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 ...

214 THE STEAM ENGINE.
sideration of another, viz. whether roads may not be constructed, by pavement or otherwise, smoother and better
adapted to carriages moved in the manner of steam carriages
than the roads now used for horse-power?
But besides the greater smoothness of railroads compared
with turnpike roads, they have another advantage, which we
suspect to have been considerably exaggerated by those who
have opposed the project for steam carriages on turnpike
roads. One of the laws of adhesion, long since developed by
experiment, and known to scientific men, is, that it is greater
between the surfaces of bodies of the same nature than between those of a different nature. Thus between two metals
of the same kind it is greater than between two metals of
different kinds. Between two metals of any kind it is greater
than between metal and stone, or between metal and wood.
Hence, the wheels of steam carriages running on a railroad
have a greater adhesion with the road, and therefore offer a
greater resistance to slip round without the advance of the
carriage, than wheels would offer on a turnpike road; for on
a railroad the iron tire of the wheel rests in contact with the
iron rail, while on a common road the iron tire rests in contact with the surface of stone, or whatever material the road
may be composed of. Besides this, the dust and loose matter which necessarily collect on a common road, when pressed between the wheels and the solid base of the road act
somewhat in the manner of rollers, and give the wheels a
greater facility to slip than if the road were swept clean, and
the wheels rested in immediate contact with its hard surface.
The truth of this observation is illustrated on the railroads
themselves, where the adhesion is found to be diminished
whenever the rails are covered with any extraneous matter,
such as dust or moist clay. Although the adhesion of the
wheels of a carriage with a common road, however, be less
than those of the wheels of a steam carriage with a railroad,
yet still the actual adhesion on turnpike roads is greater in
amount than has been generally supposed, and is quite suffi