Fig. 95.
Fig. 96.
The external fire-box, B B, is a casing nearly square in its plan, being four feet wide outside, and three feet seven and a half inches long, measured in the direction of the boiler. It is constructed of wrought-iron plates, similar to those of the boiler. This box descends about two feet below the boiler, the top being semi-cylindrical, as seen in [fig. 102.], of a somewhat greater diameter than the boiler, and concentrical with it. The front of the fire-box next the end of the boiler has a circular opening equal in size to the end of the boiler. To the edge of this opening the boiler is fastened by angle irons, and rivets in the manner represented in [fig. 96.] These rivets are seen in section in [fig. 97.]
The internal fire-box C, [fig. 97.], is similar in shape to the external, only it is flat at the top, and close every where except at the bottom. Between it and the external fire-box an open space of three inches and a half is left all round, and on the side next the boiler this space is increased to four inches. This internal fire-box is made of copper plates, 7⁄16 [Pg367] of an inch in thickness, every where except next the boiler, where the thickness is 7⁄8.
As the sides and front of the external fire-box, and all the surfaces bounding the internal fire-box, are flat, their form is unfavourable for the resistance of pressure. Adequate means are, therefore, provided for strengthening them. The plates forming the internal fire-box are bent outwards near the bottom, until they are brought into contact with those of the external fire-box, to which they are attached by copper rivets, as represented at f in [fig. 97.] The plates forming the bounding surfaces of the two fire-boxes are fastened together by stays represented at k in figs. 97. and 102. These stays, which are of copper, have a screw cut upon them through their whole length, and holes are made through the plates of both fire-boxes tapped with corresponding threads. The copper screws are then passed through them, and rivets formed on their heads within and without, as seen in [fig. 102.] These screw rivets connect all parts of the plating of the two fire-boxes which are opposed to each other: they are placed at about four inches apart over the sides and back of the internal fire-place and that part of the front which is below the boiler.
