agon 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 deg., 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
the boiler, and then ascends the chimney. The performance of this course by
the smoke is what is termed a wheel draught, as the smoke wheels once round
the boiler, and then ascends the chimney.

86. _Q._--Is the performance of this course by the smoke universal in wagon
boilers?

_A._--No; such boilers sometimes have what is termed a split draught. The
smoke and flame, when they reach the end of the boiler, pass in this case
through an iron flue or tube, reaching from end to end of the boiler; and
on arriving at the front of the boiler, the smoke splits or separates--one
half passing through a flue on the one side of the boiler, and the other
half passing through a flue on the other side of the boiler--both of these
flues having their debouch in the chimney.

87. _Q._--What are the appliances usually connected with a wagon boiler?

_A._--On the top of the boiler, near the front, is a short cylinder, with a
lid secured by bolts. This is the manhole door, the purpose of which is to
enable a man to get into the inside of the boiler when necessary for
inspection and repair. On the top of this door is a small valve opening
downward, called the atmospheric valve. The intention o