NOTE.—_Hydraulic Pressure Transmission._ Water under high pressure—500 to 3000 lbs. per square inch and upwards—affords a very satisfactory method of transmitting power to a distance, especially for the movement of heavy loads at small velocities, as by cranes and elevators. The system consists usually of one or more pumps capable of developing the required pressure; 2, accumulators, described on the next page; 3, the distributing pipes, and 4, the presses, cranes, or other machinery to be operated. This property of fluids invests us with a power of increasing the intensity of a pressure exerted by a comparatively small force, without any other limit than that of the strength of the materials of which the engine itself is constructed. It also enables us with great facility to transmit the motion and force of one machine to another, in cases where local circumstances preclude the possibility of instituting any ordinary mechanical connection between the two machines. Thus, merely by means of water-pipes, the force of a machine may be transmitted to any distance, and over inequalities of ground, or through any other obstructions. THE HYDRAULIC ACCUMULATOR. This useful and indispensable apparatus was designed by Sir William Armstrong. Its use was to secure _a uniform pressure of water in a reservoir by weight_ so that however much or little of this water was used the pressure would remain constant. [Illustration: ELEVATION. FIG. 147.] In the first accumulator which is still in use the ram was attached to the foundation while the cylinder rose and fell as the pressure was utilized. The weights were annular in shape and were hung upon the outside of cylinder. In the modern types of accumulators the cylinder is stationary and the ram supporting the weights is made to rise and fall. _By means of a hydraulic accumulator a uniform pressure can be established and maintained on all parts of a hydraulic main or system._ [Illustration: PLAN AT BOTTOM. FIG. 148.] The volume of water which is used intermittently for the purpose of operating presses—draw-benches for brass and copper tubing and the like is replaced by a pump or pumps which are started and stopped automatically by a connection between the accumulator and the throttle or belt shifter of the pump. The accumulator is used for a double purpose of maintaining a constant pressure and to store up any surplus force of the pumps. The friction loss in the transmission of power by water through mains is very small, as for example: _It has been found that water under a pressure of 700 lbs. per square inch may be transmitted through well proportioned mains, one mile with a loss of only two per cent._ The useful work stored in an accumulator may be calculated by the following rule: _Multiply the area of ram in square inches by the length of the stroke in inches by the pressure m pounds per square inch divided by 33,000 lbs. the equivalent of one H. P._