t 
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 stil! more effective. 
The outer webs of the feathers (see Pl. x1) 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 
