ry slight and hardly appreciable; or that the same
number of tons of coal will drive two ships of the same size at the
same speed in smooth water; but that the side-wheel has greatly the
advantage in a head-sea or during rough weather generally. Many
persons who do not understand the subject, have theorized in just the
contrary direction. They say that in rough weather the screw has the
advantage, because it is alway in the water, etc. Experience shows
just the reverse; and theory will bear the practice out. If, in the
side-wheel one wheel is part of the time out, the other has, at any
rate, the whole force of the engines, and the floats sink to and take
hold on a denser, heavier, and less easily yielding stratum of water;
so that the progress is nearly the same. The back current or opposing
wave can not materially affect it, because the float is at the extreme
end of the arm where the travel is greatest, and is always more rapid
than the wave. It is not so with the screw. The blade which meets the
wave is not placed at the end of a long arm where the travel is very
rapid and the motion more sudden than that of the wave. This blade
extends all the way along from its extreme end, where the motion is
rapid, to the centre, or the shaft, where there is no motion; and all
intermediate parts of this blade move so slowly, that the wave of
greater rapidity counteracts it, and checks its progress. The
side-wheel applies its power at the extreme periphery, where the
travel is greatest, while the screw applies it all along between the
point of extreme rapidity, and the stationary point in the shaft.
There is, moreover, much power lost as the oblique blades of the screw
rise and fall in a vertical line while the vessel is heaving.

In the new edition (1855) of "Bourne on the Propeller," he says in the
preface:

"Large vessels, we know, are both physically and commercially more
advantageous than small vessels, provided only they can be filled with
cargo; but in some cases in which small paddle vessels have been
superseded by large screw vessels, the superior result due to an
increased size of hull has been imputed to a superior efficiency of
the propeller. No fact, however, is more conclusively established than
this, that the efficiency of paddles and of the screw as propelling
instruments is very nearly the same; and in cases in which geared
engines are employed to drive a screw vessel, the machinery will take
up about the same a