As we watch birds--especially large birds, as hawks and gulls--winging their way about the sky, we may notice that their flight is accomplished in two ways; either they are moving through the air by flapping their wings up and down, or else with wings wide outstretched they are soaring or sailing in the air for long times together without apparently moving their wings at all. Certain birds, such as vultures and albatrosses, possess this power of soaring flight to an extraordinary degree, and the exact way in which they keep themselves poised aloft is indeed still a mystery. We cannot, however, as we watch, say, a hawk, hovering in the air with motionless wing, help being struck by its resemblance to the schoolboy’s kite, kept afloat high in the sky by the action of the wind properly applied to its surface, and we can at once see that the bird makes use of the same principle as the kite in its soaring or hovering flight. Indeed, just as a kite sinks to earth when the wind drops, so in a dead calm even an albatross has to flap its wings to keep afloat. It is to the principle of the kite, therefore, that the inventor of the flying machine must turn. He must adapt the same principle to his apparatus, and this he does in his aeroplane, which, as will be seen, is an all-important part of his machine, and which, in its simplest form, is nothing more or less than a kite. We know that if a light flat body, such as a kite, is lying upon the ground, and the wind gets under it so as to tilt it, it will be lifted by the wind into the air. The string of a kite is so adjusted that as the kite rises it is still held at an angle to the wind’s force, and so long as the kite remains tilted at the necessary angle so long it will continue to rise or poise itself in the air while the wind blows. When schoolboys fly their kites they choose an exposed spot, and a day when the wind is blowing freshly and steadily. One boy throws the kite into the air, while another, holding the string to which it is fastened, draws it tight by running with it against the wind. By this means the kite, if rightly adjusted, is held at the proper angle to the wind, and started without dragging along the ground to begin with. As soon as the wind has fairly caught the kite and carried it up into the air, the boy who holds the string need run no longer, but if the breeze suddenly fails, and the kite begins to drop, he may still keep his toy aloft by running quickly along and dragging the kite after him; the artificial wind he thus creates making up for the lack of the other. Now let us suppose that there is no string to hold the kite in proper position, and no boy to run with it; but that their places are supplied by a motor and propeller to drive it through the air; while at the same time it is so balanced as to preserve a fitting angle against a wind of its own making. We should then have a true flying machine, heavier than air, and yet capable of sailing through the sky.