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 ...

316 THE STEAM ENGINE.
XXIII.
The transition from light to darkness, the sensation of
humidity, and the change in summer from a warm atmosphere to a cold one, will always form an objection to long
tunnels on lines of railroad intended for a large intercourse
of passengers.
XXIV.
All the objections to a tunnel are aggravated when it
happens to be upon an acclivity. The destruction of vital
air in ascending it will be increased in exactly the same
proportion as the moving power is increased. Thus, if it
ascend 17 feet in a mile, the destruction of vital air will be
twice as great as on a level; if it ascend 34 feet in a mile, it
will be three times as great; 51 feet in a mile, four times as
great, and so on.
XXV.
If by an overruling necessity a tunnel is constructed on
an acclivity, its magnitude and means of ventilation should
be greater than on a level, in the same proportion as the
resistance produced by the acclivity is greater than the
resistance upon a level.
XXVI.
Tunnels should be ventilated by shafts at intervals of not
more than 200 yards.
XXVII.
While a train is passing through a tunnel, no beneficial ventilation can be obtained from shafts. The engine will leave