ils 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
and form of the Archimedean Screw, there is little doubt that the
experiment would have been attended with a different result. In
accordance with the principles here illustrated, the Archimedean
Screw properly consists of only _one_ turn; more than one being
productive of no more resistance, and consequently superfluous. A
single unbroken turn of the screw, however, when the diameter is of
any magnitude, would require a considerable length of axis, which in
its adaptation to the Balloon, would be practically objectionable;
accordingly _two half turns_, nearly equivalent in power to one
whole turn, has been preferred; as in most instances it has been by
Mr. Smith, himself, in his application of it to the navigation of the
seas,

Indeed, in all other respects, except the nature of its material, the
screw here represented is exactly analogous to that used by Mr. Smith
in its most perfect form, having been, in fact, designed, and in part
constructed under his own supervision.[A]

The model upon which these principles have been now, for the first
time, successfully, at least, tried in the air, is constructed upon
the following scale. The Balloon is, as before stated, an ellipsoid
or solid oval; in length, 13 feet 6 inches, and in height, 6 feet 8
inches. It contains, accordingly, a volume of gas equal to about 320
cubic feet, which, in pure hydrogen, would enable it to support a
weight of twenty-one pounds, which is about its r