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.]