the fire-door, communicates by large tubes with another vessel, which is removed from the furnace, and called a separator, for a reason which will presently be explained.
Now, suppose the cylinders above and below the fire-door, and the system of tubes surrounding the furnace, which communicate with them, to be filled with water, and a quantity of fuel in a state of combustion placed upon the tubes at the bottom of the furnace which form the grate bars. The heat radiated from this fire, plays on every side upon the tubes forming the back and roof of the furnace,—on the cylinders already mentioned above and below the fire-door in front,—and on the upright tubes at each side of the fire-door. Whatever quantity of heat may pass downwards is received by the water in the tubes forming the bars of the grate. The spaces between the tubes forming the roof and back of the grate are stopped; with the exception of a small space at the lowest part of the back, where the spaces between the tubes are open, and lead to the flue which carries off the draft. This flue passes immediately behind the tubes in the back, and is conducted over the tubes in the roof. The air, which, passing through the fuel, maintains it in vivid combustion, and becomes intensely heated, is thus conducted in contact with that side of the tubes forming the back and roof, which is not exposed to the action of radiant heat. As it passes, it imparts a portion of its heat to the water in these tubes, and finally issues at a reduced temperature into the chimney. Such is the contrivance by which every portion of the caloric given out by the combustion of the fuel is communicated to the water.
The water in the tubes forming the roof of the furnace, being more advantageously exposed to the
