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 of this valve is to
prevent a vacuum from being formed accidentally in the boiler, which might
collapse it; for if the pressure in the boiler subsides to a point
materially below the pressure of the atmosphere, the valve will open and
allow air to get in. A bent pipe, which rises up from the top of the
boiler, immediately behind the position of the manhole, is the steam pipe
for conducting the steam to the engine; and a bent pipe which ascends from
the top of the boiler, at the back end, is the waste-steam pipe for
conducting away the steam, which escapes through the safety valve. This