A Project for Flying | Page 5

Green has availed himself of to shew the effect of
atmospheric re-action in directing the course of the balloon. Now all
these and similar expedients are merely modifications of the same
principle, more or less perfect as they more or less resemble the perfect
screw, but all falling far short of the efficacy of that instrument in its
primitive character and construction. The reason of this deficiency can
be readily accounted for. All the modifications alluded to, which have
hitherto been applied to the purposes of locomotion, are adaptations of
plane surfaces. Now it is the character of plane surfaces to present the
same angle, and consequently to impinge upon the air with the same
condition of obliquity throughout. But the rate of revolution, and
consequently of impact, varies according to the distance from the axis;
being greatest at the outer edge, and gradually diminishing as it
approaches the centre of rotation, where it may be supposed to be
altogether evanescent. Now it is by the re-action of the air against one
side of the impinging plane, that the progressive motion is determined
in the opposite direction, which re-action is proportioned to the rate of
impact, the angle remaining the same. If then we suppose a re-action
corresponding to the greatest rate of revolution, which is that due to
the outermost portion of the impinging surface (that most removed
from the axis of rotation) we shall have a progressive motion in the
whole apparatus greater than the rate of impact of the innermost or
more central portions of the revolving plane; and accordingly the
re-action will be thereabouts transferred from the back to the front of
the propulsive apparatus, and tend to retard instead of advancing the
progress of the machine to which it is attached. This inconvenience is
felt and acknowledged by all those who have employed this principle to
obtain a progressive motion, and accordingly a provision has been
made against it in the removal or reduction of the central portion of the

revolving vanes, with a view to let the air escape or pass through as the
instrument advances; a provision which is certainly effectual to that end,
but at the cost of the surface, which is the ultimate source of the
required re-action. All this is avoided in the use of the perfect screw.
There, the rate of rotation and the angle of impact mutually
corresponding, may be said to play into each other's hands; the spiral
becoming more extended as the impact becomes less forcible, that is as
it approaches the centre, where both altogether vanish or disappear;
thus obviating the possibility of any interruption to the course of the
machine from the contrarious impact of the air, however quick or
however slow the motions, either of the screw itself or of the machine
which is propelled by its operation. In attestation of this fact and as
showing the immunity of the perfect screw from the disparaging effects
experienced by the other modes of accomplishing the same object, I
will only mention a circumstance related to me by Mr. Smith himself,
to whom I am glad to acknowledge myself indebted for so much
valuable information respecting this instrument, which, by the light he
has thrown upon its use and the improvements he has introduced into
its construction, he may be truly said to have made his own. Upon a
late occasion, when trying one of the larger class of vessels which had
just been furnished by him upon this principle, some persons not
perceiving the true nature of the figure employed, contended that some
opposition must be experienced by the central portion of the screw,
which revolved so much less rapidly than the rate of the ship itself. In
order to convince them of their error, Mr. Smith caused a portion of the
surface in question, next the axis, to a certain distance, to be cut away,
leaving an opening, by which, for the water to escape. The result was,
immediately the loss of one mile an hour in the rate of the ship; thus
shewing that even the most apparently feeble portion of the impinging
surface of this instrument contributes, in its degree, to the constitution
of the aggregate force of which it is productive.
This peculiarity of construction is the main cause of the advantage
which the Archimedean Screw possesses over all its types or imitations;
but it is not the only one. The entirety or unbroken continuity of its
surface is another, not much less influential. The value of this will be
the more readily appreciated when we consider that air, unlike water

and other non-elastic fluids, undergoes a rarefaction or impoverishment
of density, and consequently of resisting power, accordingly as it is
swept away by the rapid passage of impinging planes; the parts