a small instrument known as a steam pressure gauge is used for the purpose. This is connected to the boiler by a pipe, and is fixed on the dashboard in front of the driver, and a hand on the dial indicates the steam pressure in the boiler (fig. 15). The interior mechanism of the gauge is extremely simple. It consists in a tube bent in a loop, as shown. One end is open to the steam pressure of the boiler, and the other end is closed, the closed end being attached to a short arm, which moves a rack engaging with a small toothed wheel

Steam pressure Gauge with face removed and Bourdon Tube in section		Steam pressure Gauge, ordinary appearance

behind the dial, and so turning the hand on its face. The gauge depends for its working on the fact that a bent tube, when subjected to internal pressure, tends to straighten itself out.

The Engine.—Having seen how the heat is supplied by the burner and steam by the boiler, we turn to the engine. This description of the engine need not be read by the novice unless he likes, as it deals with a portion of the car which is quite automatic, and which is exactly the same in its operation