WING OF BIRDS. 401 



Several circumstances remain to be noticed respecting the 

 structure and actions of the wings of birds. If we attend to the 

 mode of their articulation with the scapula, we find it ])roducing 

 a motion oblique with regard to the axis of the body, so that 

 the stroke which they give to the air is directed both down- 

 wards and backwards; and the bird, while moving forwards, is 

 at the same time supported in opposition to the force of gra- 

 vity. The different portions of the wing are likewise so dis- 

 posed as to be contracted and folded together when the wing 

 is drawn up, but fully expanded when it descends in order 

 to strike the air. It is obvious that, without this provision, 

 a great part of the motion acquired by the resistance of the 

 air against the wing in its descent would have been lost by 

 a counteracting resistance during its ascent. The disposi- 

 tion of the great feathers is such that they strike the air with 

 their flat sides, but present only their edges in rising; what 

 is c2L\\edi feathering the oar in rowing is a similar operation, 

 performed with the same intention, and deriving its name 

 from this resemblance. 



As the inclination of the wing is chiefly backw^ards, 

 the greatest part of the effect produced by its action is 

 to move the body forwards. Birds of prey have a great 

 obliquity of wing, and are consequently better formed for 

 horizontal progressive motion, which is what they chiefly 

 practise in pursuing their prey, than for a rapid perpendi- 

 cular ascent. Those birds, on the contrary, which rise to 

 great heights in a direction nearly vertical, such as the Quail 

 and the Lark, have the wings so disposed as to strike di- 

 rectly downwards, without any obliquity whatsoever. For 

 the same reason, birds rise better against the wind, which, 

 acting upon the oblique surface presented by the wings 

 during their flexion, contributes to the ascent of the body 

 on the same principle that a kite is carried up into the air 

 when retained in an oblique position. This circumstance 

 is particularly observable in the ascent of birds of prey, 

 whose wings have a great obliquity, and, when fully expand- 

 ed, present a very large extent of surface. 



Vol. I. 51 



