is always a high-pressure engine; the others are, for the most part,
condensing engines.

84. _Q._--Will you explain the construction and action of the single-acting
engine, used for draining mines?

_A._--Permit me then to begin with the boiler, which is common and
necessary to all engines; and I will take the example of a wagon boiler,
such as was employed by Boulton and Watt universally in their early
engines, and which is still in extensive use. This boiler is a long
rectangular vessel, with a rounded top, like that of a carrier's wagon,
from its resemblance to which it derives its name. A fire is set beneath
it, and flues constructed of brickwork encircle it, so as to keep the flame
and smoke in contact with the boiler for a sufficient time to absorb the
heat.

[Illustration: Fig. 3]

85. _Q._--This species of boiler has not an internal furnace, but is set in
brickwork, in which the furnace is formed?

_A._--Precisely so. The general arrangement and configuration will be at
once understood by a reference to the annexed figure (fig. 3), which is a
transverse section of a wagon boiler. The line b represents the top of the
grate or fire bars, which slope downward from the front at an angle of
about 25°, giving the fuel a tendency to move toward the back of the grate.
The supply of air ascends from the ash pit through the grate bars, and the
flame passes over a low wall or bridge, and traverses the bottom of the
boiler. The smoke rises up at the back of the boiler, and proceeds through
the flue F along one side to the front, and returns along the other side of