The principle of construction of the dial gauge is that the pressure may be indicated by means of a spring and pointer upon a divided dial similar to a clock face, but marked in divisions, indicating pounds, hundreds, etc., _pressure_ instead of hours and minutes. The more approved forms of gauges are now constructed upon the principle of the _Bourdon_ spring or metallic barometer invented in 1849. (See page 113 for illus.) [Illustration: FIG. 108.] The essential principle—or discovery—is this: that a metal curved tube—oval cross section, _under pressure, tends to straighten itself_ according to the force exerted by the pressure inside. Figs. 107 and 108 show the ordinary style of gauge which consists of an elliptical tube, connected at one end to a pipe in communication with the pressure, and at the other end with toothed arc and pinion to a pointer spindle as shown in cuts. NOTE.—Hydraulic gauges are indispensable as it is often necessary to stop the pressure at points below that at which the safety valve has been set. Within the gauges—or cases, is a small coiled tube closed at one end, while the other end is attached to the socket through which the water is admitted; this tube has a tendency to straighten when under pressure, and thus its free closed end moves, and this motion is communicated to the pointer; when the pressure is relieved the tube assumes its original position and the pointer returns to zero. There are many modifications and special adaptations of the Bourdon discovery, but the principle remains, and the same useful results are obtained with both single and double tubes, the latter being the most resensitive. Fig. 107 shows the dial of a hydraulic gauge which is graduated to suit the work to which it is related. These gauges are made for pressures from 1,000 to 20,000 pounds per square inch. The springs are formed of heavy solid bar steel turned and bored to size and are of the Bourdon style. They are in use in large railroad shops, sugar refineries and cotton-seed oil mills. These gauges are also made with connections through the back of the case. The gauge illustrated in Fig. 108 is used in connection with hot water heaters, denoting the height of column of water in the tank or reservoir, one hand being painted red and the other black. As it is necessary to have at all times in the tank or reservoir a certain height of water, the red hand is set at the point on the dial which denotes this height. The black hand is connected with the working parts of the gauge and indicates on the dial the actual height of water in the tank or reservoir. The dial of this gauge is graduated in feet, instead of pounds. A check valve is almost indispensable in using a hydraulic gauge, as the pressure is often suddenly removed and the momentum of the hand will throw the pinion out of gear with the toothed arc, and is liable to break the hair-spring. A check valve prevents any trouble of this kind and should always be used.