The direct overshot wheel has the water run, without changing its direction, right over the top; which arrangement has this advantage, that as the top of the wheel moves in the same direction as the stream, it gets the benefit of the whole initial velocity and impulse of the water; but, on the other hand, the bottom of the wheel, if at all immersed. in water, which it generally is to some extent, meets with obstruction by moving against_ the current.
The overshot is a modification of the last, making the water to pass along side the wheel, and then to return and be let on the top of the wheel in a contrary di rection. This requires longer and more complicated troughs, and by the change in direction, part of the impulse from the water is lost, but the bottom of the wheel moves in the direction of the tail-water, and is not liable to be impeded by being immersed in it.
On the whole, it is generally thought better to apply the water at about 30° from the top of the wheel. In such high-breast or nearly overshot wheels, the water is let on to the buckets over the top of the sluice, which is made to open by lowering, and shut by lifting. In this way, however small may be the quantity of water, it is always ap plied at the highest possible level, which is of importance when it is its weight multi plied by the height of descent, not its impulse, that yields the effective power.
The structure of the overshot and breast wheel is nearly the same as that of the undershot, excepting in the substitution of curved or angular buckets, for straight floats; but even in the undershot wheel the floats are sometimes made with a slight curvature.
In any description of wheel, the motion may be taken off the axle by torsion, which necessarily requires rigidity in the arms; or it may be taken directly off the periphery, when the power is applied to a pinion working into segments, either external or inter nal, attached to the shrouding. In this arrangement there is no torsion of the axle, or transverse strain on the arms, and therefore the latter are more often made of round, wrought-iron rods, with a slight axle. This wheel is much lighter than with the massive. axle and the strong wooden or cast-iron arms, and is called a suspension or spid,er.whoel.
Iv reckohing the power of water, its weight being 62 lbs. to a cubic foot ; theoreti cally 529 ft. falling vertically 1 ft. a minute, would be equal to 1 Boulton and Watt horsepower of 33,000 lbs. lifted 1 ft. a minute; but the effective power is far short of that, and 60 per cent of it, requiring 880 cubic ft., falling 1 ft. a minute, is generally reckoned a fair allowance for an effective horse-power. Seventy-five per cent, requiring
704 ft., falling 1 ft. a minute, is about the highest that has ever been spoken of, and is is doubtful whether even more than 70 per cent has ever been attained; while with low falls and imperfectly constructed wheels, it is often reckoned that a horse-power requires nearly 100 cubic ft. a minute.
The velocity of the periphery of au undershot wheel is usually from 500 to 600 ft. a minute, and that of a bucket-wheel, overshot or breast, from 300 to 450 feet. It is seldom that the whole height of a fall can be advantageously made use of; for if the wheel be placed so low as to get the benefit of the whole height of the fall in low states of the water, very often it is liable in floods, to have the lower rim immersed, and to be ob structed or stopped by back-water.
The most extensive application of water-power to one work in Scotland, or probably in Britain, is that of Deanston cotton mills, on the river Teith, 6 m. above Stirling, where there are in one house four wheels, 36 ft. in diameter, and 12 ft. in breadth, and having a volume of water of 8} millions of cubit ft. in 14 hours a day—falling 33 ft. a minute. The most systematic application of water-power, however, is probably that of the Shaws water-works, now the property of the corporation of Greenock. There the yield of nearly 7,000 acres of hill-ground is stored up in reservoirs of a capacity of 320,000,000 of cubic ft., and conveyed by an aqueduct of about 6 m. in length to the outskirts of Greenock, which it reaches at the level of 512 ft. above the level of the sea, and iS then divided into two lines of falls, one having 1200cubic ft. a minute for 12 hours a day, and the other the equivalent quantity of 1066 ft. for 13 hours a day, divided each into 19 falls, for which those already appropriated pay per annum from ;t..1 15s. to £4 5s. per horse-power, according to their distance from the center of the town, and their height above its level. One foot of fall for each live is reckoned 1.8 horse-power, which is a very high computation, being 79.2 per cent of the theoretical horse-power. At the "cotton mill," where both lines of falls are combined, there is the largest, or nearly the largest water-wheel in existence. It is 70 ft. 2 in. in diameter, 13 ft. wide, with 166 buckets, having a depth of 17 in.' It has 2,266 cubic ft. of water per minute, with a fall of 64 ft. 4 in., and is therefore nearly 200 horse-power. By the Shaws computation, it would be 219 horse-power. It is a spider wheel, taking the power off the circumference.