542 



THE STEAM-ENGINE. 



Fig. 66. 



The internal fire-box C, fig. 67, is similar in shape to the external, only it 

 is flat at the top, and close everywhere 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, T 7 g- of an inch in thickness, 

 everywhere except next the boiler, where the thickness is 1. 



As the sides and front of the external fire-box, and all the surfaces bound- 

 ing the internal fire-box, are flat, their form is unfavorable for the resistance 

 of pressure. Adequate means are, therefore, provided for strengthening them. 

 The plates forming the internal fire-box are bent outward near the bottom, 

 until they are brought into contact with those of the externa] fire-box, to which 

 they are attached by copper rivets, as represented at/, in fig. 67. The plates 

 forming the bounding surfaces of the two fire-boxes are fastened together by 

 stays represented at k, in figs. 67 and 72. 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. 72. 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 in- 

 ternal fireplace and that part of the front which is below the boiler. 



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 I, in fig. 67, and 

 of which an end view is seen in fig. 72, are attached by bolts to it. These 

 ribs are hollowed out, as seen in fig. 67, 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, represented at 

 m, figs. 67 and 72, 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. 



