r the plan above described are to this day to be
seen in regular useful work upon the Killingworth Railway, conveying
heavy coal-trains at the speed of between five and six miles an hour,
probably as economically as any of the more perfect locomotives now in
use.

Mr. Stephenson's endeavours having been attended with such marked success
in the adaptation of locomotive power to railways, his attention was
called by many of his friends, about the year 1818, to the application of
steam to travelling on common roads. It was from this point that the
locomotive started, Trevithick's first engine having been constructed
with this special object. Stephenson's friends having observed how far
behind he had left the original projector of the locomotive in its
application to railroads, perhaps naturally inferred that he would be
equally successful in applying it to the purpose for which Trevithick and
Vivian had intended their first engine. But the accuracy with which he
estimated the resistance to which loads were exposed on railways, arising
from friction and gravity, led him at a very early stage to reject the
idea of ever applying steam power economically to common-road travelling.
In October, 1818, he made a series of careful experiments in conjunction
with Nicholas Wood, on the resistance to which carriages were exposed on
railways, testing the results by means of a dynamometer of his own
construction. The series of practical observations made by means of this
instrument were interesting, as the first systematic attempt to determine
the precise amount of resistance to carriages moving along railways. It
was then for the first time ascertained by experiment that the friction
was a constant quantity at all velocities. Although this theory had long
before been developed by Vince and Coulomb, and was well known to
scientific men as an established truth, yet, at the time when Stephenson
made his experiments, the deductions of philosophers on the subject were
neither believed in nor acted upon by practical engineers.

He ascertained that the resistances to traction were mainly three; the
first being upon the axles of the carriages, the second, or rolling
resistance, being between the circumference of the wheel and the surface
of the rail, and the third being the resistance of gravity. The amount
of friction and gravity he could accurately ascertain; but the rolling
resistance was a matter of greater difficulty, being subject