ANIMAL FLIGHT 



107 



the larger birds like the loons and grebes as we see them in the 

 air look ridiculously small, yet these birds can fly for enormous 

 distances at a high speed. It is, however, very difficult for them 

 to get started, and 

 many of them can- 

 not rise from the 

 ground at all. 



Bird's wings per- 

 form two functions; 

 they hft the bird 

 and they drag it 

 forward. We all 

 know that if a light 

 object is thrown it 

 will not travel so far 

 as a heavier object 

 thrown at the same 

 speed. A pitcher 

 cannot throw a ball 

 of feathers so far as 

 he can a base ball. 

 If a large and heavy 

 bird can once get 

 going at good speed 

 a relatively small 

 force will keep him 

 going. His body is 

 inclined in such a 

 way that it is kept 

 in the air through 

 its momentum on 

 the principle of a 

 kite. The wings by their motion serve to maintain the speed, 

 but have very Uttle lifting to do. The lighter and smaller the 

 bird the less is its momentum. Lessened momentum prevents it 

 from maintaining its height by incHning its body against the air. 



Figs. 202-221. Pupae of Beetles. 

 For explanations of the figures see p. xvii. 



