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
to be employed; the proper disposition or inclination of its parts
becomes a question of the first importance. According as the
_turns_ of the screw are more or less oblique with respect to the
air they strike or the axis on which they revolve, more or less of the
resistance they generate by their rotation becomes _resolved_, as
it is technically expressed, in the direction of the intended course:
in other words, converted to the purpose in view, namely, the
propulsion of the Balloon.

Our limited space here again prevents us from entering into a detail
of the experiments by means of which the true solution of this
question has been arrived at, and the proper angle determined at which
the superficial spiral exercises the greatest amount of propulsive
force of which such an engine is capable. These experiments have been
chiefly carried on by Mr. Smith, the ingenious and successful adapter
of this instrument to the propulsion of steam vessels, for a series of
years, with the greatest care, and at a very considerable expense; and
the result of his experience gives an angle of about 67 deg. or 68 deg. for
the outer circumference of the screw, as that productive of the
maximum effect; a conclusion which is further verified