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 immediately _behind_, and
to a considerable distance, being thereby relieved from the support
they had previously experienced, and extending (and consequently