152 THE FLIGHT OF BIRDS 



direction he wants, and, if there has been any loss by 

 leeway, make it good. 



Soaring in a Horizontal Wind impossible. 



Given two things, a strong upward stream of air 

 and a big bird possessed of great skill, soaring 

 becomes quite explicable. But there are still some 

 people, I believe, who hold that an up-current 

 is not an indispensable condition, but that a hori- 

 zontal wind is all that is needed if only the wind is 

 not uniform, so that somehow the bird may perpetu- 

 ally manage to be passing from a comparatively 

 slow current into a faster one. How such a process 

 can be continued for an indefinite time is more than 

 I can understand. Nevertheless some great mathe- 

 maticians, who do not, however, profess to have 

 actually watched birds soaring, maintain that it 

 is theoretically possible. Setting aside theoretic 

 possibilities for the moment, let us see what we can 

 learn by observation. Wind is least uniform close 

 to the earth, and there we find birds turning this 

 want of uniformity to account. They face the wind 

 as they rise and get help from it, owing to the fact 

 that they are perpetually passing from comparatively 

 slowly moving air into a more rapid current. But 

 they cannot get the wind to do the whole work of 

 lifting, whatever onward momentum they may have. 

 They ply their wings vigorously all the time. 



Evidently, then, a wind with varying velocities is 

 not enough to account for soaring. Imagine, too, 

 what would happen as the bird circled round. In 

 each complete turn of the helix he must, for part of 

 the time, have his back turned to the wind, and the 



