This peculiarity of construction is the main cause of the advantage which the Archimedean Screw possesses over all its types or imitations; but it is not the only one. The _entirety_ or _unbroken continuity_ of its surface is another, not much less influential. The value of this will be the more readily appreciated when we consider that air, unlike water and other non-elastic fluids, undergoes a rarefaction or impoverishment of density, and consequently of resisting power, accordingly as it is swept away by the rapid passage of impinging planes; the parts 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