WATER-POWER RBSERVOrRS, ETC.] 



APPLIED MECHANICS. 



833 



by the gravitating influence of the moon ; but this wave 

 appears only as a gentle and almost imperceptible incli- 

 nation of the water surface, except in some estuaries, 

 such as the Solway Frith, where it presents itself as an 

 elevated body of water rushing with considerable velocity 

 towards the land. 



The overshot- wheel is applicable when the water has a 

 considerable local fall, nearly equal to the diameter of 

 the wheel ; and the breast-wheel when the local fall is 

 not great less, for instance, than half the diameter of 

 the wheel but when it is of considerable volume, and 

 moves with considerable velocity. In order to apply 

 either an overshot or a breast-wheel, it is generally ne- 

 cessary to make extensive arrangements for conducting 

 the water from an elevated level to the wheel, instead of 

 permitting it to follow its natural channel. When a 

 stream has a considerable fall such as 40 or 50 feet in 

 each mite of its length a data or weir is built across it 

 at some convenient position, so as to check its progress 

 there, and a new channel is formed for conveying its 

 waters to the mill, and thence back to the bed of the 

 stream at some point below the dam. As the artificial 

 channel is made with only sufficient declivity to secure 

 the flow of the water in such quantities as may be re- 

 quired, it is thus possible to obtain at the wheel, nearly 

 the total fall which the channel of stream has, estimated 

 from the point where the dam is built, to that where the 

 tail-water of the mill re-enters. If, for instance, the 

 stream in its natural channel is found to have a fall of 

 60 feet in a mile this difference of level being made up 

 either of numerous small local falls or of a continuous 

 declivity, or both an artificial channel is formed by its 

 side, or as near it as the levels of the ground permit, 

 having a constant declivity for half a mile, amounting to 

 6 feet of difference of level ; the water acts on a wheel 

 with a fall of 20 feet, and a declivity of 5 feet is allowed 

 in the length of the tail-course. The difference of level 

 in the channel for half a mile that is to say, 30 feet is 

 thus made up, and the power due to *nls of that fall is 

 thus secured for driving machinery. The current of the 

 stream itself would probably not have so great a velocity 

 at any place as to make it practically available for an 

 undershot-wheel, on account of the irregularities of its 

 channel, and the numerous resistances opposed to its 

 [>r'i Mill 



DAMS, OR RESERVOIRS. Where there is no 

 stream of sufficient magnitude to give the necessary 

 power, and the power is not required to be constantly 

 in operation, it is usual to form a large dam or reservoir 

 for collecting the constant small tribute of the stream, so 

 that the volume of water thus accumulated may be em- 

 ployed to drive the mill as occasion requires. For moving 

 agricultural machinery, such as thrashing-machines, this 

 plan is very commonly resorted to. The corn of a farm 

 is generally thrashed in the winter season, when there is 

 the best supply of water in the streams, and from the 

 drainage of the soil. A small mill-dam or reservoir col- 

 lects, during the night, sufficient water to drive the mill 

 through the following day. And it is thus possible, even 

 in localities where apparently no water-power can be 

 obtained, to secure enough for the work to be done, by 

 executing a properly contrived dam, and turning into it 

 the drainage of the surrounding fields. 



When water is applied to manufacturing purposes, re- 

 quiring the constant supply of large volumes of water, 

 reservoirs or dams are sometimes executed on a gigantic 

 scale, in order to store up the superfluous supply of 

 rainy weather against a season of drought. In some of the 

 hilly districts of England and Scotland, these works are 

 of a most important character, and the interests of large 

 local populations are dependent on their efficiency and 

 permanence. When the differences of level in the dis- 

 trict are considerable, numerous mills are worked suc- 

 cessively by the same water, that which has driven the 

 higher, flowing along an artificial channel till it arrives 

 at the next lower ; and so on, in constant succession for 

 great distances. In such cases, the mill-owners fre- 

 quently combine to execute works for the benefit of all, 

 and of much greater magnitude than a single capitalist 



could undertake. By such arrangements, they are 

 enabled to throw an immense dam or retaining-wall 

 across some valley, and thus collect in a vast reservoir 

 the drainage of an extensive range of hills, which would 

 otherwise flow along its natural course to the sea without 

 being turned to useful account as a mover of machinery. 

 While the rains or melting of the snows contribute much 

 more water than is required, the reservoir is filled ; and 

 when the water in it attains the highest level required, 

 it is permitted to overflow into its natural channel. 

 When the season of drought arrives, the mills, that 

 would otherwise be at a stand-still, derive an ample 

 supply from the reservoir, extending, in some cases, 

 over many square miles of valley. When the depth of 

 this reservoir is considerable, great strength is required 

 in the dam ; and, notwithstanding the ingenuity and 

 labour expended on some of these structures, they some- 

 times give way ; and the enormous volume of water 

 thus suddenly set free rushes impetuously onward to the 

 sea, devastating whole districts in its course, destroying 

 crops and buildings, and too frequently causing a great 

 sacrifice of life. 



The sluices or valves for opening and closing the water- 

 channel of a mill are generally of very simple construc- 

 tion ; they consist of a plate of wood or metal, made to 

 fit against a framework fixed in the channel, and pressed 

 against it by the water. This plate is made to slide up- 

 wards in grooves fitting it at each side ; and when it is 

 of large dimensions, it is raised or depressed by racks 

 and pinions, or screws fitted with appropriate gearing, 

 as represented at K, I, and J in the engraving of the 

 compound wheel. By opening or shutting the sluice, 

 the wheel is put in motion or stopped at pleasure. A 

 channel is always provided to carry off the surplus water 

 to the tail-course of the wheel, when it rises in the spout 

 or lead, above the sill of the waste channel. 



REGULATORS. Various contrivances have been 

 applied to regulate the speed of water-wheels. The 

 most effectual is the steam-engine governor, or conical 

 pendulum. A throttle- valve, or plate, moving on an axis 



Fig. 128. 



or pivots at its middle, is fitted into the lead (Fig. 126). 

 When it presents its edge to the current, it offers very 

 little obstacle to its course ; but when it is turned into 

 an oblique position across the current, it arrests all the 

 water except what can pass through the openings left 

 between its edges and the sides of the lead. As it is 

 poised on pivots in its middle, the pressure of water on 

 each limb is the same ; and the only force required to 

 move it in either direction is that required for overcoming 

 the friction of its pivots, and the resistance of the water 

 to its movement through it, a force very inconsiderable. 

 The governor is connected, by rods and levers, to a valve 

 of this description in such a manner, that when, from 

 too great velocity of the water-wheel, the governor-balls 

 fly out from their axis, the valve is closed, or partly so, 

 and the supply of water to the wheel diminished. When 

 the wheel moves too slowly, the balls fall down to the 

 axis, and cause the valve to open for the passage of a 

 greater volume of water to the wheel. By this arrange- 

 ment, the movement of the wheel is regulated with groat 

 nicety, and the quantity of water supplied to the wheel 

 is suited exactly to the work which it has to do. The 

 consequence is, a great saving of tear and wear to the 



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