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75% as the efficiency of the motor through which the power is utilized, this rate would give 1·83d. per brake or effective h.p. hour. This cost seems high, and it is difficult to believe that it is the best hydraulic power transmission can accomplish having regard to the well-established fact that the mechanical efficiency of a steam pumping engine is greater than any other application of a steam-engine, and that the power can be conveyed through mains without any material loss for considerable distances. Still, no other system of power transmission except gas seems to be better off, and there is no method of transmission by which energy could, at the present time, be supplied retail in towns with commercial success at such an average rate when steam is employed as the prime mover.

Fig. 4.

The average rate charged for hydraulic power in London and elsewhere is much the same as the average rate charged for the supply of electrical energy to the ordinary consumer. Gas is undoubtedly cheaper, but in a large number of cases is mechanically inconvenient in its application. Hydraulic pressure, electrical energy and compressed air (with reheating) can all be transmitted throughout towns with approximately the same losses and at the same cost, because the power is obtained in each system from coal, boilers, and steam-engines, and the actual loss in transmission can be kept down to a small percentage. The use of any particular system of power does not, however, primarily depend upon the cost of running the central station and distributing the power, but mainly upon the mechanical convenience of the system for the purpose to which it is applied. One form of energy is, in practice, found most useful for one purpose, another form for another and no one can command the whole field.

Fig. 5.

When water is employed as the fluid in hydraulic transmission the effects of frost must usually be provided against. In London and other towns, the water, before being pumped into the mains, is passed through the surface condensers of the engines, so as to raise its temperature. The mains are laid 3 ft. below the surface of the ground. Exposed Precautions against Frost. pipes and cylinders are clothed, and means provided for draining them when out of use. When these simple precautions are adopted damage from frost is very rare. In special cases oil having a low B freezing point is used, and in small plants good results have been obtained by mixing glycerin and methylated spirit with the water. A few gas jets judiciously distributed are of value where there is a difficulty in properly protecting the machinery by clothing.

From the central station the hydraulic power must be transmitted through a system of mains to the various points at which it is to be used. In laying out a network of mains it is first necessary to determine what velocity of flow can be allowed. Owing to the weight of water, the medium usually employed for hydraulic transmission, a low velocity is necessary in order to avoid shocks. The loss of pressure due to the velocity is

Fig. 6.-Half section and elevation at AB. Detail of 10″ steel pipe.

independent of the actual pressure employed, and at moderate velocities of 3 to 4 ft. per second the loss per 1000 yds. is almost a negligible quantity at a pressure of 700 Tb per square inch. For practical purposes Box's formula is sufficiently accurate—

Loss of head=gallons × length in yards/(diameter of pipes in inches × 3)⁵. There is a further loss due to obstruction caused by valves and bends, but it has been found in London that a pressure of 750 ℔ at the central accumulators is sufficient to ensure a pressure of 700 ℔ throughout the system.

Fig. 6.—Half back elevation, half front elevation. Detail of 10″ steel pipe.

The greatest distance the power is conveyed from the central stations in London is about 4 m. The higher the initial velocity the more variable the pressure; and in order to avoid this variation in any large

system of mains it is usual to place additional accumulators at a