What he means by "other conditions," is evidently when the
screw is running with a fair wind, which is seldom, so as to use her
sails. Bourne also states very clearly in two places that the
propeller is by no means so efficient in a sea-way, as a side-wheel
steamer, and admits that when a vessel is steaming at eleven or twelve
knots per hour, the sails not only do not aid her, but frequently
materially retard her motion. (_See Bourne, page 237._)

All of these authorities agree that the application of a given power
produces about the same effect, whether through the side-wheel or the
screw; and if so, it is evident that the screw can not attain the same
speed as the side-wheel, without burning as much fuel, and having as
costly and as heavy engines and boilers. Indeed, taking the whole
evidence together, it appears well settled by these authorities, that
the screw is equal to the side-wheel only in smooth water, and that,
as a consequence of this distinction, it is not equal to it in general
ocean navigation. It has been seen that much of its power is lost when
it contends with head-winds and seas, and that when it has attained a
fair average mail speed, the wind will help it very little, if any,
under the most favorable circumstances. It is, therefore, reasonable
to infer that it would cost more to attain a high average mail speed
with the propeller than with the side-wheel. If in attaining this
average mail speed the advantages are clearly in favor of the
side-wheel, there is no hope that we shall accomplish the mail service
at cheaper rates than heretofore, as this agency can not be introduced
toward that end; for not only is the prime cost of the steamer the
same, as also the consumption of fuel per mile, but there are other
and numerous disadvantages connected with the propeller, which are
wholly unknown to the side-wheel.

It is a well-known fact that propellers are compelled to be placed
upon the docks three or four times as often as side-wheels. The screw
either breaks, and must be replaced by another, or it cuts the boxes
out, or works the stern of the vessel to pieces. Any one of these
requires that the steamer shall be docked, however great the expense;
and as these accidents are constantly occurring in even the best
constructed and best regulated propellers, it follows that they must
be constantly on the docks. This species of vessel being built
necessarily narrower than the side-wheel, it rolls more, and i