232, THE STRUCTURE AND LIFE OF BIRDS cuap. 
Wind and Flight. 
Before going on to describe other varieties of flight, 
it will be well to make clear, as far as possible, how 
the wind may help a bird to rise to a higher elevation. 
Many begin early to gain the skill that is necessary 
if they are to avail themselves of this assistance. 
As soon as he has the use of his wings a young gull 
may be seen for a good part of the day busily 
practising. And the great proficients in the art— 
eagles, vultures, storks, and albatrosses—have acquired 
their skill by experimenting on all varieties of currents. 
The problem presents far greater difficulties for us. 
With labour and complicated contrivances we obtain 
some notion of what wind is, of its force, its currents, 
its eddies and gusts. By long and painstaking ob- 
servation the naturalist discovers what curves the bird 
describes in the air, whether he looks down the wind, or 
in the teeth of it or across it, and what angle his body 
forms with the horizon. Mercilessly the mathematician 
applies his laws showing that the theory of soaring 
that the naturalist fondly cherishes is built on an 
absurd assumption, while he himself, without inventing 
conditions which possibly do not exist, may fail to 
show how the aerial play of the gyrating vulture is 
physically possible. 
With all these difficulties before us, let us first lay 
a sound mathematical foundation. However tempting 
the theory may be, that a Senior Wrangler has one 
system of mathematics and a Pelican a totally different 
one, we must not be beguiled by it. 
