6 THE FLIGHT OF BIRDS 



is a very elementary matter 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 



W 



Fig. 



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 



