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 sails of a
windmill. In the experiments which were made in this gallery with
several models of this proposed construction, it was found that so far
from _aiding_ the machine in its flight, the operation of these
vanes actually _impeded_ its progress; inasmuch as it was always
found to proceed to a greater distance by the mere force of acquired
velocity (which is the only force it ever displayed), than when
the vanes were set in motion to aid it--a simple fact, which it is
unnecessary to dilate upon. It is to the agency of this cause, namely,
the broken continuity of surface, that, I have no doubt, is also to be
ascribed the failure of the attempt of Sir George Cayley to propel a
Balloon of a somewhat similar shape to the present, which he made at
the Polytechnic Institution a short while since, when he employed
a series of revolving vanes, four in number, disposed at proper
intervals around, but which were found ineffectual to move it. Had
these separate surfaces been thrown into _one_, of the nature