There were three great problems in building the flying machine. First, an engine and boilers light enough and at the same time of sufficient power. Second, a structure which should be rigid and very light. Third, the enormously difficult problem of properly balancing the machine, which, Professor Langley says, took years to solve.... Professor Langley established an experimental station in the Potomac River, some miles below Washington. An old scow was obtained, and a platform about twenty feet high was built on top of it. To this spot, in 1893, the machine was taken, and here failure followed failure; the machine would not fly properly, and yet every failure, costly as it might be in time and money, brought some additional experience. Professor Langley found out that the aerodrome must begin to fly against the wind, just in the opposite way from a ship. He found that he must get up full speed in his engine before the machine was allowed to go, in the same way that a soaring bird must make an initial run on the ground before it can mount into the air, and this was, for various reasons, a difficult problem. And then there was the balancing. "If the reader will look at the hawk or any soaring bird," says Professor Langley, "he will see that as it sails through the air without flapping the wing, there are hardly two consecutive seconds of its flight in which it is not swaying a little from side to side, lifting one wing or the other, or turning in a way that suggests an acrobat on a tight-rope, only that the bird uses its widely outstretched wings in place of the pole." It must be remembered that air currents, unlike the Gulf Stream, do not flow steadily in one direction. They are forever changing and shifting, now fast, now slow, with something of the commotion and restlessness of the rapids below Niagara. All of these things Professor Langley had to meet as a part of the difficult balancing problem, and it is hardly surprising that nearly three years passed before the machine was actually made to fly--on March 6, 1896.