26 J. ANIMAL MECHANISM. 



be explained by the resistance of the air. The muscular 



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

 i!o with the course of the wing; elevation and depression arc 

 almost all the movements that it can produce. But the 

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

 i In- anterior convexity of the curve passed through, by means 

 of ,i 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- different movements. 



CHAPTER VI. 



RE- ACTIONS OF THE MOVEMENTS OF THE WING ON THE 

 BODY OF THE liJKD. 



He-actions of the movements of the vig Vertical re-actions in di fieri nt 

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

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

 flight of the bird Passive and active ]urts of the \\ing Reproduc- 

 tion of the DiechiUiL-in of the (light 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, wo must determine what are the 

 roaciimiiiry ell'rcts of each of the movements of the wing on 

 the I -oily of the animal. 



1'wo distinct effects are produced during flight: by one, the 

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

 is subjected to a proj ulsive force which carries it from OHO 

 ptace to another. Uut do we find that the bird, when sus- 

 tained in the air, keeps nt a constant level, or docs it pass 

 through oscillations in the vertical plane ? Dors it not 

 lirnco, by the intermittent eilect of the flapping of its 

 \virgs li-ing and falling nmiiuiis, of which the e\e c.m ! . tci t 



