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Rh face of No. 1, so that all the driver has to do when he wishes to increase or decrease the steam supply to his engine is to shift a lever by the seat, which moves the shaft, on which there are eight cams (fig. 13) of varying eccentricity, when the stroke of his pump will be proportionately altered. It should be added that the roller R is kept down on to the cams by a spring, which will be noticed under the lever (fig. 13).

Water Gauges.—All fire-tube and water-tube boilers, i.e. all boilers in which any appreciable quantity of water is carried, are fitted with a water gauge, as they are not sufficiently thick to resist damage from burning should the water level be allowed to become so low that the tubes and tube plate become unduly heated. A water gauge is to all intents and purposes a glass window in the boiler. It takes the form of two taps, one below the water level, and the other above. These two taps are connected by a glass tube, provision being made to prevent leakage where the glass joins the taps. In the steam cars, as the boiler is out of sight of the driver, under the seat, tubes are provided so that the water gauge can be placed nearer to him, and a looking-glass is fitted on the dashboard, reflecting the gauge to the driver's eye.

The Klinger gauge (fig. 14) is a great improvement on the ordinary type, as it is provided with a glass window of prismatic section, and the result is that the water is shown black, and its height is much more easily read than with the ordinary glass gauge-tube. Should the driver have reason to doubt the accuracy of his gauge, he can test it by opening the tap at the bottom. All water gauges have a similar tap.

Pressure Gauges.— Pressure of steam in the boiler is expressed in this country in pounds to the square inch, and