THE MACHINERY OF FLIGHT 85 



as by the resistance of the air, the feathers during 

 the up -stroke are pressed against one another and 

 the passage of the air is prevented. But this causes 

 no trouble. The wing is very rapidly swept back- 

 ward and upward ; it turns its front edge in the 

 direction of flight, and when it is oriented thus it is 

 easily straightened and moved forward into position 

 for beginning the next down-stroke. 



During the down-stroke the air helps the living 

 machinery in its work, making it still more effective. 

 The outer webs of the feathers (see PI. xi) are very 

 narrow compared with the inner ones, and the result 

 is that the air acts much more strongly upon the 

 latter during the down-stroke and also, when there 

 is much momentum, during the up-stroke, with the 

 result that each feather is rotated and has the inner 

 side of its vane pressed very closely against the one 

 that lies next to it and above it on the side nearer 

 to the body. 



Though on the surfaces of the wing nothing is 

 visible but feathers, yet for no small amount of the 

 expanse two membranes can claim the credit. What 

 is called the anterior membrane stretches from the 

 head of the clavicle (or merrythought) to the hand 

 (see fig. 24). In the Gannet it is of great breadth 

 and is so hung that it not only increases the area of 

 the wing, but sloping steeply downward, as it does, 

 to form the front margin, it deepens the wing's 

 concavity and makes it, near to the body, what a 

 parachute should be. In most birds the membrane 

 is slung with only a gentle slope from its front edge 

 backward. In all it is at once stretched when the 

 wing is spread. The other membrane lies farther 



