WIND AND FLIGHT 123 



ascend, more rapid than the one below it, the rate 

 of increase growing less as the earth is left farther 

 and farther below. As he mounts through each 

 successive stream of air the bird has always inertia ; 

 he is never, like a balloon, the toy of the breeze. And 

 this, not only because he keeps plying his wings, but 

 because he is always emerging from a slower stream 

 of air into a more rapid one. Consequently, quite 

 apart from his vigorous wing-strokes, he offers 

 resistance to the wind ; he has, in fact, the inertia 

 that is indispensable. The force of the horizontal 

 wind is broken up into two forces, one of which tends 

 to lift him. The Lark, that past-master in the art 

 of upward flight, always gets the wind to lighten 

 the work of his wings, even up to considerable alti- 

 tudes. Sometimes, when the wind fails, he will sweep 

 vigorously round in a wide circle and make the 

 velocity due to his own efforts to some extent take 

 the place of a wind. Big and small alike, all birds 

 are glad to have the help of the wind. The muscles 

 that lift their wings are by no means strong, but as 

 soon as they have got some way on the rush through 

 the air does the work of lifting. A big, heavy 

 Elevator muscle would, therefore, be a useless 

 encumbrance during horizontal flight ; it is best to 

 put all the strength into the Depressor. Not only 

 is the Elevator small, but, as I have shown, it is of 

 inferior quality ; it has not much last. All the more 

 reason, therefore, to use artifice in order to economize 

 effort in rising ; or, to put it more correctly perhaps, 

 birds have not developed high-class Elevator muscles 

 since their skill rendered them unnecessary. Big 

 birds require the help of the wind to lift them at the 



