llingworth engines he
invited 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, thereby 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
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
forming the Liverpool and Manchester Railway embankments, was
constructed with a double tube, each of which contained a fire, and
passed longitudinally through the boiler. But this arrangement
necessarily led to a considerable increase in the weight of those
engines, which amounted to about twelve tons each; and as six tons was
the limit allowed for engines admitted to the Liverpool competition, it
was clear that the time was come when the Killingworth engine must
undergo a farther important modification.

For many years previous to this period, ingenious mechanics had been
engaged in attempting to solve the problem of the best and most
economical boiler for the production of high-pressure steam.

The use of tubes in boilers for increasing the heating surface had long
been known. As early as 1780, Matthew Boulton employed copper tubes
longitudinally in the boiler of the Wheal Busy engine in Cornwall--the
fire passing _through_ the tubes--and it was found that the production
of steam was thereby considerably increased. The use of tubular boilers
afterwards became common in Cornwall. In 1803, Woolf, the Cornish
engineer, patented a boiler with tubes, with the same object of
increasing the heating surface. The water was _inside_ the tubes, and
the fire of the boiler outside. Similar expedients were proposed by
other inventors. In 1815 Trevithick invented his light high-