves just over the shuttle. When the shuttle is drawn up it comes in contact with the lower side of this piece of wood, and stops the water; but the piece D is fixed at such a height, that the water will run clear beneath it, unless its surface rises above its mean height.

The float-boards of the wheel do not point to the centre of the wheel, but are so much inclined thereto that they are exactly horizontal at the point where the water first flows upon them. In this way, the gravity of the water has its full effect upon the wheel, and the boards rise up out of the tailwater in a much better position, than if they pointed to the centre of the wheel; this is more particularly observable when the wheel is flooded by tail-water penned up in the lower part of the race, so that it cannot run freely away from the wheel. The dimensions of this wheel are as follows: diameter 18 feet to the points of the floats, and 14 feet wide; the float-boards are 40 in number, each 16 inches wide, and each rising-board 11 inches wide. The wheel is formed of four cast-iron circles or wheels, each 14 feet 8 inches diameter, placed at equal distances upon the central axis, which is 14 feet 8 inches long between the necks or bearings, and 9 inches square; the bearing-necks are 9 inches diameter. The wheel is calculated to make four revolutions per minute, which gives near 3 feet per second for the velocity with which the float-boards move. The fall of water is six feet, and the power of the wheel, when the shuttle is drawn down one foot perpendicular, equal to 28-horse power.

·BREAST-WHEEL WITH TWO SHUTTLES.

In this wheel the piece of wood marked D in the last figure, is fitted into the groove of the shuttle, and is provided with racks and pinions to slide up and down, independently of the lower shuttle. This enables the lower shuttle to rise and fall, according to the height of the water, so that the water shall always run over the top of it, in the proper quantity to work the mill with its required velocity, whilst the upper shuttle is only used to stop the mill by shutting it down upon the

lower shuttle, and preventing the water from running over it. This plan is used when the mill is to be regulated by a governor, or machine to govern its velocity; in that case the governor is made to operate upon the lower shuttle, and will raise it up, or lower it down, according as the mill takes too much or too little water, and this regulates the supply; but the upper shuttle is used to stop the mill, and by this means the adjustment of the lower shuttle is not destroyed, but when set to work again, it will move with its required velocity. Fig. 101 is a section of one of the water-wheels at the cotton-mills of Messrs. Strutt, at Belper, in Derbyshire. The width of this wheel is very great, and to render the shuttles A B firm, a strong grating of cast-iron is fixed on the top of the breast K, and the shuttles are applied at the back of the grating E, so as to slide up and down against it, the strain occasioned by the pressure of the water being borne by the grating. The lower shuttle is moved by means of long screws, a, which have bevelled wheels, b, at the upper ends, to turn them, by a connection of wheel-work with the wheel-work of the mill. The upper shuttle, A, is drawn up or down by racks and pinions, c, which are turned by a winch, or handle. The bars of the grating E are placed one above the other, like shelves, but are not horizontal; they are inclined, so that the upper surfaces of all the bars form tangents to an imaginary circle of one-third the diameter of the wheel described round the centre thereof. These bars are not above half an inch thick, and the spaces between them are 2 inches. The bars are of a considerable breadth, the object of them being to lead the water, with a proper slope from the top of the lower shuttle B, to flow upon the floats of the wheel. This disposition allows the shuttles to be placed at such a distance from the wheel as to admit very strong upright bars of cast-iron to be placed between the wheel and the shuttles, for the shuttles to bear against, and prevent them from bending towards the wheel, as the great weight of water would otherwise occasion them to do. These upright bars are very firmly fixed to the stone-work of the breast at their lower ends, and the upper ends are fastened to a large timber, D, which is supported at its ends in the side walls, and has a truss-framing applied to the back of it, like the framing of a roof, to prevent it from bending towards the wheel. The upright bars are placed at distances of five feet asunder, so as to support the shuttles in two places in the middle of their length, as well as at both ends; and large rollers are applied in the shuttle,

where it bears against these bars, to diminish the friction, which would otherwise be very great.

These precautions will not appear unnecessary when the size of the work is known. The wheel is 21 feet in diameter, and 15 feet broad; the fall of water is 14 feet, when it is at a mean height; the upper shuttle is 24 feet high, and 15 feet long; the lower shuttle is five feet high, and the same length, so that it contains 75 square feet of surface exposed to the pressure of the water; now taking the centre of pressure at two-thirds of the depth, or 3 feet, we find the pressure equal to that depth of water acting on the whole surface; that is, the weight of 34 cubic feet of water = 208 pounds, bears on every square foot of surface, which is equal to 15,600 pounds, or near seven tons, on the lower shuttle only; but if we take the two shuttles together, the surface is 112 square feet, and the mean pressure 312 pounds upon each, or 16 tons in the whole. The wheel has 40 floatboards pointing to the centre. The wheel is made of cast-iron. There are two wheels of the dimensions above stated, which are placed in a line with each other, and are only separated by a wall which supports the bearings; for they work together as one wheel, and the separation is only to obviate the difficulty of making one wheel of such great breadth as 30 feet, though this is not impossible, for there is a wheel in the same works 40 feet in breadth, but it is of wood and not iron, and is framed in a particular manner.-Dr. Rees's Cyclopædia.

DR. BARKER'S MILL.

DR. DESAGULIERS appears to have been the first who published an account of this machine. He ascribes the invention to Dr. Barker, in the following words: "Sir George Savill says, he had a mill in Lincolnshire to grind corn, which took up so much water to work it, that it sunk his ponds visibly, for which reason he could not have constant work; but now, by Dr. Barker's improvement, the waste water only from Sir George's ponds keeps it constantly to work."

Dr. Barker's mill is shown in fig. 102, where CD is a vertical axis, moving on a pivot at D, and carrying the upper millstone m, after passing through an opening in the fixed millstone C. Upon this axis is fixed a vertical tube TT, communicating with a horizontal tube A B, at the extremities of which, A, B, are two apertures in opposite directions. When water from the millcourse M N is introduced into the tube TT, it flows out of the apertures A, B, and, by the reaction or counterpressure of the issuing water, the arm AB.