To prevent this, the water is seldom permitted to run upon the wheel in a stream of more than from half an inch to an inch in thickness, and when well regulated there is scarcely a drop of water ineffectually used. The overshot wheel acts, therefore, by the gravity or weight of the water contained in the buckets, for nearly one-third of its circumference; and from the experiments of Mr. Smeaton, which were made with great accuracy, it appears that the dimensions, quantity of water, and height of fall being the same, the overshot wheel will produce double the effect of the under-shot.
The breast-wheel is by far the most common, and may be considered as a mean between the two varieties before mentioned. In this, the water, instead of passing over the top of the wheel, or entirely beneath it, is delivered about half-way up it, or rather below the level of the axis; and the race or brickwork upon which the water descends is built in a circular form, having the same common centre with the wheel itself, so as to make it parallel to the exterior edges of the float-boards, or extreme circumference of the wheel. This construction is shown in the above figure, which represents a side-view of a wheel, formed with float-boards in the same manner as the undershot wheel; but instead of the water acting upon its lower part, it is introduced upon it midway, by the sluice or pen-stock, which, by rising or falling, permits a greater or less quantity of water to act on the wheel; and as the float-boards arc made to fit as accurately as possible, without contact, into the circular hollow of the brickwork, no water can escape past the wheel,without producing its proportionate effect.
Mr. Smeaton states, that all wheels by which the water is prevented from descending, unless the wheel moves therewith, are to be considered of the nature of over-shot wheels, having power in proportion to the perpendicular height from which the water descends; while all those that receive the impulse or shock of the water, whether in an horizontal, perpendicular, or oblique direction, arc to be considered as under-shots. The breast-wheel is nearly allied to the over-shot; for notwithstanding it has only float-boards, instead of buckets, yet as the mill-course is made concentric to the outside of the wheel, and is not only there, but at the two sides, made as close as convenient, so as to prevent the escape of water as effectually as possible, the spaces between one float-board and another, become buckets for the time being, and retain the water, and thus the breast-wheel is not only impelled by the weight of water, but by its impetus or momentum also; for the water is so confined, as to be incapable of splashing or being lost, and consequently, its moving force may be exerted to great advantage.
Notwithstanding this apparent superiority, still the breast-wheel is, in effect, vastly inferior to the over-shot wheel, not only on account of the smaller height at which the water is supplied, but from the waste with which it must always be attended, even under circumstances of the most perfect workmanship. When well-constructed, and closely built in, its effect, according to Mr. Smeaton, should be the same as an under-shot wheel, whose head of water is equal to the difference of level between the surface of the stream and the point where it strikes the wheel, added to the effect of an over-shot wheel, whose height is equal to the distance from the striking point, to the tail-water of the mill, or that which runs to waste. This is, however, on the presumption that the wheel receives the impulse of the water at right angles to its radii, and that every thing is constructed to the best advantage. In practice, it is found that the breast-wheel consumes about double the quantity of water that the over-shot wheel requires, to do the same quantity of work, when all things are alike, - that is to say, the diameter and breadth of the wheel, number of float-boards, etc,-though from theory and calculation, it should rather do more; for Lambert, and others who have written on this subject, attempt to demonstrate, that the power of the over-shot, to that of the breast-wheel, is as thirteen to five; but this is upon a supposition, that no water escapes ineffectually, which is utterly impossible in practice.
In order to permit any of the above wheels to work with freedom, and to the greatest advantage, it is absolutely necessary that the tail-water, as it is called, or that which is discharged from the bottom of the wheel, after it has produced its effect, should have an uninterrupted passage to run away; for whenever this is not the case, it accumulates, and forms a resistance to the float-boards, - and consequently, abstracts considerably from the velocity and power of the wheel, sometimes indeed to so great an extent, as to prevent its working altogether. One of the simplest and most effectual means of removing this inconvenience, (says the author of the Treatise on Hydraulics, in the "Library of Useful Knowledge," for whose observations we are largely indebted in the present article,) is by an expedient, not much known or practised, and which consists of forming two drains or tunnels through the brickwork or masonry, at each side of the water-wheel, whatever may be its construction, so as to permit a portion of the upper water to flow down into the tail or lower stream immediately in front of the wheel.
The water thus brought down with great impetuosity, drives the tail-water before it, in such a manner as to form a basin or hollow place, in which the wheel can work free from interruption, even if the natural state of the water were such as might produce a tailing of from twelve to eighteen inches, without this assistance. And since the tailing of mill-streams only occurs in the winter seasons, or at times when there is a profusion of water, so the quantity that is thus thrown away without operating upon the wheel, can be spared without inconvenience. Each of the drains or tunnels is furnished with a sluice-gate or pen-stock at its upper end, by which the quantity and impetus of the water can be regulated at pleasure, or the whole be shut off, whenever water happens to he scarce.