Page:Popular Science Monthly Volume 46.djvu/647

 nearer to or remoter from the axis. This difference of speed in the turning of the screw combined with the inevitably small angles at which its slanting surfaces are presented to the wind make it incapable of developing, by the help of any of the dynamo machines now in use, a power adequate to the weight of the machine itself, its accessories, and the aëronauts.

The laws that come into play in the flying of a kite are involved in the flight of birds independent of the movement of their wings. In calm weather the kite is moved forward by the boy, who runs along, drawing the string with him. This movement, creating a wind under the kite, causes the air to gather under its slanting surface, and thus calls into play the lifting power of the atmosphere. The flying machines propelled from the rear by a motor with sufficient velocity, on rising into the air are enabled to soar on the same principle. The stronger the wind, the better the kite will rise; the quicker the horizontal motion of the flying machine, the better its aëroplanes will develop the supporting



power of the air. The exertion made by a bird on traveling a long distance is smaller, and its sailing power greater, the faster it flies.

The fastest fliers have the smallest wing surface. Some birds even reduce the area of their supporting surfaces when they increase their speed by drawing in their wings and closing their tails. Consequently, in order to keep the aëroplanes of the flying machine within a moderate size, considerable velocity has to be developed. This at the same time serves to overcome the resistance of the wind and of air currents. The air-ship is not capable of maintaining the direction pursued unless its velocity is so much greater than that of the strongest wind that it can overcome the latter and yet have velocity to spare.

While the construction of the kite machines seems to insure success through the high velocity of movement that must be attained, a new obstacle arises from the difficulty of ascent. The speed of motion being the condition of their rising power, they