vii FLIGHT 227 



surfaces face downwards and slightly backwards. 

 Why, then, if they are adjusted nearly as they arc 

 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, i.e. 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. 1 found that the lark when its wing was 

 extended could lower its front margin a great deal 

 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, 



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