412 



STEPHENSON'S PATENT 



FIG. 7. 



fire-box depends very much on the care of the engine man ; with proper use it will 

 last several years, but if the water is allowed to get too low in the boiler, so as to 

 have 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 this 

 accident a small plug of lead, m, (Plate XC. and XCII.,) is put through a hole in 

 the centre of the roof of the fire-box, and riveted over on both sides ; when the 

 water 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 very 

 liable to crack and become leaky at the joints. 



TUBES. The communication between the fire-box and the chimney is made by a 

 number of tubes, E E, (Plate XC. and XCII.,) which are fixed water-tight at one end 

 into 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 being 

 made thicker where the tubes are inserted, to allow for its being weakened by the 

 holes cut in it. There are 124 of these tubes; they are li inch in diameter 

 outside, and a space of three quarters of an inch is left between them. They are made 

 of the best rolled brass, one thirteenth of an inch thick, 

 (called No. 13. wire-gauge;) the edges of the brass 

 are properly chamfered and lapped over each other 

 and soldered together, the solder being applied inside ; 

 the tubes are then drawn through a circular steel die to 

 make them truly cylindrical. The holes to receive them 

 in the tube plates e and n, (Plate XC.,) are bored quite 

 cylindrical so as to fit the tubes exactly, which are just 

 long enough to come to the outside of both plates : the 

 ends of the tubes are then fixed by driving in a steel 

 hoop or ferrule, made slightly conical, as shewn in Fig. 

 7 ; which is a section full size of the tube A A, the 

 plate of the fire-box B B in which it is inserted, and 

 the ferrule CC; the ferrule is a little larger than the tube, 

 so that, when driven in, it compresses the tube very 

 forcibly against the sides of the hole, and makes the 

 joint completely watertight. The ferrules are sometimes made of wrought iron, but 

 they generally do not last out the tube in that case, and require replacing by new 

 ones before the tubes are worn out ; the steel ferrules are better, as they last nearly 

 twice as long. When a tube or a ferrule requires taking out, the ferrule has to be 



