VII FLIGHT 227 
surfaces face downwards and slightly backwards. 
Why, then, if they are adjusted nearly as they are 
for horizontal flight, is his progress almost vertically 
upward? The natural action of his wings is to drive 
him onward as well as upward. But not much onward 
movement can take place, since the air offers too much re- 
sistance to the expanse of his breast and tail. His on- 
ward velocity is, in part, therefore, converted into upward 
velocity ; and thus he is giving a practical illustration 
of the working of the parallelogram of forces. He 
always faces the wind, and derives, no doubt, great 
help from it ; how, I shall explain later on (see p. 239). 
Some birds can ascend much more rapidly, ze. at a 
much steeper incline than others. The reason of this 
is, I believe, that they have greater freedom at the 
shoulder joint, so that they can turn their wing further 
over, giving it a steeper slope from the front to the 
hinder margin. If a bird has but little power of 
doing this, when he inclines his body upward so as | 
to form a large angle with the horizon, his wings will 
beat backwards and forwards instead of up and down. 
~Wishing to test this I examined the wings of a good 
many birds, some of them alive, some just after they 
were shot. I found that the lark when its wing was 
extended could lower its front margin a great ded 
without any strain. The same was true, though not 
quite to the same extent, of the jackdaw, jay, crow, 
chough, magpie, rook, raven, quail, plover, eagle, all of 
which, I believe, are capable of ascending ‘at a fairly" 
steep incline. On the other hand the gannet, herring 
gull, blackheaded gull, pelican, cormorant, had none 
of them much power of rotating the wing, the cormorant, 
Q 2 
