of
adapting itself, by special effort, to the exigencies caused by the
varying nature of the road. Watch a team of horses pulling a waggon
along an undulating highway, with level stretches of easy going and
here and there a decline or a steep hill. There is a continual
adjustment of the strain which each animal puts upon itself according
to the character of the difficulties which must be surmounted, the
effort varying from nothing at all--when going down a gentle
decline--up to the almost desperate jerk with which the vehicle is
taken over some stony part right on the brow of an eminence. The whip
cracks and by threats and encouragements the driver induces each horse
to put forth, for one brief moment, an effort which could not be
sustained for many minutes save at the peril of utter exhaustion.

When the unit of nominal horse-power was fixed at 33,000 foot-pounds
per minute the work contemplated in the arbitrary standard was
supposed to be such as a horse could go on performing for several
hours. It was, of course, well recognised that any good, upstanding
horse, if urged to a special effort, could perform several times the
indicated amount of work in a minute.

Nevertheless the habit of reckoning steam-power in terms of a unit
drawn from the analogy of the horse undoubtedly tended for many years
to obscure the essential difference between the natures of the two
sources of power. Railroads were built with the object of rendering as
uniform as possible the amount of power required to transport a given
weight of goods or passengers over a specified distance; and
consequently the application of the steam-engine to traffic conducted
on the railway line was a success. Many inventors at once jumped to
the conclusion that, by making some fixed allowance for the greater
roughness of an ordinary road, they would be able to construct a
steam-traction engine that would suit exactly for road traffic. In a
rough and rudimentary way an attempt to provide for the special effort
required at steep or stony places was made by the introduction of a
kind of fly-wheel of extraordinary weight proportionate to the size of
the engine; and the same object was aimed at by increasing the power
of the engine to somewhere near the limit of the possible special
requirements. The consequence was the evolution of an immensely
ponderous and wasteful machine, which for some years only held its
ground within the domain of the heavy work of roadmaking