THE STEAM-ENGINE. 



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. 71, 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. 67, and 

 their ends are seen in the surface of the internal fire-box in fig. 72, and in the 

 remote end of the boiler where they terminate in the smoke-box in fig. 74. 

 These tubes are formed of the best rolled brass, and their thickness in the 

 engine, to which we now refer, is -fa 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. 



The tube-plates (as those parts of the boiler ends in which the tubes are in- 

 serted are called) are bored with holes in corresponding positions, truly cylin- 

 drical, and corresponding in magnitude to the tubes, so that the tubes, when 

 passed into them, will be just in contact with them. The length of the tubes 

 is so regulated, that when extending from end to end of the boiler, and passing 

 through the holes, they shall project at each end a little beyond the holes. 

 The manner of fastening them so as to be water-tight is as follows : A steel 

 hoop or ferrule, made slightly conical, a section of which is exhibited at C, 

 fig. 75, the smaller end of which is a little less than the internal diameter of 



Fig. 75. 



the tube, but which increases toward the outer end, is driven in as represented 

 in the figure. It acts as a wedge, and forces the. tube into close contact with 

 the edges of the hole in the tube-plate. 



When particular tubes in a boiler are worn out. and require to be replaced, 

 their removal is easily effected. It is only necessary to cut the steel ferrule 

 on the inside, and to bend it off from contact with the tube, by which means it 

 can be loosened and withdrawn, and the tube removed. 



In the engine to which this description refers there were one hundred and 

 twenty-four tubes, the external diameter of which was 1| inch. The distance i 



