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.