forcing the machine through the air. These machines weigh 1,000 lbs. each, and their engines are said to be 50 horse-power. The lifting effect, therefore, per horse-power is 20 lbs. If the aeroplanes were perfect in shape and set at a proper angle, and the resistance of the framework reduced to a minimum, the same lifting effect ought to be produced with an expenditure of less than half this amount of power, providing, of course, that the screw be of proper dimensions. It is said that Professor Langley and Mr. Horatio Philipps, by eliminating the factor of friction altogether, or by not considering it in their calculations, have succeeded in lifting at the rate of 200 lbs. per horse-power. The apparatus they employed was very small. The best I ever did with my very much larger apparatus--and I only did it on one occasion--was to carry 133 lbs. per horse-power. In my large machine experiments, I was amazed at the tremendous amount of power necessary to drive the framework and the numerous wires through the air. It appeared to me, from these experiments, that the air resisted very strongly being cut up by wires. I expected to raise my machine in the air by using only 100 horse-power, and my first condenser was made so that it did actually condense water enough to supply 100 horse-power, but the framework offered such a tremendous resistance that I was compelled to strengthen all of the parts, make the machine heavier, and increase the boiler pressure and piston speed until I actually ran it up to 362 horse-power. This, however, was not the indicated horse-power. It was arrived at by multiplying the pitch of the screws, in feet, by the number of turns that they made in a minute, and by the screw thrust in pounds, and then dividing the product by the conventional unit 33,000. I have no doubt that the indicated horse-power would have been fully 400. On one occasion I ran my machine over the track with all the aeroplanes removed. I knew what steam pressure was required to run my machine with the aeroplanes in position at a speed of 40 miles an hour. With the planes removed, it still required a rather high steam pressure to obtain this velocity, but I made no note at the time of the exact difference. It was not, however, by any means so great as one would have supposed. From the foregoing, it will be seen how necessary it is to consider atmospheric resistance. Although I do not expect that anyone will ever again attempt to make a flying machine driven by a steam engine, still, I have thought best to give a short and concise description of my engine and boiler, in order that my readers may understand what sort of an apparatus I employed to obtain the data I am now, for the first time, placing before the public. A full description of everything relating to the motor power was written down at the time, and has been carefully preserved. An abridgement of this will be found in the Appendix. CHAPTER II. AIR CURRENTS AND THE FLIGHT OF BIRDS.