ANIMAL MECHANIiSM. 



which they more readily furnish ; 

 namely, the number of the revolu- 

 tions of the wing, the greater or less 

 regularity of these movements, and 

 the equality or inequality of each of 

 them. 



Confining the question within 

 these limits, experiment shows that 

 the strokes of the bird's wing differ 

 in amplitude and in frequency from 

 one moment to another as they fly. 

 When they first start, the strokes 

 are rather fewer, but much more 

 energetic ; they reach, after two or 

 three strokes of the wing, a rhythm 

 almost regular, which they lose 

 again when they are about to settle 

 (fig. 96). 



TRAJECTORY OF THE WING OF THE 

 BIRD DURING FLIGHT. 



We have seen, when treating of 

 the mechanism of insect flight, that 

 the fundamental experiment was 

 that which revealed to us the course 

 of the point of the wing throughout 

 each of its revolutions. Our know- 

 ledge of the mechanism of flight 

 naturally flowed, if we may so say, 

 from this first notion. 



The same determination is equally 

 necessary for the flight of birds ; 

 but the optical method is unsuitable 

 for this purpose. In fact, the move- 

 ment of the bird's wing, although 

 too rapid to be appreciable by the 

 eye, is not sufiiciently so to furnish 

 such a persistent impression' on the 

 retina as to shbw its whole course. 



