152 THE FLIGHT OF BIRDS 
direction he wants, and, if there has been any loss by 
leeway, make it good. # 
Soaring in a Horizontal Wind impossible. 
Given two things, a strong upward stream of air 
and a big bird possessed of great skill, soaring 
becomes quite explicable. But there are still some 
people, I believe, who hold that an up-current 
is not an indispensable condition, but that a hori- 
zontal wind is all that is needed if only the wind is 
not uniform, so that somehow the bird may perpetu- 
ally manage to be passing from a comparatively 
slow current into a faster one. How such a process 
can be continued for an indefinite time is more than 
I can understand. Nevertheless some great mathe- 
maticians, who do not, however, profess to have 
actually watched birds soaring, maintain that it 
is theoretically possible. Setting aside theoretic 
possibilities for the moment, let us see what we can 
learn by observation. Wind is least uniform close 
to the earth, and there we find birds turning this 
want of uniformity to account. They face the wind 
as they rise and get help from it, owing to the fact 
that they are perpetually passing from comparatively 
slowly moving air into a more rapid current. But 
they cannot get the wind to do the whole work of 
lifting, whatever onward momentum they may have. 
They ply their wings vigorously all the time. 
Evidently, then, a wind with varying velocities is 
not enough to account for soaring. Imagine, too, 
what would happen as the bird circled round. In 
each complete turn of the helix he must, for part of 
the time, have his back turned to the wind, and the 
