STEPHENSON’S PATENT 
412 
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 if 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 C G; 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 
Fig. 7-