LOCOMOTIVE ENGINE. 415 



and lose about 6^ Ibs. in the time they are in use. The cost of both the brass and of the 

 copper tubes is about l each, and this makes the expense of repairing an engine very 

 considerable when a complete set of new tubes is required. The tubes being fixed 

 firmly into both ends of the boiler, serve to support and strengthen them ; but for an 

 additional support to the upper part, six wrought iron rods, o o, (Plates XC. and XCII.,) 

 are placed above the internal fire-box, by the side of each other and longitudinally in 

 the boiler ; and the ends are attached by a pin to a piece of wrought iron, called T 

 iron, riveted on to the end plate of the boiler and to the back plate of the fire-box. 



THE SMOKE-BOX F F, is 4 feet wide, like the fire-box, and 2 feet long, and is 

 closed on all sides ; the back of it is formed by the wrought iron plate , half an 

 inch thick, closing the end of the boiler to which it is attached by means of a piece 

 of angle iron riveted to both, like the similar joint at the fire-box. The rest of the 

 smoke-box is made of quarter inch iron plate, the front and back plates being 

 bent in round the edge, and the other plates riveted to them as in the fire-box, 

 except the front plate, which is fixed by screw bolts and nuts, because it is required 

 occasionally to take it off. 



Upon the smoke-box is fixed the chimney G, (Plates LXXXIX., XC. and XCII.) ; 

 it is 15 inches in diameter, and is made of one eighth inch iron plates, riveted 

 together and bound round by hoops, as shewn in the section ; the top is made funnel- 

 shaped to give more free egress to the hot air, and the bottom has a piece of plate 

 riveted to it, forming a flanch all round, by means of which the chimney is bolted 

 down upon the smoke-box. 



In the lower part of the smoke-box are fixed the two cylinders H H, where the 

 steam is used and motion produced; these will be described afterwards. The steam, 

 when it has been used in the cylinder and has performed its work, is no longer wanted, 

 and is let out into the air by the pipe p, (Plates XC. and XCII.) 



The tubes open into the upper part of the smoke-box, and the hot air passes from 

 them up the chimney ; no smoke is produced, except at first lighting the fire, as the 

 fuel used is coke, which does not cause any smoke in burning, but only a light dust. 

 The height of the chimney is obliged to be small, as it can never exceed 14 feet 

 height from the rails ; so that the draught produced by it is not at all sufficient to urge 

 the fire to the intense degree of ignition that is necessary to produce steam at the 

 pressure and in the quantity that is required, and some other more powerful means 

 has, therefore, to be adopted to produce the draught. This is done by making the 

 waste steam issue through the pipe pp, (Plates XC., XCI., and XCII.,) called the 

 blast pipe, which is directed up into the centre of the chimney, and is gradually con- 

 tracted throughout its length to make the steam rush out with more force ; this 

 pipe is made of copper one eighth of an inch thick, and is 3f inches in diameter 



