boiler, and the tubes belonging to one
furnace may be swept if necessary at sea without interfering injuriously
with the action of the rest. In a steam vessel it is necessary at intervals
to empty out one or more furnaces every watch to get rid of the clinkers
which would otherwise accumulate in them; and it is advisable that the
connection between the furnaces should be such that this operation, when
being performed on one furnace, shall injure the action of the rest as
little as possible.

383. _Q._--Can any constructive precautions be taken to prevent the
furnaces and tube plates of the boiler from being burned by the intensity
of the heat?

_A._--The sides of the internal furnaces or flues in all boilers should be
so constructed that the steam may readily escape from their surfaces, with
which view it is expedient to make the bottom of the flue somewhat wider
than the top, or slightly conical in the cross section; and the upper
plates should always be overlapped by the plates beneath, so that the steam
cannot be retained in the overlap, but will escape as soon as it is
generated. If the sides of the furnace be made high and perfectly vertical,
they will speedily be buckled and cracked by the heat, as a film of steam
in such a case will remain in contact with the iron which will prevent the
access of the water, and the iron of the boiler will be injured by the high
temperature it must in that case acquire. To moderate the intensity of the
heat acting upon the furnace sides, it is expedient to bring the outside
fire bars into close contact with the sides of the furnace, so as to
prevent the entrance of air through the fire in that situation, by which
the intensity of the heat would be increased. The tube plate nearest the
furnace in tubular boilers should also be so inclined as to facilitate the
escape of the steam; and the short bent plate or flange of the tube plate,
connecting the tube plate with the top of the furnace, should be made with
a gradual bend, as, if the bend be sudden, the iron will be apt to crack or
burn away from the concretion of salt. Where the furnace mouths are
contracted by bending in the sides and top of the furnace, as is the
general practice, the bends should be gradual, as salt is apt to accumulate
in the pockets made by a sudden bend, and the plates will then burn into
holes.

384. _Q._--In what manner is the tubing of boilers performed?

_A._--The tubes of marine boilers are generally