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 immediately _behind_, and
to a considerable distance, being thereby relieved from the support
they had previously experienced, and extending (and consequently
becoming thinner) in order to fill up the space thus partially cleared
away. Now it is evident that if other planes be brought into operation
in the parts of the atmosphere thus impoverished, before they have
had time to recover their pristine or natural density, they will
of necessity act with diminished vigour; the resistance being ever
proportioned to the density of the resisting medium. This is the
condition into which, more or less, all systems of revolving planes
are necessarily brought, that consist of more than one; and is a
grand cause of the little real effect they have been made capable
of producing, whenever tried. The nature of this objection, and the
extent to which it operates, will appear most strikingly from the
following fact. Mr. Henson's scheme of flight is founded upon the
principle of an inclined plane, started from an eminence by an
extrinsic force, applied and _continued_ by the revolution
of impinging vanes, in form and number resembling the sa