748 THE POPULAR SCIENCE MONTHLY. 



descend instead of ascend. A necessary condition, therefore, is 

 wind ; and, indeed, a soaring bir<J 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 ne- 

 cessary consequence of the last conditions for an even current is 

 still air to the bird immersed in it, precisely as the earth is prac- 

 tically still to us dwelling on it. If a cloud should intervene be- 

 tween 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 napping, 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 ve- 

 locity goes on increasing upward. A gentle breeze on the plain 

 becomes a furious wind on the mountain-top. Like all other cur- 

 rents, 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 cur- 

 rents, and skillfully uses them to lift himself. In soaring, the 

 bird slopes downward with the wind, acquiring thus great veloci- 

 ty, 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 prac- 

 tically only skin friction, which is also small. Observe (2) that 

 with large aeroplane 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 



