244 THE STRUCTURE AND LIFE OF BIRDS chap. 



Birds, like all matter living or dead, are subject 

 to physical laws. We must, therefore, not relegate 

 soaring to the realm of miracle, but try to find some 

 solution of the problem. 



There are three explanations possible, all of which 

 may be true, each of them being applicable to par- 

 ticular circumstances. 



(i) There may be upward currents. We have 

 already condemned as fantastic Lilienthal's notion 

 that the general tendency of the wind is upward. 

 But, as I have shown, in particular places it un- 

 doubtedly is so. A Raven or a Vulture soaring over 

 a hilltop is probably making use of alternating 

 upward and downward currents, or else, if the streams 

 of air in that particular spot flow steadily, sweeping 

 from a downward into an upward one. This means 

 of soaring is, it cannot be doubted, put at his disposal 

 through the diversion of the wind when it comes in 

 contact with a high ridge. If the results I have 

 described could be produced by a bank only a few 

 feet high, what great irregularities, what tremendous 

 up and down blasts, may be caused by such a barrier 

 as a hill of only moderate height ! Gulls are very 

 quick in finding out such currents, and enjoy playing 

 in them. When the wind is blowing hard against a 

 cliff and is diverted upward, they will fly again and 

 again to its base and let themselves be carried to the 

 top. Or they will float and circle high above a cliff, 

 profiting no doubt by the upward slant of the breeze. 

 They will often follow a steamer with wings perfectly 

 motionless, and the explanation of this, probably, is 

 that the passage of the vessel through the air causes 



