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 ...

PLAIN RULES TO RAILWAY SPECULATORS. 315
of seventeen feet in a mile doubles the resistance of the level,
thirty-four feet in a mile triples it, and eight and a half feet
in a mile adds one half its amount, and so on.
XIX.
With the speed now attainable on railways, curves should
be avoided with radii shorter than a mile. Expedients may
diminish the resistance, but, through the negligence of engine
drivers, they must always be attended with danger. Curves
are not objectionable near the extremities of a line.
XX.
The worst position for a curve is the foot of an inclined
plane, because of the velocity which the trains acquire in the
descent, and the occasional impracticability of checking
them.
XXI.
In proportion as the speed of locomotives is increased by
the improvements they are likely to receive, the objections
and dangers incident to curves will be increased.
The difficulty which attends the use of long tunnels arises
from the destruction of the vital air which is produced by
the combustion in the furnaces of the engines. Tunnels on
a level should, therefore, be from twenty-five to thirty feet
high, and should be ventilated by shafts or other contrivances.