F. W. HEADLEY : WIND AND FLIGHT. 169 



to doubt that this feat is achieved entirely by the aid of 

 an up-current. We find Gulls rising- to a great heig'ht 

 over cliifs, which undoubtedly give the wind an upward 

 incline. There they circle easily and gracefully in the air, 

 a performance which they never attemj^t out at sea, where 

 presumably the wind is horizontal. Eagles, vultures, 

 kites, buzzards, ravens, and other birds, soar to great 

 heights, and, more frequently than not, it is in hilly 

 country, where the wind must in places have an upward 

 incline. On the other hand, what is the utmost that the 

 unequal velocity of horizontal currents effects for birds in 

 the lower tracts of air? We find that in these lower 

 tracts birds always face the wind ; then they rise and so 

 get assistance from it. But in almost all cases they give 

 vigorous strokes with their wings during the whole time 

 of the ascent. It is true that Gulls, when they advance at 

 right angles to the wind {vide supra, p. 143), regain their 

 lost altitude by suddenly facing the wind — in some cases a 

 horizontal wind — which lifts them while they hold their 

 wings rigid. But they mount in this way only to a height 

 of a few yards, never, I think, more than ten or fifteen. 

 And this reminds us of the fact that the velocity of a 

 horizontal wind increases with altitude far more rapidly 

 at low levels than at high. Here, then, are facts which 

 ought to help us to form a sound theory of soaring. At 

 low levels when birds get the wind to help them, it is to 

 up-currents that their greatest achievements are due. 

 In higher regions up-currents are no less effective, for it 

 is beyond doubt that much of the noblest soaring takes 

 place when cliffs or mountains give the wind an upward 

 tilt. It seems probable, then, that in every case soaring 

 must be explained in the same way — there must be an 

 uj)-current to account for it. Mr. Peal who has watched 

 the Adjutants circling over the great plain of Upper 

 Assam, maintains stoutly that the wind there is horizontal ; 

 he has seen floating shreds of cotton pass horizontally 

 across the field of his telescope. But it is very difficult to 

 judge when you see shreds of cotton from far below. 



