146 THE FLIGHT OF BIRDS 



must take the argument for what it is worth. But 

 there is direct evidence. Aviators are now exploring 

 the air just as birds for ages past have done, and I 

 read in an article on military aviation* that remous 

 (up and down currents of air) " are seldom met with 

 when flying over a uniform surface such as the sea." 

 In hot countries the air is heated by contact with the 

 sun-baked soil. It ascends and when, at some 

 height, it gets chilled by contact with colder air, it 

 forms cumulus clouds. It is found that the rate at 

 which balloons ascend varies much when there are 

 clouds of this kind about, and that is good evidence 

 that the air in places is streaming upward. f At a 

 low level there are no distinct upward and downward 

 streams, but at some height above the plains such 

 streams begin to form and the great cumulus clouds 

 tell us in what parts the movement is upward. One 

 day in the Nile delta, a very hot day with a blazing 

 sun, I was watching the Kites soaring. They were 

 wheeling and wheeling round and not a motion of 

 wing was to be seen. Suddenly a cloud obscured 

 the sun, and very soon all the Kites began beating 

 with their wings, and descended to a lower level. 

 It may be maintained that they do not care to soar 

 unless it is bright and sunny, and that they gave it 

 up because, there being no longer any sunshine, they 

 had no pleasure in continuing. Still I have seen 

 Ravens soaring over hills in cloudy weather, and 

 one day in the Alps, when snow or sleet fell at 

 intervals and the wind was raw and nasty, I saw a 



* By Captain Brooke-Popham, Army Review, Jan. 1912, p. 88. 



^ See "Methods for Observing Pilot Balloons," by C. J. P. 

 Cave, Journal of the Royal Meteorological Society, Jan., 1910. 



