42 THE FLIGHT OF BIRDS 



bird's wings, f being the front and b the back margin. 

 The resistant air (w in the figure) will be equivalent 

 to a wind blowing vertically upward. It will act 

 at right angles to the plane of the wing, and the line 

 representing its action will point not only upward 

 but forward. During horizontal flight the front 

 edge of the wing is slightly at a lower level than the 

 back. But when the bird is rising and taking very 



Fig. 16. 



Diagram showing the effect of the lowering of the front margin 

 of the wing. 



energetic strokes, then the required incline is not 

 obtained by that method only. The wing moves 

 forward as it descends, so that at the end of the 

 down-stroke, instead of making a right angle with 

 the body, it points more forward than outward. 

 When the wing is in this position, there is the down- 

 ward slope that is wanted, from its base at the 

 shoulder to the tip. One method passes gradually 

 into the other ; indeed, the wing is inevitably shoved 

 forward when the air lifts its hinder margin. On 



