The best way is to have the fire-box at least as wide as the boiler, and have as much heating surface as possible; but below the water-line all passages should be made large, so as to allow a free passage to the heated gases, and where they leave the boiler, a damper should be provided. The bridge-wall should be high enough to prevent coal from being thrown over, and the grates low enough to allow ample room for combustion. Nothing can be gained by putting the fire near the boiler or contracting any of the passages; it is better to let the heat diffuse itself fully throughout the entire heating surface.