GAUGE, gaj, the name of many different instruments and appliances used for measuring various dimensions, forces, etc. The various kinds of gauge are distinguished by means of special names indicating the use to which they are applied. Among the most important con trivances of this nature are the instruments fixed to engine boilers for registering the force of the steam and the level of the water. In one of its simplest forms the pressure or steam gauge consists of a bent siphon-tube, with two unequal legs, partly filled with mercury. The top of the shorter limb is connected to a short pipe, which enters that part of the boiler which contains the steam; the other end is open to the atmosphere. A stop-cock is generally placed between the gauge and the boiler, so that it may be put in communication with the boiler at pleasure. When the stop-cock is open, the steam, acting on the mercury in one leg of the gauge, presses it down, and the mercury in the other leg rises. The difference between the two columns is the height of mercury which corre sponds to the excess of the pressure of the steam in the boiler above the pressure of the atmosphere. For high-pressure engines, how. ever, the steam-gauge usually works in the man ner of an aneroid barometer, a pointer moving on a circular scale under the influence of the motion of a corrugated diaphragm; or, as in the Bourdon gauge, the tendency of a bent tube to straighten itself tinder the influence of . the steam pressure communicates movement in a similar manner to a pointer or index hand. The water-gauge is a vertical glass tube called a gauge-glass, communicating above and below with the boiler. The gauge-glass is not fixed
directly to the boiler, but to a brass column known as the gauge-column, communicating with the boiler by two copper tubes of consider able length, the upper leading to the steam space and the lower to the water space. These tubes are fitted with cocks or valves. Two gauge glasses of different lengths are sometimes fitted to the one column. Gauge-cocks are used as checks on the water-gauges. There are usually three of them on the front of the boiler, one at the normal level of the water, one above and one below. As applied to railroad the term gayge signifies the clear distance between the rails. The usual distance in the ordinary or narrow gauge is four feet eight and one-half inches. The broad gauge of the Great Western Railway of England was formerly seven feet; the Irish, Indian and Spanish gauge is five feet six inches. Special narrow gauges have been adopted for certain lines, especially for moun tain and mineral lines, such as the three feet six inch Norwegian gauge. Gauge is also the name applied to various contrivances for meas uring any special dimension, such as the wire gauge, an oblong plate of steel, with notches of different Widths cut on the edge, and numbered, the size of the wire being determined by trying it in the different notches till one is found which it exactly fits. The thickness of sheet metal is tried by a similar gauge. See ANEMOM ETER; CALIPERS ; RAILWAYS.