6 THE FLIGHT OF BIRDS 
is a very elementary mee to be made clear. Let 
us picture the bird to ourselves as gliding in an 
exactly horizontal direction. The resistance of the 
air to his forward progress is equivalent to a 
horizontal wind blowing against him, and what 
is wanted is support, to counteract the downward 
pull of gravity. From a horizontal wind he must 
somehow get this support. The question presents 
no difficulty to anyone who has studied elementary 
mechanics, and in the application of mathematical 
principles the bird is a wonderful proficient. He 
inclines his aeroplane (his expanded wings and his 
Ww 
<< OW 
Fig. 2. 
body) slightly upward, and the result is that the 
air supports him more than it resists him. It acts 
at right angles to the plane that he opposes to it. 
In fig. 2, B represents the gliding bird, w the 
rush of air against his expanded surface. If the air 
is still, there will, nevertheless, be the wind due to 
the bird’s own velocity. W’, at right angles to the 
bird, shows the direction in which the wind acts ; 
its lifting power, when the bird inclines himself 
thus, being greater than its resistance. The action 
of the air at right angles to a plane moving through 
it, is illustrated by an experiment sometimes made 
by very unscientific persons. A stone is thrown 
