hen it was first discovered that a smooth-faced driving wheel,
running on a smooth-faced rail, would "bite," the era of iron railways
and locomotive engines may be said to have fairly commenced. The
correction of a single radical error was, in this case, the dawn of a
new system of travel, so extensive in its growth and marvelous in its
results, that even the wildest dreamer could not, at that time, have
imagined the consequences of so simple a discovery.

A popular and somewhat similar error regarding the bite of wheels on
rough and uneven surfaces, has also prevailed. We say popular error,
because engineers have not shared it, and it has obtained, to any
notable extent, only among those unfamiliar with mechanical science.
The error in question is, that hard-surfaced wheels will not bite on
a moderately rough surface, sufficiently to give an efficient tractile
power. It seems strange that this error should have diffused itself
very extensively, when it is remembered that a certain degree of
roughness is essential to frictional resistance. The smoothness of the
ordinary railway track is roughness compared to that of an oiled or
unctuous metallic surface; and it has been amply demonstrated that
the resistance of friction, of two bearing surfaces depends, not
upon their extent, but upon the pressure with which they are forced
together. A traction wheel, of given weight, resting upon two square
inches of hard earth or rock, would develop the same tractile power
as though it had a bearing surface of two square feet of similar
material.

On very rough and stony ways, however, another element practically of
no importance on moderately rough ways, like a macadam surface or a
concrete road, where the prominences are nearly of uniform hight, and
so near together as to admit between their summits only very small
arcs of the circumference of the wheel; comes into action. This
element is the constantly recurring lifting of the superincumbent
weight of the machine. Even this would not result in loss of power,
could the power developed in falling be wholly applied to useful work
in the direction of the advance of the engine. The fact is, however,
that it is not so applied, and in any method of propulsion at present
known to engineering science, cannot be so applied. Above a certain
point where friction enough is developed to prevent slip, the more
uneven the road surface is, the greater the power demanded for the
propulsion of the