corrected by the intervention of a sufficient rudder.

The second object, after establishing a proper form for the floating
body, was to contrive a disposition of striking surface that should
be able to realise the greatest amount of propulsive re-action, in
proportion to its magnitude and the force of its operation, which it
is possible to accomplish. To shew by what steps and in consequence of
what reasoning this point was determined as in the plan adopted, would
occupy considerably more space than the few pages we have to spare
would admit of our devoting to it. Suffice it to say that of all the
means of creating a resistance in the atmosphere capable of being
applied to the propulsion of the Balloon, the Archimedean Screw
was ascertained to be undoubtedly the best. It is true that by a
_direct_ impact or stroke upon the air, as for instance by the
action of a fan, or the wafting of any _flat_ surface at _right
angles_ to its own plane, the maximum effect is accomplished
which such a surface is capable of producing with a given power. The
mechanical difficulties, however, which attend the employment of such
a mode of operation are more than sufficient to counterbalance any
advantage in point of actual resistance which it may happen to
possess; at least in any application of it which has hitherto been
tried or proposed: so that here, as in the case of ships propelled
by steam, the _oblique_ impact obtained by the rotation of the
striking surface is found to be the most conducive to the desired
result; and of these, that arrangement which is termed the Archimedean
Screw is the most effective.

The result aimed at, being the development of the greatest amount of
re-action in the direction of the axis of revolution, it is not enough
to have determined the _general_ character of the instrument