_Q._--What is the benefit of bridges?

_A._--Bridges are found in practice to have a very sensible operation in
increasing the production of steam, and in some boilers in which the brick
bridges have been accidentally knocked down by the firemen, a very
considerable diminution in the supply of steam has been experienced. Their
chief operation seems to lie in concentrating the heat within the furnace
to a higher temperature, whereby the heat is more rapidly transmitted from
the furnace to the water, and less heat has consequently to be absorbed by
the flues. In this way the bridges render the heating surface of a boiler
more effective, or enable a smaller amount of heating surface to suffice.

381. _Q._--Are the bridges behind the furnaces the only bridges used in
steam boilers?

_A._--It is not an uncommon practice to place a hanging bridge, consisting
of a plate of iron descending a certain distance into the flue, at that
part of the flue where it enters the chimney, whereby the stratum of hot
air which occupies the highest part of the flue is kept in protracted
contact with the boiler, and the cooler air occupying the lower part of the
flue is that which alone escapes. The practice of introducing a hanging
bridge is a beneficial one in the case of some boilers, but is not
applicable universally, as boilers with a small calorimeter cannot be
further contracted in the flue without a diminution in their evaporating
power. In tubular boilers a hanging bridge is not applicable, but in some
cases a perforated plate is placed against the ends of the tubes, which by
suitable connections is made to operate as a sliding damper which partially
or totally closes up the end of every tube, and at other times a damper
constructed in the manner of a venetian blind is employed in the same
situation. These varieties of damper, however, have only yet been used in
locomotive boilers, though applicable to tubular boilers of every
description.

382. _Q._--Is it a benefit to keep the flues or tubes appertaining to each
furnace distinct?

_A._--In a flue boiler this cannot be done, but in a tubular boiler it is
an advantage that there should be a division between the tubes pertaining
to each furnace, so that the smoke of each furnace may be kept apart from
the smoke of the furnace adjoining it until the smoke of both enters the
chimney, as by this arrangement a furnace only will be rendered inoperative
in cleaning the fires instead of a