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 
