[Illustration: FIG. 5.]

Still, a lot of the heat gets away. In a steam-boiler the burning fuel
is enclosed either by fire-brick or a "water-jacket," forming part of
the boiler. A water-jacket signifies a double coating of metal plates
with a space between, which is filled with water (see Fig. 6). The fire
is now enclosed much as it is in a kitchen range. But our boiler must
not be so wasteful of the heat as is that useful household fixture. On
their way to the funnel the flames and hot gases should act on a very
large metal or other surface in contact with the water of the boiler, in
order to give up a due proportion of their heat.

[Illustration: FIG. 6.--Diagrammatic sketch of a locomotive type of
boiler. Water indicated by dotted lines. The arrows show the direction
taken by the air and hot gases from the air-door to the funnel.]

THE MULTITUBULAR BOILER.

[Illustration: FIG. 7.--The Babcock and Wilcox water-tube boiler. One
side of the brick seating has been removed to show the arrangement of
the water-tubes and furnace.]

To save room, boilers which have to make steam very quickly and at high
pressures are largely composed of pipes. Such boilers we call
multitubular. They are of two kinds--(1) _Water_-tube boilers; in which
the water circulates through tubes exposed to the furnace heat. The
Babcock and Wilcox boiler (Fig. 7) is typical of this variety. (2)
_Fire_-tube boilers; in which the hot gases pass through tubes
surrounded by water. The ordinary locomotive boiler (Fig. 6) illustrates
this form.

The Babcock and Wilcox boiler is widely used in mines, power stations,
and, in a modified form, on shipboard. It consists of two main
parts--(1) A drum, H, in the upper part of which the steam collects; (2)
a group of pipes arranged on the principle illustrated by Fig. 5. The
boiler is seated on a rectangular frame of fire-bricks. At one end is
the furnace door; at the other the exit to the chimney. From the furnace
F the flames and hot gases rise round the upper end of the sloping tubes
TT into the space A, where they play upon the under surface of H before
plunging downward again among the tubes into the space B. Here the
temperature is lower. The arrows indicate further journeys upwards into
the space C on the right of a fire-brick division, and past the down
tubes SS into D, whence the hot gases find an escape into the chimney
through the opening E. It will be noticed that the greatest heat is
brought t