!32 THE STRUCTURE AND LIFE OF BIRDS chap. 



Wind and Flight. 



Before going on to describe other varieties of flight, 

 it will be well to make clear, as far as possible, how 

 the wind may help a bird to rise to a higher elevation. 

 Many begin early to gain the skill that is necessary 

 if they are to avail themselves of this assistance. 

 As soon as he has the use of his wings a young gull 

 may be seen for a good part of the day busily 

 practising. And the great proficients in the art — 

 eagles, vultures, storks, and albatrosses — have acquired 

 their skill by experimenting on all varieties of currents. 

 The problem presents far greater difficulties for us. 

 With labour and complicated contrivances we obtain 

 some notion of what wind is, of its force, its currents, 

 its eddies and gusts. By long and painstaking ob- 

 servation the naturalist discovers what curves the bird 

 describes in the air, whether he looks down the wind, or 

 in the teeth of it or across it, and what angle his body 

 forms with the horizon. Mercilessly the mathematician 

 applies his laws showing that the theory of soaring 

 that the naturalist fondly cherishes is built on an 

 absurd assumption, while he himself, without inventing 

 conditions which possibly do not exist, may fail to 

 show how the aerial play of the gyrating vulture is 

 physically possible. 



With all these difficulties before us, let us first lay 

 a sound mathematical foundation. However tempting 

 the theory may be, that a Senior Wrangler has one 

 system of mathematics and a Pelican a totally different 

 one, we must not be beguiled by it. 



