GLIDING 17 



Very fast flyers, such as the Swift, have very little 

 concavity. There is another fact, too, that points 

 to the same conclusion : in a bird's wing the part 

 that is near the body is the most concave, while 

 towards the extremity it becomes nearly flat, in 

 some cases quite flat. Now the extremity of the 

 wing, when a stroke is given, moves with great 

 velocity forward as well as downward, the near 

 part comparatively slowly. Thus birds seem to be 

 exponents of the principle that the curve should 

 vary inversely as the speed. And here I must 

 point out a great advantage the bird has over his 

 upstart rival, the aeroplane. The aeroplane is 

 rigid, with at most the rear edge only of its plane 

 flexible. If the principle to which I have called 

 attention is sound, it must be built either with a 

 curve appropriate to great speed or with one which 

 suits comparatively slow travelling. It cannot 

 be varied during flight according as the pace is 

 varied. But when the bird takes a very hard 

 stroke, the front-to-back curve of his wings, which 

 is mainly the curve of the feathers, is much reduced, 

 so that by plying his wings with great vigour he to 

 some extent modifies his configuration and adapts 

 it to his high velocity. (For birds in rapid flight, 

 see Pis. iv-vn.) 



It is always well to ask " Why is it so ? " when- 

 ever experiment discovers a fact. Why, then, 

 with a curved surface is the lift more and the drift 

 less ? It is very difficult to see how the air does 

 its work, and theory on such a subject may prove 

 to be only random guesswork. But the lift would 

 seem to be greater from the simple fact that the 



