I found that there was a great deal of misunderstanding regarding the action of aeroplanes, and also of screws working in the air. I procured all the literature available on the subject, both English and French, and attempted to make a thorough study of the question; but I was not satisfied, on account of the wide difference in the views of the writers and the conflicting formulæ that were employed. I therefore decided to make experiments myself, and to ascertain what could be done without the use of anybody’s formula. Although this was nearly twenty years ago, I find that there is still a great deal of discussion regarding the action of aeroplanes and screws, in which the majority taking part in the discussion are in the wrong. However, several good works on the subject have recently been published. [Illustration: Fig. 11.--The three best screws. The screw on the right has a uniform pitch throughout, the middle screw has increasing pitch, and the left screw compound increasing pitch.] Having designed and put my boiler and engine in hand, I commenced a series of experiments for the purpose of ascertaining the efficiency of screw propellers working in the air, and the form and size that would be best for my proposed machine. The illustration Fig. 9 shows a photographic group of the screws and other objects with which I experimented. Fig. 10 shows some of the leading types which, as will be seen, have blades of different shape, pitch, and size. Fig. 11 shows three of the best screws employed. It will be observed that one has uniform pitch, another increasing pitch, and the third compound increasing pitch. In order to test the efficiency of my screws I made the apparatus shown in Fig. 12. The power for running the screw was transmitted by means of a belt to the straight cylindrical pulley _c_, _c_. Shaft _b_, _b_ was of steel, rather small in diameter, and ran smoothly, and practically without friction, through the two bearings _d_, _d_. When the first screw, _a_, _a_, was run at a high velocity, the axial thrust pushed the shaft _b_, _b_ back and elongated the spiral spring _e_. The degree of screw thrust was indicated in pounds by the pointer _g_. The power was transmitted through a very accurate and sensitive dynamometer, so that the amount consumed could be easily observed by a pointer similar to the one employed for indicating the screw thrust. A tachometer was also employed to observe the number of turns that the screw was making in a minute. The whole apparatus was carefully and accurately made and worked exceedingly well. I was thus enabled, with my various forms of screws and other objects, to make very accurate measurements, some of which are exceedingly interesting. [Illustration: Fig. 12.--Apparatus for testing the thrust of screws--_a_, _a_, the screw; _b_, _b_, shaft sliding freely in the bearing _d_, _d_; _c_, cylindrical pulley; _e_, spiral spring; _f_, steel rod; _g_, pointer for indicating the thrust in pounds.]