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254 Its mission is to, as far as possible, prevent the escape of heat, as any loss of heat means that more fuel must be burned to maintain steam.

As the heat passes up the boiler tubes, after heating the bottom of the boiler, the water is heated and boiled, and the steam rises, filling the space between the water-level and the top of the boiler. This space is very much less than the steam would naturally occupy, and, consequently, pressure soon becomes high. The heat from the fire in the boiler tubes not



only boils the water around the portion of each tube surrounded by water, but in the upper part its heat tends to dry the steam and keep it from being too wet for satisfactory use in the engine.

The hot gases then pass into a box on the top of the boiler, called the 'smoke-box,' and thence into the down-take, or chimney projecting below the car. The steam from the boiler passes along the pipe to the throttle-valve to engine, the handle of which is by the side of the driver, and as this throttle