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-