42 THE FLIGHT OF BIRDS 
bird’s wings, F being th@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 
Ww 
Ww 
Fia. 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 
