COCK, or STOP-COCK. The stop-cocks in most common use consist of a short tube of brass, intersected by a nearly cylindrical plug, capable of being turned on its axis at pleasure, and so perforated or cut that, while in one position it completely prevents the passage of fluid through the pipe, it may be so turned as to permit the fluid to pass through it, and consequently to flow through the pipe. The kind of cock usually employed, under the name of a tap, for drawing off the contents of barrels, usually terminates in a curved nose, or spout, from which the liquor maybe conveniently received in a jug or other open vessel; but, where it is desired to trans fer the liquor from the cask immediately to bottles, the nose is prolonged into a long slender tube, which will enter the neck of a bottle, and obviate the necessity for a funnel. Other cocks arc adapted for insertion in me tallic cisterns or boilers, and fitted with a flange to bear against the outer face of the cistern, and a nut, wormed on to the portion of the tube of the cock which passes into the cistern, by which they may be screwed up firmly. Others again are adapted for solder ing to the end of a leaden pipe, such as the service-pipe for laying water on to a house. Some stop-cocks, again, have no nose or spout, but are made alike at each end, for the purpose of being soldered between two lengths of leaden pipe.
In some cocks the plug is so contrived as either to contain a piece of cork, which may be turned so as to close the pipe, or to revolve within a hollow cylinder of cork, with arrangements for opening or closing a passage through it at pleasure. In others the princi
ple of' the revolving plug is entirely departed from, and the pipe is closed by means of a flat disc of •metal, covered with a leather washer, and screwed up firmly against the end of the pipe, or by a conical or globular valve similarly applied. In such cocks eaoutchouc forms an excellent substitute for leather. Another ingenious class of cocks comprises those furnished with contrivances for admit ting air to a barrel as fast as the liquor is drawn off, and thereby superseding the use of a vent-peg. Another class are furnished with apparatus for locking the cock.
Cocks are most commonly made of tap metal, a peculiar alloy of inferior brass, or rather copper saturated with lead. Fine brass is sometimes used; as also a white alloy made to resist the action of vinegar and other cor rosive liquors. For such liquors, however, cocks of porcelain or stoneware are to be pre ferred. Even wood is occasionally employed ; and attempts have been made to manufacture cocks of cast-iron.
The four-way cock consists of a plug perfo rated with two distinct passages, and mounted in a barrel into which four separate pipes or channels open. Its object is to connect the four passages with each other in alternate couples, either by a continuous revolution upon its axis, or by an alternating motion through one-fourth part of a revolution ; and it has frequently been applied to steam en gines as a means of alternately admitting steam to and allowing it to escape from each end of the engine-cylinder.