Transmission of Power. It is generally necessary to convey the power developed by a steam-eugine or other prine mover, to a greater or less distance before it call be made available. Thus in factories rotating shafts and belt- or rope-gearing (q.v.) are employed to distribute to various machines the power of a main engine. In many cases power can be cheaply generated at some place where it is not wanted; and it is, therefore, of great importance to have some efficient method of transmitting it to places, possibly at a considerable distance, where it can be made use of. Rope-gearing, on an extended scale, may be used, but a long length of wire rope must be supported at frequent intervals upon pulleys, and the consequent friction produces a serious loss. The system is, however, rather extensively used for the propulsion of tram-cars, as the excess of the cost of power produced by horses over that of a steam engine is sufficient to allow of a large loss in transmission. A continuous loop of steel wire cable travels in a slotted tube, buried in the roadway, being, of course, supported in suitable pulleys, and is kept in motion by passing round a drum in the engine-house, this drum being turned by an engine. A clutch on each car projects into the tube, and can be caused to grip the moving cable as required. Fluid pressure - of steam, air, or water - is used for power transmission in various ways. Steam from a boiler may be conveyed for a limited distance in pipes if precautions are taken to avoid loss of heat by radiation, and may, of course, supply an engine of any kind, but the loss due to condensation is so serious that the distance can only be comparatively small. Compressed air is frequently used to supply power at the bottoms of mines, an engine on the surface pumping the air into a pipe leading to an air-engine below. The heat produced during the compression of the air [THERMODYNAMICS], is, however, a source of loss. Hydraulic power is of much more general application, for water at a pressure of 700 or more pounds per square inch. may be conveyed for long distances in pipes with but little loss by friction, and can be conveniently applied to the operation of lifts, cranes, riveting and other machines. Power is in this way distributed over a considerable portion of London and other large towns, as well as in docks, on board ships, etc. For most purposes electricity seems the most efficient agent for the transmission of power, [DYNAMO MACHINE.] The substitution of small electric motors for the shafting and belts of many factories would result in an economy, and it is proved by experience in America and other places that, when the conditions are suitable, power can be supplied to tram-cars by electricity more efficiently than by any other means. The fact that a high-tension current can traverse a long wire with a trifling loss has already rendered it possible to transmit power efficiently to a distance of more than 100 miles, and the work of distributing some of the power of Niagara Falls is now in progress, with every prospect of being, scientifically at any late, a success.