When the Wrights determined to fly, two problems which had beset earlier experimenters had been partially solved. Experience had brought out certain facts regarding the wings; and invention had supplied an engine. But the laws governing the balancing and steering of the machine were unknown. The way of a man in the air had yet to be discovered. The starting point of their theory of flight seems to have been that man was endowed with an intelligence at least equal to that of the bird; and, that with practice he could learn to balance himself in the air as naturally and instinctively as on the ground. He must and could be, like the bird, the controlling intelligence of his machine. To quote Wilbur Wright again: "It seemed to us that the main reason why the problem had remained so long unsolved was that no one had been able to obtain any adequate practice. Lilienthal in five years of time had spent only five hours in actual gliding through the air. The wonder was not that he had done so little but that he had accomplished so much. It would not be considered at all safe for a bicycle rider to attempt to ride through a crowded city street after only five hours' practice spread out in bits of ten seconds each over a period of five years, yet Lilienthal with his brief practice was remarkably successful in meeting the fluctuations and eddies of wind gusts. We thought that if some method could be found by which it would be possible to practice by the hour instead of by the second, there would be a hope of advancing the solution of a very difficult problem." The brothers found that winds of the velocity they desired for their experiments were common on the coast of North Carolina. They pitched their camp at Kitty Hawk in October, 1900, and made a brief and successful trial of their gliding machine. Next year, they returned with a much larger machine; and in 1902 they continued their experiments with a model still further improved from their first design. Having tested their theories and become convinced that they were definitely on the right track, they were no longer satisfied merely to glide. They set about constructing a power machine. Here a new problem met them. They had decided on two screw propellers rotating in opposite directions on the principle of wings in flight; but the proper diameter, pitch, and area of blade were not easily arrived at. On December 17, 1903, the first Wright biplane was ready to navigate the air and made four brief successful flights. Subsequent flights in 1904 demonstrated that the problem of equilibrium had not been fully solved; but the experiments of 1905 banished this difficulty.