[Illustration: FIG. 141.--Cross-section of a single-acting hydraulic ram.] When inactive the valve _D_ stands open and as the water enters from the pipe _A_, it flows through the valve to the waste pipe but as soon as the full force of the water bears on the valve it will suddenly close. This sudden stop of the flowing water will lift the valve _E_, and the energy of flow, due to its sudden stopping, will force some of the water into the chamber _G_. As this action occurs the upward pressure against the valve _D_ is released and it reopens but immediately closes again as the water begins to flow. This process is kept up, each closure of the valve sending a little water into the air chamber. As the water gradually fills the air chamber, it is subjected to the same action as was described in the pressure tank, the air above the surface being compressed and the pressure developed in the space _G_ forces the water out through the delivery pipe where it attains a force that is a factor of the height of the original fall. The air in the chamber _G_, is subject to the same conditions of loss as that of the pressure tank, and to be assured of a supply to give pressure to the water, some air must be carried into the chamber with the water. For this purpose the valve _F_ provided. After the chamber is partially filled, there occurs a reaction in the flow of water at each closure of the valve, which causes a little air to be drawn in through the valve _F_ with each impulse. This air bubbles up through the water and enters the chamber where it assures an elastic cushion for closing the valve _E_. The flow of water from the supply pipe is regulated at _H_ by a nut on the stem of the impetus valve which permits its regulation. Closing the valve slightly causes a less supply of water to be delivered; opening the valve wider gives a greater supply. [Illustration: FIG. 142.--Sectional view of a double-acting hydraulic ram.] =The Double-acting Hydraulic Ram.=--The diagram of Fig. 142 illustrates the working principle of the double-acting hydraulic ram mentioned above; where the water from a muddy stream is used to drive the ram and that from a separate source, as a spring is delivered. The construction of the double-acting ram is similar to the single-acting ram, but a separate pipe _S_ discharges spring water directly below the valve which acts just as though it had entered at the drive pipe. The ram in this case is receiving water from the drive pipe _D_, which operates the valve and furnishes power for elevating the spring water. The spring water enters the ram through the pipe _S_, to keep the space _T_ filled, directly under the valve. The water which enters the air chamber is, therefore, only that from the spring. A standpipe is arranged as shown in the figure, with a check valve to prevent the water in the ram from being forced back into the spring water pipe after entering the ram. DOMESTIC WATER-SUPPLY PLANTS