WINGS IN ACTION ^^ 



wing taken as a whole; in fact, it is the 

 valve principle. 



To see how this plumous adaptation actually 

 behaves when in operation it will be necessary 

 to proceed slowly in order to follow each 

 movement separately. 



The first act is the spreading of the wing. 

 Unfolding at the elbow, the " arm " is ex- 

 tended and held in a rigid position at right 

 angles to the body ; set in motion it beats 

 the air with a uniform straight up-and-down 

 action. Now, for a moment, supposing in the 

 place of the wing, we substitute a fiat piece of 

 wood of the same shape and subject it to the 

 same movemait ; at the utmost speed it 

 would never develop lifting power in the 

 sUghtest degree, simply because the air presses 

 equally on both sides ; but in the case of the 

 wing, the air pressure is not equal on both 

 surfaces. At the up-stroke the whole wing 

 bends to the pressure, which reduces the 

 surface area whilst the air sUdes easily from 

 off the rounded form ; at the same time, the 

 primary and secondary feathers, being un- 

 supported for their greater length, open out, 

 allowing passage to the air. Thus it will be 

 seen that the upper surface of the wing offers 

 very little resistance. But all this is reversed 

 at the down-stroke, when the under-surface 



