It is not generally realized that water in motion also acquires kinetic energy. Whenever a faucet is turned off very quickly, there is a hammering sound which is due to the fact that the moving water in the water pipe is brought to an abrupt stop. This puts a severe strain on the piping. A great deal of trouble was experienced from this source in the early days of plumbing. At a hospital in Bristol, England, there was a lead pipe leading from a cistern in one of the upper stories to the kitchen. Every time the faucets were turned off abruptly the momentum of the water caused the lead pipe to expand, and every now and then the pipe was burst. In order to relieve the situation, a plumber connected a pipe to the faucet and carried it up the side of the building to the level of the cistern. His idea was that whenever the water was turned off suddenly it would have a vent leading up to the level of the water reservoir. Much to his surprise, the water issued from the pipe in a jet of considerable height. To prevent the escape of the water, he extended the pipe considerably, and still a jet of water would issue from it. Eventually the relief pipe was carried up twice the height of the cistern and even then the water would squirt out occasionally when the faucets in the kitchen were turned off very suddenly. Then the idea was conceived of placing a reservoir on one of the upper floors of the hospital and letting the jet of water fill this reservoir. Every time the faucet was operated in the kitchen a certain amount of water flowed into the new cistern, and in this way it was kept supplied with enough water to furnish that which was required for the upper floors of the hospital. [Illustration: FIG. 33.--SECTIONAL VIEW OF A WATER RAM] It is on this principle that the hydraulic ram operates. Water from a stream is made to flow down a pipe, and as it gains velocity a check valve suddenly stops the flow which produces enough pressure to force open a valve in an air chamber and let some of the water enter the chamber. As soon as the pressure is relieved the check valve opens and the valve into the chamber closes automatically until a moment later the stream of water has gained sufficient velocity to repeat the performance. Thus an intermittent jet of water is forced into the air chamber and thence through a pipe to a reservoir. The height to which the water will rise depends entirely upon the velocity of the water flowing through the system. The air chamber is necessary to cushion the action of the hydraulic ram and provide a fairly steady pressure upon the water that flows up through the vent pipe. The check valve is entirely automatic. It is held open against the pressure of the water by a spring or a weight, but when the water is in motion is dragged shut, only to spring open again when the pressure is reduced by the escape of the water into the air chamber. [Illustration: FIG. 34.--THE GYRATING WATER METER]