at first barely economical; and at the end
of the year the steam power and the horse power were ascertained to be as
nearly as possible upon a par in point of cost. The fate of the
locomotive in a great measure depended on this very engine. Its speed
was not beyond that of a horse's walk, and the heating surface presented
to the fire being comparatively small, sufficient steam could not be
raised to enable it to accomplish more on an average than about four
miles an hour. The result was anything but decisive; and the locomotive
might have been condemned as useless, had not our engineer at this
juncture applied the steam-blast, and by its means carried his experiment
to a triumphant issue.

The steam, after performing its duty in the cylinders, was at first
allowed to escape into the open atmosphere with a hissing blast, to the
terror of horses and cattle. It was complained of as a nuisance; and an
action at law against the colliery lessees was threatened unless it was
stopped. Stephenson's attention had been drawn to the much greater
velocity with which the steam issued from the exit pipe compared with
that at which the smoke escaped from the chimney. He conceived that, by
conveying the eduction steam into the chimney, by means of a small pipe,
after it had performed its office in the cylinders, allowing it to escape
in a vertical direction, its velocity would be imparted to the smoke from
the fire, or to the ascending current of air in the chimney, thereby
increasing the draft, and consequently the intensity of combustion in the
furnace.

The experiment was no sooner made than the power of the engine was at
once more than doubled; combustion was stimulated by the blast;
consequently the capability of the boiler to generate steam was greatly
increased, and the effective power of the engine augmented in precisely
the same proportion, without in any way adding to its weight. This
simple but beautiful expedient was really fraught with the most important
consequences to railway communication; and it is not too much to say that
the success of the locomotive has in a great measure been the result of
its adoption. Without the steam-blast, by means of which the intensity
of combustion is maintained at its highest point, producing a
correspondingly rapid evolution of steam, high rates of speed could not
have been kept up; the advantages of the multi-tubular boiler (afterwards
invented) could never have been fairly test