led combustion, is the
act which heats the boiler. Only when the carbon present has combined
with the greatest possible amount of oxygen that it will take into
partnership is the combustion complete and the full heat-value (fixed by
scientific experiment at 14,500 thermal units per pound of carbon)
developed.

Now, carbon may unite with oxygen, atom for atom, and form _carbon
monoxide_ (CO); or in the proportion of one atom of carbon to _two_ of
oxygen, and form _carbon dioxide_ (CO_2). The former gas is
combustible--that is, will admit another atom of carbon to the
molecule--but the latter is saturated with oxygen, and will not burn,
or, to put it otherwise, is the product of _perfect_ combustion. A
properly designed furnace, supplied with a due amount of air, will cause
nearly all the carbon in the coal burnt to combine with the full amount
of oxygen. On the other hand, if the oxygen supply is inefficient, CO as
well as CO_2 will form, and there will be a heat loss, equal in
extreme cases to two-thirds of the whole. It is therefore necessary that
a furnace which has to eat up fuel at a great pace should be
artificially fed with air in the proportion of from 12 to 20 _pounds_ of
air for every pound of fuel. There are two methods of creating a violent
draught through the furnace. The first is--

The _forced draught_; very simply exemplified by the ordinary bellows
used in every house. On a ship (Fig. 10) the principle is developed as
follows:--The boilers are situated in a compartment or compartments
having no communication with the outer air, except for the passages down
which air is forced by powerful fans at a pressure considerably greater
than that of the atmosphere. There is only one "way out"--namely,
through the furnace and tubes (or gas-ways) of the boiler, and the
funnel. So through these it rushes, raising the fuel to white heat. As
may easily be imagined, the temperature of a stokehold, especially in
the tropics, is far from pleasant. In the Red Sea the thermometer
sometimes rises to 170 deg. Fahrenheit or more, and the poor stokers have
a very bad time of it.

[Illustration: FIG. 10.--Sketch showing how the "forced draught" is
produced in a stokehold and how it affects the furnaces.]

[Illustration: SCENE IN THE STOKEHOLD OF A BATTLE-SHIP.]

The second system is that of the _induced draught_. Here air is
_sucked_ through the furnace by creating a vacuum in the funnel and in a
chamber opening into it. T