170 BRITISH BIRDS. 



whether they are sailing horizontally or travelling- slightly 

 upward or downward. And here is another argument, 

 which any one who has watched birds soaring will, I 

 think, account a strong one. They circle slowly, calmly, 

 comfortably, round. Mr. Peal believed that they slept 

 high aloft as they formed their spirals. Any one who has 

 watched the constant slight adjustments for purposes of 

 balance made by a Gull following a steamer with wings 

 outstretched, will hardly agree with this. Still the 

 soaring bird sweeps slowly, majestically I'ound. Its 

 movements suggest dependable up-currents, not the seizing- 

 of occasional opportunities, occasional differences of 

 velocity, by j)assing from one part of an eddy to another, 

 or by taking advantage of a gust from time to time. 



If the soaring bird is poised upon an ascending current 

 of air, it might be thought that there would be nothing to 

 gain by circling round and forming a spiral or helix. But 

 if an up-current is only local, the circling movement would 

 be necessary in order that the bird may keep within its 

 limits. I once used a vane made to work vertically in 

 order to discover the extent and nature of the up-currents 

 in the neighbourhood of a bank about six feet high. 

 Several trials were made. On each occasion a fresh 

 breeze was blowing. Before it reached the bank, at a 

 distance of four yards from it, the wind was deflected so 

 that it had a decided upward incline. On the other side, 

 at a distance of five yards from the bank, it blew down- 

 wards, making a large angle with the horizon. Occasion- 

 ally there was an upward gust. At a distance of ten yards 

 the direction was much more of the downward than 

 upward. At points between twenty and thirty yards from 

 the bank there were wild gusts, now upward, now down- 

 ward. I also tested the effect of a bank only two feet 

 high. The vane was raised by its tripod four feet above 

 the bank, yet it indicated a decided upward incline of the 

 wind. A mountain barrier must cause up and down 

 currents such as these, only on a far larger scale. The 

 soaring Eagle is, I believe, practising the same art as the 



