A Project for Flying | Page 4

greatest amount of propulsive re-action, in proportion to its magnitude and the force of its operation, which it is possible to accomplish. To shew by what steps and in consequence of what reasoning this point was determined as in the plan adopted, would occupy considerably more space than the few pages we have to spare would admit of our devoting to it. Suffice it to say that of all the means of creating a resistance in the atmosphere capable of being applied to the propulsion of the Balloon, the Archimedean Screw was ascertained to be undoubtedly the best. It is true that by a direct impact or stroke upon the air, as for instance by the action of a fan, or the wafting of any flat surface at right angles to its own plane, the maximum effect is accomplished which such a surface is capable of producing with a given power. The mechanical difficulties, however, which attend the employment of such a mode of operation are more than sufficient to counterbalance any advantage in point of actual resistance which it may happen to possess; at least in any application of it which has hitherto been tried or proposed: so that here, as in the case of ships propelled by steam, the oblique impact obtained by the rotation of the striking surface is found to be the most conducive to the desired result; and of these, that arrangement which is termed the Archimedean Screw is the most effective.
The result aimed at, being the development of the greatest amount of re-action in the direction of the axis of revolution, it is not enough to have determined the general character of the instrument to be employed; the proper disposition or inclination of its parts becomes a question of the first importance. According as the turns of the screw are more or less oblique with respect to the air they strike or the axis on which they revolve, more or less of the resistance they generate by their rotation becomes resolved, as it is technically expressed, in the direction of the intended course: in other words, converted to the purpose in view, namely, the propulsion of the Balloon.
Our limited space here again prevents us from entering into a detail of the experiments by means of which the true solution of this question has been arrived at, and the proper angle determined at which the superficial spiral exercises the greatest amount of propulsive force of which such an engine is capable. These experiments have been chiefly carried on by Mr. Smith, the ingenious and successful adapter of this instrument to the propulsion of steam vessels, for a series of years, with the greatest care, and at a very considerable expense; and the result of his experience gives an angle of about 67�� or 68�� for the outer circumference of the screw, as that productive of the maximum effect; a conclusion which is further verified by the experiments of Sir George Cayley, of Mr. Charles Green, the most celebrated of our practical aeronauts, and others who have employed their attention upon the subject. This conclusion requires only one modification, which ought to be noticed; namely, that in cases of extreme velocity, the number of the angle may be still further increased with advantage, until an inclination of about 73�� be obtained; when it appears any further advance in that direction is attended with a loss of power. With these facts in view, the impinging surface of the Archimedean Screw, in the model under consideration, has been so disposed as to form, at its outer circumference, an angle of 68�� with the axis of revolution, gradually diminishing as it approaches the centre, according to the essential character of such a form of structure.
The novelty of the application of this instrument to the propulsion both of ships and balloons, suggests the propriety of a few more explanatory remarks to elucidate its nature and meet certain objections which those who are ignorant of its peculiar qualities are apt to raise in respect of it.
Previous to the adoption of this particular instrument, various analogous contrivances had been resorted to in order to produce the same effects. Of these, examples are afforded in the sails of the windmill, the vane of the smoke jack, and of more modern introduction, the propellers designed by Mr. Taylor for the equipment of steam-boats, and which Mr. Green has availed himself of to shew the effect of atmospheric re-action in directing the course of the balloon. Now all these and similar expedients are merely modifications of the same principle, more or less perfect as they more or less resemble the perfect screw, but all falling far short of the efficacy of that instrument in its primitive character and construction. The reason of this deficiency can