The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

FURNACE. 12:9
When fresh fuel for feeding the boiler is first introduced,
it is merely laid in the feeding mouth. Here it is exposed
to the action of a part of the heat of the burning fuel on the
grate, and undergoes, in some degree, the process of coking.
The door of the feeding mouth is furnished with small apertures for the admission of a stream of air, which carries the
smoke evolved by the coking of the fresh fuel over the burning fuel on the grate, by which this smoke is ignited, and
becomes flame, and in this state enters the flue, and circulates round the boiler. When the furnace is to be fed, the
door of the feeding mouth is opened, and the fuel which had
been laid in it, and partially coked, is forced upon the front
part of the grate. At first, its combustion being imperfect,
but proceeding rapidly, a dense black smoke arises from i..
The current of air from the open door through the feeding
mouth carries this over the vividly burning fuel in the back
part of the grate, by which the smoke, being ignited, passes
in a state of flame into the flue. When the furnace again
requires feeding, every part of this fuel will be in a state of
active combustion, and it is forced to the back part of the
grate next the flue, preparatory to the introduction of more
fuel from the feeding mouth.
The apertures in the door of the feeding mouth are furnished with covers, so that the quantity of air admitted
through them can be regulated by the workmen. The efficiency of these furnaces in a great degree depends on the
judicious admission of the air through the feeding mouth;
for if less than the quantity necessary to support the combustion of the fuel be admitted, a part of the smoke will
remain unconsumed; and if more than the proper quantity
be admitted, it will defeat the effects of the fuel by cooling
the boiler. If the process which we have just described be
considered, it will not be difficult to perceive the total impossibility in such a furnace of exactly regulating the draught
of air, so that too much shall not pass at one time, and too
little at another. When the door is open to introduce fresi
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