Fire Box is that portion of the boiler of a steam-engine theet contains the fire. For economy of the heat generated therein the fire-box is best surrounded on nearly all sides by water-spaces. In England, and wherever coal is burnt, the fire-box of a locomotive is built of copper plates, for flaws in iron become larger under the action of intense heat, and the heat-conductivity of iron is not so good as that of copper. The box is flat-sided, and therefore liable to be crushed in by the great external steam pressure. It is in consequence strongly braced to the outer shell of the boiler by copper or iron bolts, and the top or crown is stiffened by ribs of wrought-iron. The grate is a few inches above the ash-pan, and is provided with a series of parallel wrought-iron fire-bars. Besides the fire-door through which the fuel is passed, a door opens below to the ash-pan to act as a damper in regulating the air-supply to the furnace.