F. W. HEADLEY: WIND AND FLIGHT. 171 



Gull that hangs with wings held rigid over the stern of a 

 moving steamer. Each is poised upon an up-current. 

 But the soaring bird circles round in order to keep himself 

 in the most favourable region. In a mountain country 

 there are all the necessary conditions for this. The 

 question is as to soaring over a wide-stretching level plain. 

 There, too, there may be ascending currents, owing to the 

 ground being heated by the sun. 



The great cumulus clouds, with which we are familiar, 

 are believed to be the tops of ascending currents of air. 

 The warm air as it ascends, tends to collect in definite 

 streams ; it becomes chilled, its moisture is condensed, 

 and the clouds are formed. Till recently I was not 

 inclined to believe that there were any uj^-currents over 

 wide-stretching plains strong enough to make soaring a 

 possibility. I held to the view that the Adjutants over 

 the jjlain of Upper Assam described their majestic spirals 

 by the help of eddies in a horizontal breeze. But when 

 we fix our attention on what we know takes place, rather 

 than on theoretical possibilities, we must, I think, give up 

 this view altogether. Even from the great plain of 

 Upper Assam we have very strong evidence against the 

 horizontal wind theory, and so for the up-current theory. 

 Mr. Peal states (see especially Nature, 21st May, 1891), 

 that the Adjutants and other soarers always rise the first 

 hundred or two hundred feet by flapping their wings. 

 But it is at the lower levels that the velocity of the wind 

 increases most rapidly with altitude. It is at the lower 

 levels, I believe, that the stream of the wind is fullest of 

 eddies ; they are due to the fact that the surface of the 

 earth is not smooth or level. At low levels, then, hori- 

 zontal winds have the irregularity of velocity that is said 

 to be the all-important condition for soaring, and yet 

 soaring does not take j)lace there. At great altitudes, 

 where we have reason to believe that the wind is much 

 more uniform in velocity, we see soaring in perfection. 

 It is true that birds may be seen circling round under 

 circumstances that mio'lit be thouo-ht to exclude the 



