WATElt-WHisELS. The machines, by means of which the weight, or the impulse of water, is converted into circular, vertical, or hori zontal motion. are known in mechanics by the name of water-wheels and as the power tips employed is furnished by nature, and requires but little outlay to be rendered available, these machines have at all times attracted much attention from the manufacturing interests. They are of various descriptions, according to the conditions under which the waters flow, or the uses to which the power thus converted is to be aPplied ; but they are all alike in this respect, namely, that the motion they receive is always circular, whereas in the other modifications of water-power, the motion is rectilineal, and alternate. Waterwheels are in fact subdivided into those with floats or blades; and those with buckets. The wheels with floats are divided into the classes which have either straight or curved floats; and Into those working in free water, in races, or water troughs either straight r or curved. The bucket-wheels may be adapted to receive the water at the summit, or at an intermediate point of their height ; hut the float-wheels are exclusively of the kind known as the undershot wheels, whilst the bucket-wheels are either overshot- or breast wheels ; the latter distinctions depending upon the height at which the water strikes the wheel, for in the undershot-wheels the water acts below the horizontal line passing through the centre of the wheel : in the over shot-wheel it acts above the said horizontal line, and on the downside (to the current) of the vertical line passing through the centre : and in the breast-wheel the water seta at a variable angle above the horizontal line through the centre, but on the same side as the stream flows naturally. In addition to these are the horizontal wheels, which are either set in motion by an isolated vein of water, or are placed in a cylinder, or are placed around a cylinder conducting the water, in the manners already described under TURBINES. Sometimes a modifica tion of the overshot-wheel is used, under the name of the wheel, in which the water is carried beyond the summit, and thence returns to strike the wheel at a point situated a little below that level, on the upper side of the vertical line passing through the centre of the wheel.
The parts of a water-wheel and its adjuncts which it may be desirable to define, are as follows. In the water channel there is, generally speaking, a mid-head, with tumbling bay and waste weir to regulate the supply of water to the fore-buy, or the channel leading the water directly upon the wheel ; this fore-bay has a hatch, or sluice, by means of which the quantity of water falling upon the wheel can be regulated at will. The mill-race is the part of the channel immediately under the wheel ; the tad-bay is the part by which the water escapes after it has produced its effect. The wheel consists of a shaft, upon which are fastened the arms, bearing in their turn the periphery, to which are attached the floats or the buckets ; sometimes this periphery is close boarded, or soled, sometimes it has openings for the purpose of ventilating the buckets ; the sides of the buckets which keep the water upon the wheel are called the shrouds. The motion is taken off from the wheel either by gearing working upon the segments of the wheel, or by first motion, or bevilled wheels fixed upon the axle ; the last are generally known by the name of pit-wheels.
Formerly the vertical wheel, with straight floats working in a straight mill-race, was the description of undershot-wheel most generally adopted, but it is now seldom used, unless in cases where there is an abundance of water, and a fall not exceeding five feet (fig. I.) In
these wheels, the water acts entirely by its shock, and evidently this must be proportionate to the velocity with which it issues from the hatch, under the pressure of the head in the fore-bay. The distance between the hatch and the wheel must therefore be made as small as possible, because the water in spreading out in the mill-race is exposed to lose a portion of its velocity. The fore-bay should be made with converging sides, upon the principle of ordinary conical ajutages, wider towards the dam than to the sluice; and from Poncelet's experiments, It would appear that a great increase of useful effect is obtained by making the hatches incline, so as to offer an angle, towards the dam, of 45'. Immediately beyond the centre line of the wheel, the race must fall away, so as to leave a more uninterrupted discharge into the mill tail ; its width must be regulated by the quantity of water to be discharged, because it is found practically that the depth of the stream ought not to exceed 10 inches when the course is clear ; the depth also should never be less than 6 inches, and in order to lose as little of the water as possible, the space between the outer edge of the wheel and the mill floor, should not be more than or of an inch. In the most carefully constructed mills at the present day, a great improve ment in the efficiency of the wheels is obtained by giving an inclination to the floor of the race from the hatch, to the level of the lower edge of the second float on the upside of the vertical diameter, of about 1 in 12, or 1 in 15; the bottom then curves away from that point, con centrically to the wheel, until it meets the vertical diameter line prolonged; and it then falls away with a sudden drop of 4 inches, and every precaution is taken to ensure the rapid withdrawal of the tail water. The width of the race is made somewhat smaller than that of the floats until a short distance before the position of the wheel ; and beyond the centre line it widens out still more than the width of the floats.
As the water, on escaping from the hatch, heaps up against the float, the latter must be made deeper than the normal thickness of the fluid vein; and generally speaking this is effected by making the floats about three times the depth of the water in the race, provided the width does not exceed 2 feet, or 2 feet 2 inches. The distance from one float to another should be rather less than the depths of the floats, their number being regulated principally by the purposes to which the machinery is to be applied. Thu diameter of the wheel is fixed in most cases by the number of revolutions it may be required to perform In a given time, in order to drive the connected machinery at the desired speed, and with the intervention of the smallest number of parts; and to a certain extent it is desirable to make the wheel act as a fly-wheel, to regulate the movement of the machinery. In the best wheels, the velocity of the extremity of the floats is made r = N/11, in which r = the velocity, and ft = the fall of water ; and the diameter ^ 3 is made D = \/ n, in which v = the number of revolutions per rc minute ; and n = the diameter sought : it is rarely, however, that the diameter of undershot-wheels varies beyond the respective limits of 13 and 26 feet. The numbers of floats usually adopted aro, for the dia. meters given, as follows : they are multiples of four, because millwrights prefer distributing them equally upon the quadrants of the wheel ; but it is to be understood that the number may be increased without inconvenience, if such a course were desired.