can be opened or shut at pleasure; when a head of water presses against the gates they will open great part of the way of themselves, by only letting the catches that keep them shut be lifted out of their place. X, Y, are two knees of cast-iron, to support the posts that the gates are fixed to. The walls of the building are represented at a, b, c, and d.

The reader will now be able to form an estimate of the comparative value and ingenuity of the two kinds of tidemills here described. The simplicity of construction of the wheels of Gosset, De la Deuille, and Dryden, recommend them strongly; but we entertain some doubts of their being completely successful in practice had the curious wheel, with the folding-gates, &c. fig. 104 and 106, been placed with its axle perpendicular, instead of parallel, to the course of the river, the water might then have always been admitted to act upon the same side of it, and the hydrostatic pressure would have operated as completely in lowering it continually during the time of ebb, as in raising it continually during the rising of the tide; thus, as appears to us, would the labour of a man be saved, who, according to the present construction, must attend the water-wheel; and all the additional apparatus now requisite to shift the spur-wheels, would at the same time be saved, and a consequent diminution of original expense. Dr. Gregory's Mechanics, vol. ii.

In selecting a site for the erection of a mill, the engineer must be careful not to make choice of a spot that is liable to be flooded. When the water in the mill-tail will not run off freely, but stands pent up in the wheel-race, so that the wheel must work or row in it, the wheel is said to be tailed, or to be in back-water or tail-water; which greatly impedes the velocity of the wheel, and, if the flood be great, completely stops it.

Every mill that is well and properly constructed, will clear itself of a considerable depth of tail-water, provided there is, at the time, an increase in the height of the water in the mill-dam or head, and an unlimited quantity of water to draw upon the wheel. Common breast-mills will bear two feet of tail-water, when there is an increase of head, and plenty of water to be drawn upon the wheel, without prejudice to their performance; and mills that are well constructed, with slow moving wheels, will bear three and even four feet and upwards of tail-water. Mr. Smeaton mentions having seen an instance of six feet; and it is a common thing in level countries, where tail-water is most annoying, to lay the wheel from six to twelve inches below the water's level of the

pond below, in order to increase the fall of water; and, if judiciously applied, is attended with good effect, as it increases the diameter of the wheel, and though it must always work in that depth of tail-water, it will perform full as well, because the water ought to run off from the bottom of the wheel, in the same direction as the wheel turns.

ON THE CONSTRUCTION OF THE WHEEL-RACE AND WATER-COURSE.

THE wheel-race should always be built in a substantial manner with masonry, and if the stones are set in Roman cement, it will be much better than common mortar. The earth, behind the masonry, should be very solid, and if it is not naturally so, it should be hard rammed and puddled, to prevent percolation of the water. This applies more particularly to breast-wheels, in which the water of the dam or reservoir is usually immediately behind the wall or breast in which the wheel works, a sloping apron of earth being laid from the wall in the dam to prevent the water leaking. The wall of the breast should have pile planking driven beneath, to prevent the water from getting beneath, because that might blow up the foundation of the race. The stones of the race are hewn to a mould, and laid in their places with great care; but afterwards, when the side walls are finished, and the axis of the wheel placed in its bearings, a gauge is attached to it and swept round the curve, and by this the breast is dressed smooth, and hewn to an exact arch of a circle; the side walls, in like manner, are hewn flat and true at the place where the float-boards are to work. It is usual to make the space between the side walls two inches narrower at each side, in the circular part where the float acts, than in the other parts.

In some old mills the breast is made of wood planking, but this method has so little durability that it cannot be recommended.

In modern mills, the breast is lined with a cast-iron plate, but we do not approve of this, because it is next to impossible to prevent some small leakage of water through the masonry; and this water, being confined behind the iron breast, cannot escape, but its hydrostatic pressure to force up the iron is enormous; and if the water can ever insinuate itself behind the whole surface of the plate, rarely fails to break it, if not to blow it up altogether. This is best guarded against by making deep ribs projecting from the back of the plate, and bedding them with great care in the masonry; these not only strengthen the plate, but also cut off the communication

of the water, so that it cannot act upon larger surfaces at once, than the strength and weight of the plate can resist. Stone is undoubtedly the best materials for a breasting. In overshot-wheels the loss of water, by running out of the buckets as they approach the bottom of the wheel, may be considerably diminished by accurately forming a sweep or casing round the lower portion of the wheel, so as to prevent the immediate escape of the water, and causing it to act in the manner of a breast-wheel. While this improvement remains in good condition, and the wheel works truly, it produces a very sensible effect; but it is frequently objected to, because a stick or a stone falling into the wheel would be liable to tear off part of its shrouding, and damage the buckets; and again, a hard frost frequently binds all fast, and totally prevents the possibility of working during its continuance; but we do not think the latter a great objection, for the water is not more liable to freeze there than in the buckets, or in the shuttle, and may be prevented by the same means, viz. by keeping the wheel always in motion, a very small stream of water left running all night will be sufficient. Mr. Smeaton always used such sweeps, and with very good effect; it is certainly preferable to any intricate work in the form of the buckets.

