It is important that the whistle shall sound as soon as the steam is turned on; to ensure this great care must be taken to keep the whistle-pipe free from water.
THE STEAM GAUGE.
The principle of construction of the dial steam gauge is, that the pressure may be indicated by means of a pointer in a divided dial similar to a clock face, but marked in division, indicating pounds pressure per square inch instead of hours and minutes.
Figs. [87] and [88] show the ordinary style of gauge which consists of an elliptical tube, connected at one end to a steam pipe in communication with the boiler pressure and at the other end with gearing to a pointer spindle as shown in cut.
An inverted syphon pipe is usually formed under the gauge, its object being to contain water and thus prevent the heat of the steam injuring the machinery of the gauge, or distorting its action by expansion.
Fig. 87.
Fig. 88.