es into
the flues at the after end of the bars, whereby the efficacy of the boiler
is diminished. Where the bars are very long it will generally be found that
an increased supply of steam and a diminished consumption of coal will be
the consequence of shortening them, and the bars should always lie with a
considerable inclination to facilitate the distribution of the fuel over
the after part of the furnace. When there are two lengths of bars in the
furnace, it is expedient to make the central cross bar for bearing up the
ends double, and to leave a space between the ends of the bars so that the
ashes may fall through between them. The space thus left enables the bars
to expand without injury on the application of heat, whereas without some
such provision the bars are very liable to get burned out by bending up in
the centre, or at the ends, as they must do if the elongation of the bars
on the application of heat be prevented; and this must be the effect of
permitting the spaces at the ends of the bars to be filled up with ashes.
At each end of each bed of bars it is expedient to leave a space which the
ashes cannot fill up so as to cause the bars to jam; and care must be taken
that the heels of the bars do not come against any of the furnace bearers,
whereby the room left at the end of the bars to permit the expansion would
be rendered of no avail.

379. _Q._--Have you any remarks to offer respecting the construction and
arrangement of the furnace bridges and dampers of marine boilers?

_A._--The furnace bridges of marine boilers are walls or partitions built
up at the ends of the furnaces to narrow the opening for the escape of heat
into the flues. They are either made of fire brick or of plate iron
containing water: in the case of water bridges, the top part of the bridge
should be made with a large amount of slant so as to enable the steam to
escape freely, but notwithstanding this precaution the plates of water
bridges are apt to crack at the bend, so that fire brick bridges appear on
the whole to be preferable. In shallow furnaces the bridges often come too
near the furnace top to enable a man to pass over them; and it will save
expense if in such bridges the upper portion is constructed of two or three
fire blocks, which may be lifted off where a person requires to enter the
flues to sweep or repair them, whereby the perpetual demolition and
reconstruction of the upper part of the bridge will be prevented.

380.