264 



ANIMAL MECHANISM. 



be explained by the resistance of the air. The muscular 

 apparatus of the bird, like that of the insect, has nothing to 

 do with the course of the wing ; elevation and depression are 

 almost all the movements that it can produce. But the 

 resistance of the air during the phase of descent gives rise to 

 the anterior convexity of the curve passed through, by means 

 of a mechanism which we already understand. The posterior 

 convexity which belongs to the ascensional phase is also 

 explained by the action of the air on the lower surface of the 

 wing, which it carries backward at the same time as it raises 

 it. We must seek for the demonstration of this theory in the 

 artificial representation of these difierent movements. 



CHAPTER VI. 



RE- ACTIONS OF THE MOVEMENTS OF THE WING ON THE 

 BODY OF THE BIRD. 



Re-actions of the movements of the wing— Vertical re-actions in different 

 species ; horizontal re -actions or changes in the rapidity of flight ; 

 simultaneous study of the two orders of re-actions— Theory of the 

 flight of the bird— Passive and active parts of the wing — Reproduc- 

 tion of the mechanism of the flight of the bird. 



In order that we may follow, in studying the flight of the 

 bird, the same plan which has guided our researches on the 

 other kinds of locomotion, we must determine what are the 

 reactionary effects of each of the movements of the wing on 

 the body of the animal. 



Two distinct effects are produced during flight : by one, the 

 bird is sustained in opposition to its weight ; by the other, it 

 is subjected to a propulsive force which carries it from one 

 place to another. But do we find that the bird, when sus- 

 tained in the air, keeps at a constant level, or does it pass 

 through oscillations in the vertical plane ? Does it not 

 experience, by the intermittent effect of the flapping of its 

 wiDgs, rising and falling motions, of which the eye can detect 