Mill-courses. As it is of the highest importance to have the height of the fall as great as possible, the bottom of the canal or dam which conducts the water from the river should have a very small declivity; for the height of the water-fall will diminish in proportion as the declivity of the canal is increased; on this account, it will be sufficient to make A B, fig. 100, slope about one inch in 200 yards, taking care to make the declivity about half an inch for the first 48 yards, in order that the water may have a velocity sufficient to prevent it from flowing back into the river. The inclination of the fall, represented by the angle G C R, should be 25° 50′; or CR, the radius, should be to GR, the tangent of this angle, as 100 to 48, or as 25 to 12; and since the surface of the water Sb is bent from ab into a c, before it is precipitated down the fall, it will be necessary to incurvate the upper part BCD of the course into B D, that the water at the bottom may move parallel to the water at the top of the stream. For this purpose, take the points B, D, about 12 inches distant from C, and raise the perpendiculars BE, DE; the point of intersection E will be the centre, from which the arch BD is to be described; the radius being about 10 inches.

Now, in order that the water may act more advantageously upon the float-boards of the wheel W W, it must assume a horizontal direction HK, with the same velocity which it would have acquired when it came to the point G: but, in falling from C to G, the water will dash upon the horizontal part H G, and thus lose a great part of its velocity; it will be proper, therefore, to make it move along FH, an arch of a circle, to which D F and KH are tangents in the points F and H. For this purpose, make G F and G H each equal to three feet, and raise the perpendiculars HI, FI, which will intersect one another in the points I, distant about four feet nine inches and ths from the points F and H, and the centre of the arch FH will be determined. The distance HK, through which the water runs before it acts upon the wheel, should not be less than two or three feet, in order that the different portions of the fluid may have obtained a horizontal direction; and if HK be much larger, the velocity of the stream would be diminished by its friction on the bottom of the course. That no water may escape between the bottom of the course KH and the extremities of the floatboards, KL should be about three inches, and the extremity o of the float-board n o, should be beneath the line HKX, sufficient room being left between o and M for the play of the wheel, or KLM may be formed into the arch of a circle KM, concentric with the wheel. The line L M V, called by M. Fabre the course of impulsion, (le coursier d'impulsion,) should be prolonged, so as to support the water as long as it can act upon the float-boards, and should be about nine inches distant from O P, a horizontal line passing through O, the lowest point of the fall; for if O L were much less than nine inches, the water, having spent the greater part of its force in impelling the float-boards, would accumulate below the wheel and retard its motion. For the same reason, another course, which is called by M. Fabre the course of discharge, (le coursier de décharge,) should be connected with LMV by the curve VN, to preserve the remaining velocity of the water, which would otherwise be destroyed by falling perpendicular from V to N. The course of discharge is represented by V Z, sloping from the point O. It should be about 16 yards long, having an inch of déclivity in every two yards. The canal, which reconducts the water from the course of discharge to the river, should slope about four inches in the first 200 yards, three inches in the second 200 yards, decreasing gradually till it terminates in the river. But if the river, to which the water is conveyed, should, when

swollen by the rains, force the water back upon the wheel, the canal must have a greater declivity, in order to prevent this from taking place. Hence it will be evident, that very accurate levelling is necessary for the proper formation of the mill-course.

ON SETTING OUT WATER-COURSES AND DAMS.

THE most ancient mills were undershot-wheels placed in the current of an open river, the building containing the mill being set upon piles in the river. It would soon be observed that the power of the mill would be greatly increased if all the water of the river was concentrated to the wheel, by making an obstruction across the river which penned up the water to a required height; and also to form a pool or reservoir of water. A sluice or shuttle would then become necessary to regulate the admission of water to the wheel, and other sluices would be necessary to allow the water to escape in times of floods; for though in ordinary times the water would run over the top of the obstruction or dam, yet a very great body of water running over might carry away the whole work, by washing away the earth at the foot of the dam, and then overturning it into the excavation. This is an accident which frequently happens to mills so situated; and the danger is so obvious, that most water-mills are now removed to the side of the river, and a channel is dug from the river to the mill to supply it with water, and another to return the water from the mill to the river. The difference of level between these two channels is the fall of water to work the mill, and this is kept up by means of a wear or dam entirely across the river, but the water can run freely over this dam in case of floods, without at all affecting the mill, because the entrance to the channel of supply is regulated by sluices and side walls.

The dam should be erected across the river at a broad part, where it will pen up the water so as to form a large pond or reservoir, which is called the mill-pond or dam-head. This reservoir is useful to gather the water which comes down the river in the night, and reserve it for the next day's consumption; or for such mills as do not work incessantly, but which require more water, when they do work, than the ordinary stream of the river can supply in the same time. The larger the surface of the pond is, the more efficient it will be, but depth will not compensate for the want of surface, because, as the surface sinks, when the water is drawn off, the fall on descent of the water, and consequently the power of the water, diminishes.

The dam for a large river should be constructed with the