ed, and
the progress of the undertaking was watched with intense interest.

During the progress of the discussion with reference to the kind of power
to be employed, Mr. Stephenson was in constant communication with his son
Robert, who made frequent visits to Liverpool for the purpose of
assisting his father in the preparation of his reports to the Board on
the subject. They had also many conversations as to the best mode of
increasing the powers and perfecting the mechanism of the locomotive.
These became more frequent and interesting, when the prize was offered
for the best locomotive, and the working plans of the engine which they
proposed to construct came to be settled.

One of the most important considerations in the new engine was the
arrangement of the boiler and the extension of its heating surface to
enable steam enough to be raised rapidly and continuously, for the
purpose of maintaining high rates of speed,--the effect of high-pressure
engines being ascertained to depend mainly upon the quantity of steam
which the boiler can generate, and upon its degree of elasticity when
produced. The quantity of steam so generated, it will be obvious, must
depend chiefly upon the quantity of fuel consumed in the furnace, and by
necessary consequence, upon the high rate of temperature maintained
there.

It will be remembered that in Stephenson's first Killingworth engines he
invented and applied the ingenious method of stimulating combustion in
the furnace, by throwing the waste steam into the chimney after
performing its office in the cylinders, thus accelerating the ascent of
the current of air, greatly increasing the draught, and consequently the
temperature of the fire. This plan was adopted by him, as we have
already seen, as early as 1815; and it was so successful that he himself
attributed to it the greater economy of the locomotive as compared with
horse power. Hence the continuance of its use upon the Killingworth
Railway.

Though the adoption of the steam-blast greatly quickened combustion and
contributed to the rapid production of high-pressure steam, the limited
amount of heating surface presented to the fire was still felt to be an
obstacle to the complete success of the locomotive engine. Mr.
Stephenson endeavoured to overcome this by lengthening the boilers and
increasing the surface presented by the flue-tubes. The "Lancashire
Witch," which he built for the Bolton and Leigh Railway, and used in
f