172 BRITISH BIRDS. 



assumption of up-currents. I have sometimes seen birds 

 soaring' over j)laiiis — e.g., in Egypt and in Spain — early in 

 the day, before the sun had gained strength to cause the 

 ascent of heated columns of air. But it is possible that 

 the wind was deflected uj)ward by hills that were not 

 very many miles distant. If a small bank can cause the 

 results that I have described, what cannot a high moun- 

 tain do, a mountain which causes a mighty stream of air, 

 a thousand or several thousand feet in dex^th, to flow 

 upward ? 



There is one marvellous performance of which an up- 

 current must undoubtedly have the credit. Every one is 

 familiar with the hovering of the Kestrel. As a rule there 

 is a wonderfully rapid beating of the wings, but occasionally, 

 when the conditions are favourable, they are held out- 

 stretched and motionless. The bird hangs in mid-air as 

 if gravitation were a myth. In Algeria I once saw an 

 Egyptian Vulture thus jnoised on air. In this case, a hori- 

 zontal breeze with varying velocity will avail us nothing. 

 But in the case of the soaring bird it is no better. Let us 

 consider how it will act. As he circles round he must 

 now face the breeze, now turn his back towards it. When 

 facing" it, if he is passing from a slow-moving current into 

 a rapid one, he will, no doubt, be supported, if not lifted. 

 But how will it be when he turns his face away from the 

 wind ? Just after making- the turn he will necessarily 

 move more slowly than the wind, which striking upon his 

 back will drive him downward. Gravitation will pull, the 

 wind will push him down. In each circle he must lose far 

 more than he can gain in altitude. But we know that as 

 a fact he often completes a circle without losing altitude 

 at any stage. A strong upward, slanting current would 

 account for this. We know that Gulls maintain their 

 level and occasionally glide upward as they hang with 

 outstretched wings over the stern of a steamer, having aU. 

 the while an upward slanting wind blowing in their faces, 

 and I have described how a Gull in Alderney advanced 

 without movement of his wings by the aid of a wind that 



