A Project for Flying | Page 5

be readily accounted for. All the modifications alluded to, which have hitherto been applied to the purposes of locomotion, are adaptations of plane surfaces. Now it is the character of plane surfaces to present the same angle, and consequently to impinge upon the air with the same condition of obliquity throughout. But the rate of revolution, and consequently of impact, varies according to the distance from the axis; being greatest at the outer edge, and gradually diminishing as it approaches the centre of rotation, where it may be supposed to be altogether evanescent. Now it is by the re-action of the air against one side of the impinging plane, that the progressive motion is determined in the opposite direction, which re-action is proportioned to the rate of impact, the angle remaining the same. If then we suppose a re-action corresponding to the greatest rate of revolution, which is that due to the outermost portion of the impinging surface (that most removed from the axis of rotation) we shall have a progressive motion in the whole apparatus greater than the rate of impact of the innermost or more central portions of the revolving plane; and accordingly the re-action will be thereabouts transferred from the back to the front of the propulsive apparatus, and tend to retard instead of advancing the progress of the machine to which it is attached. This inconvenience is felt and acknowledged by all those who have employed this principle to obtain a progressive motion, and accordingly a provision has been made against it in the removal or reduction of the central portion of the revolving vanes, with a view to let the air escape or pass through as the instrument advances; a provision which is certainly effectual to that end, but at the cost of the surface, which is the ultimate source of the required re-action. All this is avoided in the use of the perfect screw. There, the rate of rotation and the angle of impact mutually corresponding, may be said to play into each other's hands; the spiral becoming more extended as the impact becomes less forcible, that is as it approaches the centre, where both altogether vanish or disappear; thus obviating the possibility of any interruption to the course of the machine from the contrarious impact of the air, however quick or however slow the motions, either of the screw itself or of the machine which is propelled by its operation. In attestation of this fact and as showing the immunity of the perfect screw from the disparaging effects experienced by the other modes of accomplishing the same object, I will only mention a circumstance related to me by Mr. Smith himself, to whom I am glad to acknowledge myself indebted for so much valuable information respecting this instrument, which, by the light he has thrown upon its use and the improvements he has introduced into its construction, he may be truly said to have made his own. Upon a late occasion, when trying one of the larger class of vessels which had just been furnished by him upon this principle, some persons not perceiving the true nature of the figure employed, contended that some opposition must be experienced by the central portion of the screw, which revolved so much less rapidly than the rate of the ship itself. In order to convince them of their error, Mr. Smith caused a portion of the surface in question, next the axis, to a certain distance, to be cut away, leaving an opening, by which, for the water to escape. The result was, immediately the loss of one mile an hour in the rate of the ship; thus shewing that even the most apparently feeble portion of the impinging surface of this instrument contributes, in its degree, to the constitution of the aggregate force of which it is productive.
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