Fig. 103. ELEVATION OF THE FOREMOST END OF THE ENGINE.
Fig. 104. CROSS VERTICAL SECTION OF ENGINE THROUGH THE SMOKE-BOX.
As the top of the internal fire-box cannot be strengthened by stays of this kind, ribs of wrought-iron, which are seen in their length at l, in [fig. 97.], and of which an end view is seen in [fig. 102.], are attached by bolts to it. These ribs are hollowed out, as seen in [fig. 97.], between bolt and bolt, in order to break their contact with the roof of the fire-box, and allow a more free passage to the heat through it. If they were in continuous contact with the fire-box, the metal composing them would become more highly heated, and would soon wear out, besides intercepting heat from the water. This part of the fire-box is subject to rapid wear, unless care be taken that the level of the water be preserved at its proper height in the boiler. Even when the boiler is properly filled, the depth of water above the roof of the fire-box is not considerable, and on the least neglect the roof may be exposed to the contact of steam, in which case it will soon be destroyed.
To prevent accidents arising from this cause, a leaden plug, [Pg368] represented at m, figs. 97. and 102., is inserted in the roof of the internal fire-box. If the water be allowed to subside, this plug will melt out before the copper is very injuriously heated, and the steam rushing out at the aperture will cause the fire to be extinguished.
Copper fire-boxes are almost universally used; but sometimes, from the consideration of cheapness, the internal fire-box is constructed of iron.
In the plating which forms the back of the external fire-box, an oval aperture is formed, as represented in the back view of the engine, [fig. 101.], for the fire-door g. The plating of the internal fire-box around this aperture is bent at right angles to meet that of the external fire-box, to which it is fastened by a row of copper rivets. The fire-door is formed of two plates of wrought-iron, riveted together with a space of nine inches and a half between them. The air between these plates being an imperfect conductor of heat, keeps the outer plate of the fire-door at a moderate temperature.
In that part of the surface of the internal fire-box which forms the end of the boiler, holes are made to receive the extremities of the tubes, by which the air proceeding from the fire is drawn to the smoke-box at the remote end of the boiler. These tubes are represented in longitudinal section at E, [fig. 97.], and their ends are seen in the surface of the internal fire-box in [fig. 102.], and in the remote end of the boiler where they terminate in the smoke-box in [fig. 104.] These tubes are formed of the best rolled brass, and their thickness in the engine, to which we now refer, is 1⁄13 of an inch. After the brass plating is bent into the form of a tube, and being overlapped, is properly soldered together, and the edges smoothed off, the tubes are made perfectly cylindrical by being drawn through a circular steel die.