A Project for Flying | Page 4

position
is altered with respect to the propelling power, its course is
immediately affected, and it ceases to progress in a straight line,
following the direction of its major axis, unless 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 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° or 68° for the outer
circumference of the screw, as that productive of the maximum effect;
a conclusion which is further verified by the experiments of Sir George
Cayley, of Mr. Charles Green, the most celebrated of our practical
aeronauts, and others who have employed their attention upon the
subject. This conclusion requires only one modification, which ought to
be noticed; namely, that in cases of extreme velocity, the number of the
angle may be still further increased with advantage, until an inclination
of about 73° be obtained; when it appears any further advance in that
direction is attended with a loss of power. With these facts in view, the
impinging surface of the Archimedean Screw, in the model under
consideration, has been so disposed as to form, at its outer
circumference, an angle of 68° with the axis of revolution, gradually
diminishing as it approaches the centre, according to the essential
character of such a form of structure.
The novelty of the application of this instrument to the propulsion both
of ships and balloons, suggests the propriety of a few more explanatory

remarks to elucidate its nature and meet certain objections which those
who are ignorant of its peculiar qualities are apt to raise in respect of it.
Previous to the adoption of this particular instrument, various
analogous contrivances had been resorted to in order to produce the
same effects. Of these, examples are afforded in the sails of the
windmill, the vane of the smoke jack, and of more modern introduction,
the propellers designed by Mr. Taylor for the equipment of steam-boats,
and which Mr.