BOILERS FOR MARINE PURPOSES. 53

from getting rubbed away by the mere friction of the tools forraking out the ashes.

Some boilers are constructed, as it were, back to back, in onelarge boiler. By this means two ends are saved, but the greatweight of the mass deters many from adopting this plan; but wherelarge power is required in small space, the arrangement has certainadvantages. The stoke holes must be “ fore and aft;” and in generalthe fore part of the boiler is the best steam producer, owing to the

Fig. 27.—Double Boilers. Longitudinal Section and Front Elevation.aa, Furnaces. B b, Combustion chambers. cc, Tubes. d d, Smoke-boxes, ee, Uptakes.

air getting better circulated in the stoke hole, but, with suitableair funnels from the deck, the aft furnaces of the boiler can be pro-vided with the plentiful supply of air so necessary for combustion,and for keeping the stoke hole cool. There is a passage left be-tween the two boilers, forming a communication between the foreand aft stoking-rooms; two funnels are fitted, and the generalarrangement is best suited for paddle-wheel ships.

Another modification differs materially from the former example,having one combustion chamber common to both sets of furnaces.This will tend, in a great measure, to effect complete combustion,and the prevention of smoke; that is to say, if the furnaces areproperly constructed and fired—the fore and aft furnaces beingfired alternately, so that one fire is bright while the other is receiv-ing fresh fuel. To assist combustion, air is admitted through thebridge, thus getting partially heated before mixing with the flamein the combustion chamber. These boilers are made high to insureample steam room, while the large area of the uptakes inside ofthe boiler dries the steam. Indeed, some think this is by far the