42 ALTITUDE IN FLYING. 



rookeries, to the distance at which the pasture upon 

 which they are to feed lies ; and the swallow tribe 

 wheel about far more rapidly and gracefully when 

 they hawk high before rain, than when they skim 

 the surfaces of the pools in fine weather. If we may 

 judge from their appearance when we see them on 

 the wing (the only means we have of judging), it 

 appears that birds, when they are not in search of 

 any thing upon the ground near them, mount up till 

 they come to that density of atmosphere which is 

 best suited to their weight and wings, and then con- 

 tinue onwards. There may be another reason : those 

 upper regions to which the birds ascend on their 

 long flights are in a great measure exempted from 

 the momentary gusts and squalls which war upon 

 the surface under them. 



This buoyancy, arising from the structure of the 

 wings, is a very beautiful portion of the mechanics of 

 birds, but it is one which man would find it very 

 difficult to imitate. The general form of the wing, 

 the characters of the feathers, the articulations of all 

 the joints, the relative power of the muscles, and even 

 the general form and action of the whole body, are 

 concerned in it. They are all living too all exerting 

 their peculiar animal actions, in unbidden harmony, 

 with each other ; for even the feathers are alive, and 

 the skin in which they are inserted can communicate 

 an individual motion to each. Thus the process is 

 one which we cannot analyse so as to bring it within 

 the scope of our very limited notions of mechanics, 

 though there is no doubt that both the compound 

 motion, and all the individual motions, of which it is 

 mad^,J||te are in strict accordance with mechanical 

 principles. 



The most singular part of the whole process of 



