The many early conversations that I had with my father on the subject kept the matter constantly before me, and I think it was in 1872, after having seen Roper’s hot-air engine and Brayton’s petroleum engine, that I took the matter up, and commenced to make drawings of a machine of the Hélicoptère type, but instead of having one screw above the other, I saw at once that it would be much better if the two screws were widely separated, so that each would engage new air, the inertia of which had not been disturbed. The designing of the machine itself was a simple matter, but the engine gave me trouble. No matter from what point I examined the subject, the engine was always too heavy. It appears that the Brayton engine was shown at the Centennial Exhibition at Philadelphia in 1876, and that Otto visited this exhibition. Up to that time, he had been making a species of rocket engine--that is, an engine in which an explosive mixture shot the piston upward and then sucked it back, a rack and pinion transmitting movement to the rotating shaft by means of a pawl and ratchet. He appears to have been much interested in the Brayton engine, as it was evidently very much in advance of his own. It actually developed, even at that time, one horse-power per hour for every pound of crude petroleum consumed, but it was very heavy indeed, very difficult to start, and not always reliable. The shaft that worked the valve gear was parallel to the cylinder, and placed in the exact position occupied by a similar shaft in the present Otto engine, but instead of revolving only half as fast as the crank shaft, it made the same number of revolutions. On Otto’s return to Germany, he evidently profited by what he had seen, and made a new engine, which in reality was a cross between his own and the Brayton; the result was a very important invention, which has been of incalculable value to mankind. It is this engine which is now propelling our motor cars, and it is the only engine suitable for employment on a flying machine; but even this motor was not in a sufficiently high state of development as far as lightness was concerned, to be of any use to me. The drawings which I made in 1873, although of little or no value, kept my thoughts on artificial flight, and while I was away from home attending to business, especially when in foreign countries, I often amused myself by making mathematical calculations. Quite true, the formula which I used at the time--Haswell’s--was not correct; still, it was near enough to the mark to be of considerable value. Moreover, the error in this formula affected the Hélicoptère quite as much as the aeroplane system, and as I was working with the view of ascertaining the relative merits of the two systems, the error, although considerable, did not have any influence at all in the decision which I arrived at--namely, that the aeroplane system was the best. The machine that I thought out at that time