WATER-POWER. The value of water-power depends much on the nature of the source of supply, whether steady or otherwise. Where streams supplying water-power are liable to fall off much in dry weather, large impounding reservoirs are necessary to keep the mills from being stopped during summer. These, however, being generally expen sive, concerns, are seldom made for one mill, but rather by some association of mill owners; and often by a water company or commission for supplying a town with water, the afford compensation to the mills by storing up flood-water, for what is abstracted for h y e use of the town. On small streams there is generally a Pond provided fit to hold a night's water, or, perhaps, even a Sunday's, in addition; but in the case of large rivers, there is, in general, only a weir or dam across the river to direct the water into the intake lade. When the inclination in the bed of the stream is small, the lades require to be proportionally long, to give sufficient fall, and are often above a mile long or more from the intake to the lower end of the tail or discharge lade, where the water is re.
turned to the stream. The rise and fall of the tide has been frequently used for drivino.
a water-wheels.
The most usual, and generally the most eligible, mode of applying water to the ing of machinery is by means of a vertical wheel; and the wheel is put in motion either' by the water acting on blades or floats by impulse derived from its velocity acquired in falling, or by the weight of water being applied to one side of the wheel. The former' mode of applying the water is generally adopted in low falls, say under six ft. or there abouts, and to what is called an undershot wheel—i.e., a wheel where the effective head of water is below the level of the center; and to make the application efficient, that portion of the periphery of the wheel measuring from the point of impact of the water to a point directly below the center, requires to surrounded by a casing generally of stone, but sometimes of cast-iron, called the arc, closely fitted to the extremity of the floats, so as to prevent any considerable escape of water.
The wheel, which may be either of timber or of cast-iron, or partly of both, consists of axle, arms, floats, which generally radii of the circle, but are sometimes set a lit tle obliquely to the radius, pointing up stream; and generally there are also a sole, being a lining round the circumference at the lower edge of the floats, having openings for the escape of air; and a shrouding or circular plate at each side of the wheel, and of the same depth as the floats.
Sometimes, when there is very little fall beyond the mere current of the stream, the floats simply dip into the water like the paddles of a steamer, in which case no sole or shrouding is required; and to -make allowance for the rise of the water in the tail-lade during floods, which is generally called and seriously impedes and sometimes stops the motion of the wheel, occasionally the wheel and its arc are so constructed as to be capable of or depressed together, without throwing the machinery out of gear. This is done in the case of the Inverness water-works, where the wheel is lia ble to be much affected by the rising and falling of the river Ness.
Sometimes in this country, and often on the continent, the machinery is all on board a vessel moored in a river, so as to rise and fall with the level of the water, and thereby keep its water-wheel always immersed to the proper depth. At the old London bridge water-works the wheels, which rose and fell with the tide, were worked by the current of both the flood and ebb.
The other mode of applying the water to a vertical wheel by making it act by its gravity, is the more perfect and economical mode, where circumstances will admit of it, and is generally adopted in falls of any considerable height, say of six ft. and upward, and where the water can be let on above the level of the center. The wheels are called respectively breast and overshotwheels, according as the water is let on more near to the level. of the center or to the crown of the wheel; and they have, instead of straight floats, curved or kneed buckets, as they may be made of iron-plate or of wood, and' of such a shape as to retain the water down to the lowest possible point. There are gen erally in good wheels ventilating openings in the sole for the escape of air. The over shot wheel has this disadvantage that, as the water has little or no power until consider ably past the top center, the wheel is burdened with a useless weight of water.