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 ...

268 THE STEAM ENGINE.
1000 tons measurement furnished with engines from 200 to
250 horse-power.* Such vessels-could take a supply of from
300 to 400 tons of coals, which, being consumed at the rate
of from 20 to 25 tons per day, would last about fifteen
days.
Applying these results, however, to particular cases, it
will be necessary to remember that they are average calcu
lations, and must be subject to such modifications as the circumstances may suggest in the particular instances: thus, if
a voyage is contemplated under circumstances in which an
adverse wind generally prevails, less than the average speed
must be allowed, or' what is the- same, a greater consumption of fuel for a given distance. Against a strong: head wind,
in which a sailing vessel would double-reef her top-sails,
even a powerful steamer cannot make more than from 2
to 3 miles an hour, especially if she has a head sea to en-:
counter.
(121.) In considering the general economy of fuel, it may:
be right to state, that the results of experience obtained in
the steam navigation of our channels, and particularly in the
case of the Post-office packets on the Liverpool station, have
clearly established the fact, that by increasing the ratio of the
power to the tonnage, an actual saving of fuel in a given distance is effected, while at the same time the speed of the
vessel is increased. In the case of the Post-office steamers
called the DOLPHIN and the THETIs, (Liverpool station,) the
power has been successively increased, and the speed proportionably augmented; but the consumption of fuel per
voyage'between Liverpool and Dublin has been diminished.
This, at first view, appears inconsistent with the known
theory of the resistance of solids moving through fluids;
since this resistance- increases in the same proportion as the
square of the speed. But this physical principle is founded
* Engines in steam vessels generally work considerably above their nominal
power. The power, however, to which we uniformly refer is the nominal power,
or that power at which they would work with steam of the ordinary pressure.