This is the problem of the man who would navigate the air. He launches himself in a treacherous, unstable element: constantly, beneath his wings, the air pressure changes and varies in its strength; constantly is he losing his balance--and as constantly must he regain it. Imagine a man walking a tight-rope, and seeking incessantly to keep himself in equilibrium, and you have a notion of what the first man faced when he strove to fly. And his case, really, was worse than that of the tight-rope walker. The latter is concerned mainly with the danger of falling to one side or the other; he need not trouble himself unduly with the problem of his fore and aft stability. But the aerial acrobat may fall forward or backward, or from side to side. Hence his trick, once he masters it, is the more skilful. The art, as has been shown, is to bring together these centres of gravity and pressure; and it can be done in either of two ways. One is to alter the centre of gravity should the machine begin to fall, and the other to move the centre of pressure. Lilienthal, and others of the early gliders, adopted the plan first mentioned; they shifted the centre of gravity. But others who followed them, and notably the Wright brothers, finding that they needed to build larger craft, made use of movable planes by which they could shift the centre of pressure; but this, of course, will be dealt with in its place. To alter the centre of gravity of a machine it is necessary to move in some way the weight it carries--to shift the load forward or backward, say, or from side to side. In Lilienthal’s glider the load was the weight of his own body, and he learned to move this when wind-gusts struck his craft. His body, as he passed through the air in flight, hung free from the shoulders below the wings of his machine; he was therefore able to swing himself forward or backward, or from side to side. And this he did, counteracting the rolling movements of his machine, and seeking always to prolong the glide. Should his craft be struck by a sudden gust, for example, and heel to one side, he swung the weight of his body towards the rising wing; should he dive abruptly, or threaten to rise at an acute angle, he was ready with a movement of his body to check the falling tendency and restore the machine to an even keel. But the point to be considered is this: all these movements, when a craft is in flight, have to be made with a lightning rapidity. There is not an instant to lose; not a fraction of a second to be wasted while a man thinks what he is to do. His balancing, if he would glide through the air with wings, must be instinctive--instantaneous; as, indeed, is the balancing of the birds. Here, then, is the difficulty: to learn to make these balancing movements with sufficient quickness; and this Lilienthal found to be the stumbling-block. Time after time, while gliding close to the ground, his machine lost its balance and, before he could correct