216 ANIMAL MECHANISM. 



Form of the bird. All those who have studied the flight of 

 birds have very properly paid great attention to the form of 

 these creatures, as rendering them eminently adapted to flight. 

 They have recognised in them perfect stable equilibrium in 

 the aerial medium. They have thoroughly understood the 

 part played by the large surfaces formed by the wings, which 

 may sometimes act as a parachute, to produce a very slow 

 descent; while at other times these surfaces glide through the 

 air, and following the inclination of their plane, allow the 

 bird to descend very obliquely, and even to rise, or to hover 

 while keeping its wings immovable. Some observers have 

 gone so far as to admit that certain species of birds play 

 an entirely passive part in flight, and that giving up their 

 wings to the impulse of the wind, they derive from it a force 

 capable of carrying them in every direction, even against the 

 wind. It seems to us interesting to discuss, in a lew words, 

 this important question in the theory of flight. 



The stable equilibrium, of the bird lias been well explained ; 

 there is nothing for us to add to the remarks which have been 

 made on this subject. The wings are attached exactly at 

 the highest part of the thorax, and consequently when the 

 outstretched wings act upon the air as a fulcrum, all the 

 weight of the body is placed below this surface of suspension. 

 We know also that in the body itself, the lightest organs, the 

 lungs and the air vessels, are in the upper part ; while 

 the mass of the intestines, which is heavier, is lower; also 

 that the thoracic muscles, which are so voluminous and heavy, 

 occupy the lower part of the system. Thus the heaviest 

 part is placed as low as possible beneath the point of sus- 

 pension. 



The bird, as it descends with its wings outspread, will thus 

 present its ventral region downwards, without its being neces- 

 Miry to make an effort to keep its equilibrium ; it will take 

 this position passively, like a parachute set free in space, or 

 like the shuttlecock when it falls upon the battledore. 



But this vertical descent is an exceptional case ; the bird 

 which allows itself to fall is almost always impelled by some 

 previous horizontal velocity; it therefore slides obliquely upon 

 iJie air, us every light body of large surface docs whew placed 



