is open or shut, steam is admitted or shut off from the engine. The type of boiler we have been describing is fitted to nearly all the smaller and lighter steam carriages, though in the majority of cases steel enters more largely into the construction of the boiler, so that the wire winding is not used.
Boilers with tubes surrounded by water, up which the heat of combustion passes, are known as fire-tube boilers, and are the same in principle as those used on a railway locomotive, though in the latter case the boiler is horizontal, instead of vertical. There is another type of boiler which is largely used on steam lorries and other heavy steam automobiles, which is known as the water-tube type. In this the water is contained in tubes, which the fire surrounds—the
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Fig. 8.—Section of Toledo Boiler
exact opposite to the arrangement in the fire-tube boiler.
Fig. 8 is a section of the Toledo boiler, which is a combination of the two types, as an internal chamber is surrounded by water and filled with heat from the burner, but to increase the heating surface a coil of spiral tube is introduced, starting near the bottom of the water, and passing up into the steam space. This induces a very violent circulation of the water, and, although only one coil is shown, eight are actually used, so that the central vessel of the boiler is almost filled with these coils. All boilers are fitted with an automatic safety valve, which releases surplus steam long before the pressure can become dangerously high. The way in which the boilers are supplied with water will be dealt with later.
The Flash Boiler.—All fire-tube and water-tube boilers carry a considerable quantity of water, but the flash generator is not a boiler at all in the ordinary sense of the word, and contains practically no water. So far as steam generation is concerned, the principle of the flash boiler may be likened to dropping water on a red-hot iron. A small stream of water is
