three
inches in diameter, extended from one end of the boiler to the other,
the heated air passing through them on its way to the chimney; and the
tubes being surrounded by the water of the boiler, it will be obvious
that a large extension of the heating surface was thus effectually
secured. The principal difficulty was in fitting the copper tubes in the
boiler ends so as to prevent leakage. They were manufactured by a
Newcastle coppersmith, and soldered to brass screws which were screwed
into the boiler ends, standing out in great knobs. When the tubes were
thus fitted, and the boiler was filled with water, hydraulic pressure
was applied; but the water squirted out at every joint, and the factory
floor was soon flooded. Robert went home in despair; and in the first
moment of grief he wrote to his father that the whole thing was a
failure. By return of post came a letter from his father, telling him
that despair was not to be thought of--that he must "try again;" and he
suggested a mode of overcoming the difficulty, which his son had
already anticipated and proceeded to adopt. It was, to bore clean holes
in the boiler ends, fit in the smooth copper tubes as tightly as
possible, solder up, and then raise the steam. This plan succeeded
perfectly, the expansion of the copper tubes completely filling up all
interstices, and producing a perfectly water-tight boiler, capable of
withstanding extreme external pressure.

The mode of employing the steam-blast for the purpose of increasing the
draught in the chimney was also the subject of numerous experiments.
When the engine was first tried, it was thought that the blast in the
chimney was not sufficiently strong for the purpose of keeping up the
intensity of fire in the furnace, so as to produce high-pressure steam
with the required velocity. The expedient was therefore adopted of
hammering the copper tubes at the point at which they entered the
chimney, whereby the blast was considerably sharpened; and on a farther
trial it was found that the draught was increased to such an extent as
to enable abundance of steam to be raised. The rationale of the blast
may be simply explained by referring to the effect of contracting the
pipe of a water-hose, by which the force of the jet of water is
proportionately increased. Widen the nozzle of the pipe, and the jet is
in like manner diminished. So it is with the steam-blast in the chimney
of the locomotive.

Doubts were, however, express