10

STEPHENSONS’ PATENT

fire-box depends very much on the care of the engine man; with proper use it willlast several years, but if the w T ater is allowed to get too low in the boiler, so as tohave but little depth over the roof, the plate will be liable to get frequently un-covered, from the motion of the engine, and be rapidly destroyed. To prevent thisaccident a small plug of lead, m, (Plate XC. and XCII.,) is put through a hole inthe centre of the roof of the fire-box, and riveted over on both sides; when thewater gets so low as to uncover this plug it is melted by the heat, and the steam,rushing into the fire-box, extinguishes the fire. The internal fire-box is made some-times of wrought iron, and is generally found to last nearly as long as a copper one;the iron fire-box costs considerably less, but requires more care in using and is veryliable to crack and become leaky at the joints.

Tubes.— The communication between the fire-box and the chimney is made by anumber of tubes, E E, (Plate XC. and XCII.,) which are fixed water-tight at one endinto the front plate of the fire-box e, and at the other into the plate n, (Plate XC.,)which closes the front end of the boiler; the tube plate, e, of the fire-box beingmade thicker where the tubes are inserted, to allow for its being weakened by theholes cut in it. There are 124 of these tubes; they are If inch in diameteroutside, and a space of three quarters of an inch is left between them. They are madeof the best rolled brass, one thirteenth of an inch thick,

(called No. 13. wire-gauge;) the edges of the brassare properly chamfered and lapped over each otherand soldered together, the solder being applied inside;the tubes are then drawn through a circular steel die tomake them truly cylindrical. The holes to receive themin the tube plates e and n, (Plate XC.,) are bored quitecylindrical so as to fit the tubes exactly, which are justlong enough to come to the outside of both plates : theends of the tubes are then fixed by driving in a steelhoop or ferrule, made slightly conical, as shewn in Fig.

7; which is a section full size of the tube A A, theplate of the fire-box B B in which it is inserted, andthe ferrule G G\ the ferrule is a little larger than the tube,so that, when driven in, it compresses the tube veryforcibly against the sides of the hole, and makes thejoint completely watertight. The ferrules are sometimes made of wrought iron, butthey generally do not last out the tube in that case, and require replacing by newones before the tubes are worn out; the steel ferrules are better, as they last nearlytwice as long. When a tube or a ferrule requires taking out, the ferrule has to be

Fig. 7.