The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

160 THE STEAM ENGINE.
and of the improvements of which they seem to us to be
susceptible.
(85.) It is a singular fact, that in the history of this invention, considerable time and great ingenuity were vainly
expended in attempting to overcome a difficulty which in
the end turned out to be purely imaginary. To comprehend
distinctly the manner in which a wheel carriage is propelled
by steam, suppose that a pin or handle is attached to the
spoke of the wheel at some distance from its centre, and that
a force is applied to this pin in such a manner as to make
the wheel revolve. If the face of the wheel and the surface
of the road were absolutely smooth and firee from friction,
so that the face of the wheel would slide without resistance:upon the road, then the effect of the force thus applied would
be merely to cause the wheel to turn round, the carriage
being stationary, the surface of the wheel would slip or slide
upon the road as the wheel is made to revolve. But if, on
the other hand, the pressure of the face of the wheel upon
the road is such as to produce between them such a degree
of adhesion as will render it impossible for the wheel to
slide or slip upon the road by the force which is applied to
it, the consequence will be, that the wheel can only turn
round in obedience to the force which moves it by causing
the carriage to advance, so that the wheel will roll upon the
road, and the carriage will be moved forward, through a distance equal to the circumference of the wheel, each time it
performs a complete revolution.
It is obvious that both of these effects may be partially
produced; the adhesion of the wheel to the road may be
insufficient to prevent slipping altogether, and yet it may be
sufficient to prevent the wheel from slipping as fast as it
revolves. Under such circumstances the carriage would
advance, and the wheel would slip. The progressive motion
of the carriage during one complete revolution of the wheel
would be equal to the difference between the complete circumference of the wheel and the portion through which in
one revolution it has slipped.