Professor Langley, in attacking the same problem, first studied the principle and behaviour of a well-known toy--the model invented by Penaud, which, driven by the tension of india-rubber, sustains itself in the air for a few seconds. He constructed over thirty modifications of this model, and spent many months in trying from these to as certain what he terms the "laws of balancing leading to horizontal flight." His best endeavours at first, however, showed that he needed three or four feet of sustaining surface to a pound of weight, whereas he calculated that a bird could soar with a surface of less than half a foot to the pound. He next proceeded to steam-driven models in which for a time he found an insuperable difficulty in keeping down the weight, which, in practice, always exceeded his calculation; and it was not till the end of 1893 that he felt himself prepared for a fair trial. At this time he had prepared a model weighing between nine and ten pounds, and he needed only a suitable launching apparatus to be used over water. The model would, like a bird, require an initial velocity imparted to it, and the discovery of a suitable apparatus gave him great trouble. For the rest the facilities for launching were supplied by a houseboat moored on the Potomac. Foiled again and again by many difficulties, it was not till after repeated failures and the lapse of many months, when, as the Professor himself puts it, hope was low, that success finally came. It was in the early part of 1896 that a successful flight was accomplished in the presence of Dr. Bell, of telephone fame, and the following is a brief epitome of the account that this accomplished scientist contributed to the columns of Nature:-- "The flying machine, built, apparently, almost entirely of metal, was driven by an engine said to weigh, with fuel and water, about 25 lbs., the supporting surface from tip to tip being 12 or 14 feet. Starting from a platform about 20 feet high, the machine rose at first directly in the face of the wind, moving with great steadiness, and subsequently wheeling in large curves until steam was exhausted, when, from a height of 80 or 100 feet, it shortly settled down. The experiment was then repeated with similar results. Its motion was so steady that a glass of water might have remained unspilled. The actual length of flight each time, which lasted for a minute and a half, exceeded half a mile, while the velocity was between twenty and twenty-five miles an hour in a course that was constantly taking it 'up hill.' A yet more successful flight was subsequently made."