246 THE STRUCTURE AND LIFE OF BIRDS chap. 



be seen doing when they advance at right angles to 

 the wind without moving their wings. Descending 

 with all the velocity of the upper current, he would 

 rebound from the slower one below, and the inertia 

 due to this undercurrent would make him rise like a 

 kite when he re-entered the more rapid stream at a 

 higher level. But, as I have shown, it is very improb- 

 able that there is a regular progressive increase in 

 the velocity of the wind at high altitudes. We know, 

 too, that it comes in gusts, and this would bring it 

 about that the bird, if he depended on the arrange- 

 ment of the air in strata of increasing velocity, would, 

 when the breeze happened to strengthen inopportunely, 

 find himself in a slower instead of a more rapid 

 current. 



Whatever the difficulties that meet us when we 

 try to explain by this principle the phenomenon of 

 soaring, it is certain that near the surface of land or 

 sea the increase of velocity with altitude is rapid 

 enough and constant enough to assist a bird in rising 

 or in making progress at right angles to the wind. 



(3) I believe myself that the irregularity of the 

 wind may supply the explanation of soaring. The 

 wind is a " chartered libertine," and, even when 

 steadiest, blows, as Professor Langlcy has shown, with 

 great fitfulness. A bird, when soaring, if this ex- 

 planation be sound, will face a strengthening breeze : 

 when it begins to slacken will turn and go with it, 

 until conscious it is freshening again, when once more 

 he will sweep round and face it, his aim being always 

 to feel the wind blowing in his face, sure evidence 

 that he has momentum that will lift him. To do 



