urning to Fig. 6, we see a pipe through which
the exhaust steam from the locomotive's cylinders is shot upwards into
the funnel, in which, and in the smoke-box beneath it, a strong vacuum
is formed while the engine is running. Now, "nature abhors a vacuum," so
air will get into the smoke-box if there be a way open. There
is--through the air-doors at the bottom of the furnace, the furnace
itself, and the fire-tubes; and on the way oxygen combines with the
carbon of the fuel, to form carbon dioxide. The power of the draught is
so great that, as one often notices when a train passes during the
night, red-hot cinders, plucked from the fire-box, and dragged through
the tubes, are hurled far into the air. It might be mentioned in
parenthesis that the so-called "smoke" which pours from the funnel of a
moving engine is mainly condensing steam. A steamship, on the other
hand, belches smoke only from its funnels, as fresh water is far too
precious to waste as steam. We shall refer to this later on (p. 72).

BOILER FITTINGS.

The most important fittings on a boiler are:--(1) the safety-valve; (2)
the water-gauge; (3) the steam-gauge; (4) the mechanisms for feeding it
with water.

THE SAFETY-VALVE.

Professor Thurston, an eminent authority on the steam-engine, has
estimated that a plain cylindrical boiler carrying 100 lbs. pressure to
the square inch contains sufficient stored energy to project it into the
air a vertical distance of 3-1/2 miles. In the case of a Lancashire
boiler at equal pressure the distance would be 2-1/2 miles; of a
locomotive boiler, at 125 lbs., 1-1/2 miles; of a steam tubular boiler,
at 75 lbs., 1 mile. According to the same writer, a cubic foot of heated
water under a pressure of from 60 to 70 lbs. per square inch has _about
the same energy as one pound of gunpowder_.

Steam is a good servant, but a terrible master. It must be kept under
strict control. However strong a boiler may be, it will burst if the
steam pressure in it be raised to a certain point; and some device must
therefore be fitted on it which will give the steam free egress before
that point is reached. A device of this kind is called a _safety-valve_.
It usually blows off at less than half the greatest pressure that the
boiler has been proved by experiment to be capable of withstanding.

In principle the safety-valve denotes an orifice closed by an
accurately-fitting plug, which is pressed against its seat on the boiler
top by a w