descend instead of ascend. A necessary condition, therefore, is wind; and, indeed, a soaring bird always drifts with the wind. The spiral is never upright, but always inclined to leeward. In soaring, the bird slopes downward with the wind, then turns and rises, facing the wind. How does wind help him?
3. The feat is physically impossible in an even current of air. It is strange that this is not seen at once; and yet excellent writers have become confused on this point. I confess that until recently I have been confused myself. It really follows as a necessary consequence of the last conditions for an even current is still air to the bird immersed in it, precisely as the earth is practically still to us dwelling on it. If a cloud should intervene between the bird and the earth, it would be impossible for him to know whether he was in a current or not.
4. Therefore, in order to rise in a spiral without doing work by flapping, there must be differential currents, which the bird takes advantage of to do the work.
Explanation. Now, there are such differential air currents. "Wind, like all other currents, increases in velocity from bottom upward. Experiments show that on a grass meadow the velocity eight feet above ground is double of that at one foot, and the velocity goes on increasing upward. A gentle breeze on the plain becomes a furious wind on the mountain-top. Like all other currents, too, there are differential currents side by side, the velocity along some stream lines being greater than along others. Also in air currents, especially, the velocity varies in time i. e., the wind blows in puffs. These differential currents, both side by side and in altitude in time, would be evident at once if we could see the air. Now, the bird feels these invisible differential currents, and skillfully uses them to lift himself. In soaring, the bird slopes downward with the wind, acquiring thus great velocity, passes into a lower current of less velocity, then turns facing the wind, and shoots up a slope which carries him higher than the level of the start, then turns again in a current of still higher velocity, then descends again along a slope and repeats the same cycle.
To explain more definitely: Observe (1) that the lines of a bird are so fine that the front resistance is almost zero. There is practically only skin friction, which is also small. Observe (2) that with large aëroplane and rapid motion the fall by gravity is also very small. This is proved by experiments of Langley, to be described presently. Therefore, if the differential force of the air currents through which he circles is precisely equal to the skin friction plus the downward tendency, the bird will just rise to the level of the starting point; if greater, he will rise above that level. In order to rise, therefore, the differential force of the successive
