Professor Langley made his first experiments more than twelve years ago at Allegheny, Pennsylvania.... Professor Langley formed the general conclusion that by simply moving any given weight in plate form fast enough in a horizontal path through the air it was possible to sustain it with very little power. It was proved that, if horizontal flight without friction could be insured, 200 pounds of plates could be moved through the air and sustained upon it at the speed of an express train, with the expenditure of only one horse-power, and that, of course, without using any gas to lighten the weight. Every boy who has skated knows that when the ice is very thin he must skate rapidly, else he may break through. In the same way, a stone may be skipped over the water for considerable distances. If it stops in any one place it sinks instantly. In exactly the same way, the plate of brass, if left in any one place in the air, would instantly drop to the earth; but if driven swiftly forward in a horizontal direction it rests only an instant in any particular place, and the air under it at any single moment does not have time to give way, so to speak, before it has passed over a new area of air. In fact, Professor Langley came to the conclusion that flight was theoretically possible with engines he could then build, since he was satisfied that engines could be constructed to weigh less than twenty pounds to the horse-power, and that one horse-power would support two hundred pounds if the flight was horizontal. That was the beginning of the aerodrome. Professor Langley had worked out its theory, and now came the much more difficult task of building a machine in which theory should take form in fact. In the first place, there was the vast problem of getting an engine light enough to do the work. A few years ago an engine that developed one horse-power weighed nearly as much as an actual horse. Professor Langley wished to make one weighing only twenty pounds, a feat never before accomplished. And then, having made his engine, how was he to apply the power to obtain horizontal speed? Should it be by flapping wings like a bird, or by a screw propeller like a ship? This question led him into a close study of the bird compared with the man. He found how wonderfully the two were alike in bony formation, how curiously the skeleton of a bird's wing was like a man's arm, and yet he finally decided that flapping wings would not make the best propeller for his machine. Men have not adopted machinery legs for swift locomotion, although legs are nature's models, but they have, rather, constructed wheels--contrivances which practically do not exist in nature. Therefore, while Professor Langley admits that successful flying machines may one day be made with flapping wings, he began his experiments with the screw propeller.