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 real power when
recently inflated, and before the gas has had time to become
deteriorated by the process of _endosmose_.[B] The whole weight
of the machine and apparatus is seventeen pounds; consequently there
is about four pounds to spare, in order to meet this contingency.