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an upward current. I have tried to detect such a 
current by means of small scraps of paper let fly from 
the stern of asteamer. 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- 
currents 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. 
(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 shown2 The Adjutant wheeling upward 
would be only doing on a grand scale what Gulls may 
1 See Mature, October Ist, 1891. 
2 See Nature, May, 1883, May 9, 1889. 
