HYDRAULIC TRANSMISSION. The fact that water confined within an unyielding encasement transmits equally in all directions any pressure put upon it gives rise to the principle of hydraulic transmission—the transference of power by means of a column of water under pressure. In the industrial appli cation of this idea the column of water is held in a pipe, which may be vertical, horizontal, in clined or in any combination of these positions: wherever the water reaches, the pressure under which it has been placed is available as power at any of the pipe line. In large cities this principle is made use of for supplying many power users from central stations by means of water pipes laid in the streets. Lon don has the largest development of this form of power transmission, having upwards of 150 miles of pressure mains, some of them seven inches in diameter, carrying water under 750 pounds to the square inch. At Glasgow and Manchester the pressure carried is 1,120 pounds to the square inch. Many other British cities have similar hydraulic power service. The pressure upon the water is supplied by steam pumps at the central stations. The steam is used expansively in high and low pressure cylinders working directly upon single-aoting pumps of small diameter. The action is a direct reversal of the hydraulic press (q.v.). The
water from the pumps is forced into storage cylinders known as hydraulic accumulators. These cylinders are fitted with pistons which are weighted with loads per square inch equal i to the pressure the confined water is to carry. The water from the pumps enters at the bot tom of the accumulator at a little higher pres sure than is represented by the weight, and lifts the piston with its load, thus storing power in proportion to the amount of the confined water. In the case of the London service the pres sure in the steam cylinders is 80 pounds to the square inch, and in the accumulators 800 pounds to the square inch — the areas of the steam cylinders to the pump cylinders being as 10 to 1. The high pressure power is used geni erally through Pelton wheels. The cost of hydraulic power in London is equivalent to about five cents per horse power per hour — from 30 to 60 per cent less than the cost of electric power.
The natural gravity pressure of water in pipe lines leading from elevated reservoirs is equally available for power, and is usually passed through an accumulator to render the pressure uniform. Each 32 feet of elevation contributes about 14 pounds pressure to the square inch.