The reason tubes of smaller diameter than two inches are not ordinarily used is because they are then liable to become stopped up with cinders and pieces of unconsumed fuel. QUESTION 90. _How is the fire-box of a locomotive constructed?_ _Answer._ It usually consists of a rectangular box (_G_, figs. 41 and 42) about three feet wide[24] and, for the size of engine we have selected as an example, about five or five and a half feet long inside. This box is composed of metal plates, either iron, steel or copper, which, excepting on the front side, are from ⁵⁄₁₆ to ³⁄₈ of an inch thick. This box is called the _inside shell_ of the fire-box, and is surrounded by another shell, _A B C D E F_, fig. 42, of either iron or steel plates, of about the same thickness as those composing the inside. This is called the _outside shell_ of the fire-box and, as already explained, is so much larger than the inside that there is a space, called the _water-space_, from 2¹⁄₂ to 4¹⁄₂ inches wide, on all the sides of the fire-box between the inner and outer plates. [24] The width is dependent upon the distance between the rails, or _gauge_ of the road, as it is called. The above size is for a 4 feet 8¹⁄₂ in. gauge. The top _g_, _g_, of the inside shell, which is called the _crown-sheet_ or _crown-plate_, is flat, whereas the outside shell is arched, as shown in fig. 42. To the front plate of the inside shell the tubes _a a′_, _a a′_ are attached. For this reason its thickness is usually made greater than that of the other plates, and is usually from ³⁄₈ to ³⁄₄ of an inch. The edges of one of the plates at each corner of the fire-box, where they are united together, as shown in figs. 41 and 42, are bent at right angles, and the other is fastened to it with rivets from ⁵⁄₈ to ³⁄₄ of an inch in diameter.