But here, at all events, is a demonstration of this theory of gliding flight--one that can be carried out without a motor. Of course such flying has its restrictions: a man must start from the summit of a hill, and the glide is in the form of a descent towards the ground below; but still he is passing through the air; and above all--and this proved the advantage of the scheme for such a pioneer as Lilienthal--there is no need, during any such glide, to pass high above the ground. The operator may, in fact, if the side of his hill slopes gently, skim within only a few feet of its surface; and this means that, should he lose his balance at first, as he may expect to do, he will not share the fate of those who leapt from towers, but will be able to alight without mishap. [Illustration: FIG. 22.--Lilienthal Glider.] This, then, was Lilienthal’s plan; he would build a machine with wings and a tail, stand facing the wind as the storks had done, then seek to glide through the air in the manner of a soaring bird. The idea underlying his scheme was that he hoped, by a series of such gliding flights, to learn the adjusting movements he knew would be necessary to preserve his balance in the air. The gliding craft Lilienthal built, as illustrated in Fig. 22, has become an historical machine. The framework of its wings and the supports of its tail were of willow, and the wings and tail, to give them their grip upon the air, were covered with a smoothly stretched fabric. Then the whole structure was braced and tightened; and though it weighed less than 50 lbs., it was strong enough to bear its operator though the air. Lilienthal could raise the apparatus upon his shoulders--passing his head through the aperture between the planes, which will be noted in the sketch--and walk or run forward; and to hold the machine, as he carried it thus, he gripped two wooden rods. The tail was flexible, being allowed an automatic movement, thus giving the craft a certain natural stability. The main wings had a span of 24 feet, and the machine measured 18 feet from front to tail. The wings were cambered, according to the curve Lilienthal had decided most efficient, and contained about 180 square feet of lifting surface. In giving them this area, Lilienthal was relying upon experiments he had made; these showed that, as his machine glided through the air, each square foot of its surface should bear a weight equal to about 1 lb.