CHANGES IN THE PLANE OF THE BIRD'S WING. 203 



The direction of tlie movement of the wing is read from above 

 and forward, from H to Av. 



Fig-. Ill shows that the wing during its ascent assumes 

 an inclined position which allows it to cut the air so as to 

 meet with the minimum of resistance ; while in its descent, 

 on the contrary, the position of its plane is reversed, so that 

 its louer surface turns downwards and slightly backwards. 

 It Jollows, that in its period of depression, the wing, by its 

 obliquity, acts upon the resistance of the air, and while raising 

 the body of the bird, carries it forward. We see, also, that 



Flo. 111. Inclinations of the plane of the wing with reference to the axis 

 (Av) ot llie budy during flight. 



the inclination of (lie wing changes gradually, in the different 

 phases of its elevation and of its descent. Especially in this 

 hitter pha^e, the influence of the air in shaping the course of 

 the wing is more evidently seen; it is, in fact, at the moment 

 when the rapidity of its depression attains its maximum that 

 we see the posterior edge of the wing turn up the more strongly. 



The wing, when it has reached the end of its descending 

 course, changes its plane very suddenly. The explanation of 

 this movement is very natural. As soon as the resistance of 

 the air censes to raise the feathers, these, by their elastic-it}', 

 return to their ordinary position, which they occupy during all 

 the phase of elevation. 



Even the ellipse which forms the trajectory of the wing cm 



