146 THE FLIGHT OF BIRDS 
must take the argumeny 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.t 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. 
