"Nature has made her flying-machine in the bird, which is nearly a thousand times as heavy as the air its bulk displaces, and only those who have tried to rival it know how inimitable her work is, for the "way of a bird in the air" remains as wonderful to us as it was to Solomon, and the sight of the bird has constantly held this wonder before men's minds, and kept the flame of hope from utter extinction, in spite of long disappointment. I well remember how, as a child, when lying in a New England pasture, h watched a hawk soaring far up in the blue, and sailing for a long time without any motion of its wings, as though it needed no work to sustain it, but was kept up there by some miracle. But, however sustained, I saw it sweep in a few seconds of its leisurely flight, over a distance that to me was encumbered with every sort of obstacle, which did not exist for it.... How wonderfully easy, too, was its flight! There was not a flutter of its pinions as it swept over the field, in a motion which seemed as effortless as that of its shadow. After many years and in mature life, I was brought to think of these things again, and to ask myself whether the problem of artificial flight was as hopeless and as absurd as it was then thought to be"... In three or four years Langley made nearly forty models. "The primary difficulty lay in making the model light enough and sufficiently strong to support its power," he says. "This difficulty continued to be fundamental through every later form; but, beside this, the adjustment of the center of gravity to the center of pressure of the wings, the disposition of the wings themselves, the size of the propellers, the inclination and number of the blades, and a great number of other details, presented themselves for examination." By 1891 Langley had a model light enough to fly, but proper balancing had not been attained. He set himself anew to find the practical conditions of equilibrium and of horizontal flight. His experiments convinced him that "mechanical sustenation of heavy bodies in the air, combined with very great speeds, is not only possible, but within the reach of mechanical means we actually possess." After many experiments with new models Langley at length fashioned a steam-driven machine which would fly horizontally. It weighed about thirty pounds; it was some sixteen feet in length, with two sets of wings, the pair in front measuring forty feet from tip to tip. On May 6, 1896, this model was launched over the Potomac River. It flew half a mile in a minute and a half. When its fuel and water gave out, it descended gently to the river's surface. In November Langley launched another model which flew for three-quarters of a mile at a speed of thirty miles an hour. These tests demonstrated the practicability of artificial flight.