Both iron and brass pipes must be put up with care to allow for expansion, by arranging long, right-angled turns in them, which can spring a little if necessary. Without this precaution the contraction of a long, straight line of such pipe strains the joints, and soon causes leakage. Lead pipes are not liable to leaks from this cause, as the metal is soft enough to yield under the strain; but, if improperly put up, with iron hooks instead of wide and smooth brass bands, or hard-metal "tacks" soldered to the pipes, the alternate expansion and contraction of the pipe as it passes over the hooks causes these to cut into the soft lead, so that in course of years they may be cut so nearly through as to be unable to resist the pressure of the water in them, and a bad leak is the result.

Defects in supply-pipes.

The softness of lead exposes pipes made of it to another danger, from the so-called "water-hammer." It is a familiar property of water that, being practically incompressible, it transmits through its whole substance with undiminished force a blow struck anywhere upon it. Where the water-supply of a house is under a high pressure, the sudden closing of a faucet to shut off a flowing stream operates as a blow upon the water, the force of which is transmitted through the entire system of cold-water pipes, causing the concussion so familiar to housekeepers. As the concussion acts through all portions of the system, it produces most effect at the weakest part; and, where it has not been properly guarded against, some pipe, often remote from the faucet to which the trouble is due, will begin to swell from the internal pressure. As soon as it begins to yield, the force of the water-hammer, acting on a larger surface, increases its effect, and the bulge enlarges until the pipe bursts. This is a less common accident now than it was in former times, as the compression-faucets at present generally used shut off the water much more gradually than the old-fashioned ground-cocks; but it still sometimes occurs, particularly where self-closing cocks are used, and the way to avoid it, or to prevent its repetition, is to arrange "air-chambers," or extensions of the supply-pipes, generally over or near the faucets in the lower stories of the house, where the pressure is greatest. These air-chambers are simply extra lengths of pipe, closed at the end, and usually turned upward. They are full of imprisoned air, which cannot escape at the bottom against the pressure of the water, or at the top, this being closed, and therefore remains, like an elastic cushion, ready to receive and absorb the shocks transmitted to it through the water. To be of much use, such air-chambers should be eighteen inches or two feet long; but, if of proper size, they act very efficiently to reduce water-hammer, so long as any air remains in them. Under heavy pressures, however, the air in them is gradually dissolved in the water until they are completely filled with water, and their action as air-cushions ceases. The water-hammer then reappears; but, in order to restore the efficacy of the air-chambers, it is only necessary to shut the water off at the stopcock always provided just inside the cellar wall, and let the house-pipes drain out through the waste-tube of the stop-cock. On opening the stop-cock again so as to let the water into the house-pipes, the air-chambers will be found filled with air, ready for renewed usefulness.

Water-hammer.

Air-chambers.

Where the supply-pipes are intact, the faucets may drip, or leak more copiously, wasting water in an annoying way. The old-fashioned "ground-cocks," still extensively used, contain a tapering brass plug, with a hole through it (Fig. 27) which can be turned to coincide with the bore of the faucet, or at any angle with it, either by a handle attached to the top of the plug, or, in the case of "swing basin-cocks," by swinging the faucet itself, the plug remaining stationary. In either case, particles of sand in the water soon cut grooves around the plug and its seating, through which water finds its way, causing a dripping which is practically incurable.

As the leakage of a ground-cock can be remedied only by putting in a new faucet, most modern houses are fitted with "compression-cocks,"

Fig. 27.

Faucets.

Ground-cocks.

Compression-cocks.

which are made on a different principle (Fig. 28), the flow of water through them being checked by pressing a spindle, armed with a leather or rubber washer, upon the end of a tube, through which the water passes on its way to the outlet of the faucet. The pressure is usually applied by screwing down the spindle, by means of a handle at the top; but a variety is often used in which a lever takes the place of the screw. The washer is attached to the end of the spindle by a short, brass screw, passing through a hole in the washer, and it is the work of a few moments only to take off a washer and replace it by a new one. This is an operation frequently necessary, as the washers wear rapidly, especially where exposed to hot water. The first step in the process is to shut off the water temporarily from the faucet to be operated upon. This is not absolutely necessary, as, in case of emergency, a worn washer may be replaced without shutting off the water; but, as the water spouts in streams through the top of the faucet during the operation, it is more comfortable for the operator to have it shut off.

Where the patient is a hot-water faucet, as is most commonly the case, the easiest way to shut off the water, where the hot-water system is supplied from a tank in the attic, is generally to close the opening in the bottom of the tank, through which the water descends to the boiler. Usually, this is controlled by a "cistern-valve," which is simply a heavy weight, with a leather washer on the bottom, and a guide to direct its movement, which can be dropped upon the end of the descending pipe. In most cases the cistern-valve has a copper wire attached to it, with two loops at the end, which can be hooked over a nail, or screw, somewhere at the top of the tank, to keep the wire from falling into the tank. When the hot-water system is in operation, the wire is hung by the lower loop, which holds the cistern-valve up, away from the mouth of the pipe; and, to close the latter, the wire is simply unhooked, the valve dropped, and the wire again hung by the upper loop, to prevent it from being lost in the water.