•582 THE MECHANICAL FUNCTIONS. 



gravity. The difFerent portions of the wing are 

 likewise so disposed 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 disposition of 

 the great feathers is such that they strike the air 

 with their flat sides, but present only their edges 

 in rising : what is called 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 back- 

 wards, 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 

 perpendicular ascent. Those birds, on the con- 

 trary, 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 directly 

 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, contri- 



