vii KLiCIIT 245 



an upward current. I have tried to detect such a 

 current by means of small scraps of paper let fly from 

 the stern of a steamer. They showed that immediately 

 behind the vessel the air rushes violently downward 

 to fill the vacuum left as she moves onward. But 

 the rebound of this current from the water was by no 

 means so marked as I should have expected, though 

 there were unmistakable signs of a certain amount 

 of updraught. 



It cannot be denied that when beneath a tropical 

 or sub-tropical sun a plain is heated in various degrees 

 at different places, according to the nature of the 

 surface, there are upward movements of air, and it has 

 been held by some writers that the up-currents thus 

 formed are sufficient to render soaring possible. It 

 is urged in support of this view that birds soar chiefly 

 during the great heat of the day, and more in summer 

 than in winter, and it is certainly remarkable that to 

 see soaring at its best you have to go far south. But 

 before we can believe in the adequacy of these up- 

 currcnts we want further evidence. The fact, too, 

 that soaring does not begin till 100 — 200 feet up 

 seems to me to tell against this view. 1 



(2) Increase of the wind's velocity with altitude will 

 explain much, but the principle must be cautiously 

 applied. If this progressive increase extended to 

 altitudes of a mile or two, the grandest feats of soaring 

 birds might easily be explained, as Lord Rayleigh and 

 others have shown. 2 The Adjutant wheeling upward 

 would be only doing on a grand scale what Gulls may 



1 Sec Nature, October 1st, 1891. 



2 See Nature, May, -1883, May 9, 1889. 



