A Project for Flying | Page 6

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 interposition of a metallic covering; as for instance, by gilding the Balloon, which would be effectual could it be contrived to endure the constant friction and bending of the material itself.]
Beneath the centre of the Balloon, and about two-thirds of its length, is a frame of light wood, answering to the hoop of an ordinary Balloon; to which are attached the cords of the net which encloses the suspending