vii FLIGHT 247 



this he would have to be perpetually feeling the 

 pulse of the wind, and, moreover, soaring would 

 be a much less regular progress upward than it 

 is supposed to be. Some turns of the helix would 

 be failures. There would be a loss instead 

 of a gain of elevation, or nothing more than a 

 maintenance of level would be achieved. But when 

 we watch a bird circling at a great height, what can 

 we tell of his progress during a particular minute ? 

 We only know that his general tendency is upward. 

 Gulls make many only partially successful turns when 

 they soar, and it is possible that similar failures in 

 a nobler performer like the Adjutant may remain 

 undetected. 



Supposing the irregularity of the wind to be due to 

 eddies, the bird may still be able to avail himself of 

 it, turning to account the difference in velocity of the 

 outer and inner rings of the whirl. This supposes that 

 the eddy is of a convenient size. It may be either so 

 large or so small as to be useless to him. The com- 

 plications of the problem, when we introduce into it 

 the question of eddies, are almost insurmountable. 

 In the present state of our knowledge we can only 

 say that there are great irregularities of velocity, to 

 whatever cause they may be due, and that it is highly 

 probable that a bird in soaring turns them to account. 

 We are apt to speak of soaring as being effected 

 without an)- muscular effort. But it is by muscles 

 that the wings are held fixed and immovable, and the 

 strain must be considerable. Apparently, however, 

 there is hardly any limit to the time during which a 

 bird can continue an exertion of this kind. No such 



