WATER POWER.

Thames Guardian and Mining Record, Volume I, Issue 238, 13 July 1872, Page 3

WATER POWER.

We take the following from a letter in the Ross News , by 11. A. Levestam, of Nelson. We give it a place in our columns, believing it will be acceptable to those who inter -st themselves in a water supply for the Thames : A sluice-head delivers 30 cubic feet of water per minute, which, with a fall of 10 feet, gives theoretically 057 h.p. The highest per ceutage obtainable from the best constructed pressure-engine is 80 per centum ; from a reaction wheel, 76 p:r cent. ; from a turbine, 70 per cent. ; and from a well-constructed overshot wheel, 68 per cent. The actual power derived from one sluice-head, with a fall of 10 feet, would, therefore, be—with the pres-sure-engine, 5.45 h.p., or with an overshot wheel, 6.38 h.p. A saving of 12 per cent. may, therefore, be effected by using the engine in preference to the wheel, which is a considerable object where water is scarce. The difference is, however, in most instances, far greater, as most waterwheels are faulty in their construction. The average per centage obtained from wooden wheels rarely reaches 50 per cent, of the power expended. An additional loss of power is frequently caused by intermediate gearing, required for the purpose of accelerating the motion. The proper speed of wheels is very slow, thus, a wheel 10 feet diameter should only make ten revolutions per minute, while a 50 feet wheel would only make one and a half revolutions per minute. The number of revolutions per minute of the water-pressure engine may be fixed by the length of stroke, the proper piston speed being 1 foot per second. The reaction wheels, or turbines, are, however, equally unmanageable regarding the speed, as they make, with high falls, thousands of revolutions per minute. There are other considerations, which may sometimes influence the choice of the motor to be employed—such as the space available, the state of roads, and the general nature of country, whether timbered or not, and many other considerations. The conclusion to be drawn from the foregoing is that the engines ought to have the preference, as being more economical, and mure manageable regarding the speed. There is, however, an objection to their more general use—namely, the expense of the pipes required, as the water must be conveyed to them in close piles, and their weight, which would make their transport in a country like this very expensive—unless a large main were laid down, to supply a large number of machines, each of which would bear their share of the expense. There is another subject connected with water-power which is far less known—but far mure important.—than any of the foregoing remarks, —namely the loss of power by friction in the pipes, which is, when the 1-ngtli of pipe is great, enormous. Tnis loss may almost be totally avo'd d in sonic localities, by erecting a water-mill as near the source ot power as possible, and then to conduct the power to the spot wh°re it is desired to render it available, by forcing water through a pipe at a high pressure, under which conditions the loss by friction would be less. I may here remark that the loss of power remains constant uudar all pressure—that is, if through a given pipe, at any particular pressure, 10 h.p. can be transmitted with a loss of say 1 h.p : then, if the pressure were to tie increased 1000 times, 10,000 h.p. could be transmitted through the same pipe, with a loss only of 1 h.p. Experiments were made upon one of the pipes by which the city of Edinburgh is supplied. The length of the pipe is 14,930 feet, its diameter 44 inches, and the head 51 feet. The actual discharge of this pipe was found to be 11.3 cubic feet per minute, which is equal to 1.1h.p., whereas, the discharge from a short 4£ inch pipe, with a head of 51 feet, ought to be 306 cubic feet per minute, which is equal to 28£ h.p. The friction of the water in passing through the pipe has, therefore consumed 2L} h.p. It will be seen from the foregoing that it is not advantageous to make use of the natural weight or pressure of water for originatitg power, by conducting it through a pipe of uniform size to a distance from its source, as a very considerable proportion of power will be expended on Diction.