RE-ACTIONS DURING FLIGHT. 



269 



the slackening of the bird's speed will cause a descent of the 

 lever by an inverse action. 



Experiments tried upon the species of birds before men- 

 tioned, have furnished us with tracings analogous with those 

 of the vertical oscillations. 



If it be true, as we have supposed, that the vertical 

 oscillation of the bird, at the moment of the ascent of the 

 wing, is due to the transformation of speed into elevation, 

 we shall have the means of verifying this supposition, by 

 collecting simultaneously the tracings of the vertical oscilla- 

 tions and those of the variations of rapidity. 



Thus, by registering at the same time the two orders of 

 oscillation in the flight of a buzzard, we find that the phase 

 of depression of the wing produces at the same time the 

 elevation of the bird and the acceleration of its horizontal 

 swiftness. This effect is the natural consequence of the 

 inclination of the plane of the wing at the moment of its 

 descent ; this we already know from having obtained it in the 

 flight of the insect. As to the elevation of the wing, it is 

 found that during the slight ascent which accompanies it, the 

 swiftness of the bird diminishes. In fact, the curve of the 

 variations of rapidity is depressed at the moment when the 

 bird rises. This is, therefore, a confirmation of the theory 

 which we have propounded concerning the transformation of 

 the horizontal rapidity of the bird into ascensional force. 

 Thus by this mechanism, the stroke of the descending wing 

 produces the force which will cause the two oscillations of the 

 bird in the vertical plane. It produces directly the ascent 

 which is synchronous with it, and indirectly prepares the 

 second vertical oscillation of the bird by creating rapidity. 



Simultaneous tracing of the two orders of the oscillations of the 

 bird. — Instead of representing separately the two kinds of 

 oscillation executed by the bird as it flies, it is more instruc- 

 tive to seek to obtain a single curve representing together the 

 movements which the body of the bird makes as it advances 

 in space. 



The method which we have employed to obtain the move- 

 ments of the point of the wing may, with certain modifica- 

 tions, furnish the simultaneous tracing of the two orders of 



