A Project for Flying | Page 6

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 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 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.
[Footnote A: The frame was made at Mr. Smith's request, by Mr.
Pilgrim, of the Archimedes; the original experimental vessel in which
this mode of propulsion was first tried upon the large scale. Mr. Pilgrim
has been long versed in all that relates to the mechanism of this
instrument, and is indeed a most expert and ingenious artist.]
[Footnote B: Endosmose is that operation by which gases of different
specific gravities are enabled, or rather forced to come together through
the pores of any membranous or other flexible covering by which it is
sought to restrain them. As above referred to, it is the introduction of
atmospheric air into the body of the Balloon through the pores of the
silk, however accurately varnished, by which the purity of the hydrogen

gas is contaminated, and its buoyant power ultimately exhausted This it
is impossible to prevent by any process, except the