_Answer._ As has already been explained, they are made usually of cast-iron bars,[40] _A_, _A_, _A_, figs. 82 and 83, called _grate-bars_. Fig. 82 is a plan, and fig. 83 a horizontal section of one form of grate. The bars in this kind of grate are usually cast in pairs, or some times three or more are cast together. They are made wider on the top than on the bottom edges, as shown in the section, fig. 83, so that cinders and ashes will fall through easily, and also to give free access to the air from below. They are usually from ³⁄₄ to 1¹⁄₂ inches wide on the top, and about ³⁄₄ inch on the lower edges. The spaces between the bars are made from ¹⁄₂ to 1¹⁄₄ inches wide. For burning wood the bars are placed comparatively close together and are stationary, but for burning bituminous coal they are usually made so that they can be moved, in order to shake or stir up the fire, just as is necessary in an ordinary stove or grate fire. In the grate we have illustrated the bars, _A_, _A_, are cast in pairs, and run crosswise of the fire-box. The ends are made with a sort of journals, _b_, _b_, which rest on two supports, _B_, _B_, called _bearing bars_, which have suitable indentations to receive the ends of the grate-bars. The latter have arms, _C_, _C_, fig. 83, cast on the under side, to which a bar, _D D_, is attached. By moving this bar back and forth, the grate-bars have a rocking motion imparted to them, as shown in fig. 84. It is evident that in this way the fire over the whole surface of the grates will be disturbed or shaken. The bar, _D D_, is moved by a lever, _m m_, shown in fig. 71. An extension piece, not shown in fig. 71, is used with the lever _m m_, so as to increase its length; but it is removed after it has been used, so as not to be in the way of the fireman. Grates which have movable bars are called _shaking_ or _rocking grates_. A great variety of such grates are made and in use, to describe which would require more room than is available here. [40] In Europe and in some few cases in this country they are made of wrought iron. For burning anthracite coal what are called _water-grates_ are used. These consist of wrought-iron tubes, 2 inches in diameter outside, which are fastened in the front and back plates of the fire-box and are inclined upward from the front end, so that there will be a continued circulation of water through them to keep them cool and thus prevent them from being burned out by the intense heat of the fire. QUESTION 153. _How is the fire removed from the fire-box when it is necessary to do so?_ _Answer._ In bituminous coal burning engines, what is called a _drop-door_, _E E_, figs. 82, 83 and 84, is provided for that purpose. This door is supported partly on journals, _d_, _d_, similar to those in the grate-bars, on which it can turn, and is held up or prevented from dropping by arms, _e_, _e_, attached to a shaft, _F F_. This shaft is operated by a lever, _f f_, fig. 82, outside the fire-box.