vit ‘FLIGHT 213 
incline from the front to the hinder margin has 
been brought about, the resistance of the air will 
no longer act vertically, but at right angles: to the 
plane of the wing. And the force thus acting may 
be resolved into two, one raising the wing, the other 
urging it forward. When the end of the descent has 
been reached, muscular action and the air acting 
mainly on the front margin,! cause the wing to change 
front and face forward and downward. The wind 
due to the bird’s own velocity will act on the oblique 
‘surface, and lift it backward and upward. Thus when 
‘the bird is flying rapidly the air relieves the Elevator 
muscle of a great part of its work, and this accounts 
for its small size. The velocity due to the action of 
‘the great Depressor muscle comes to the assistance 
of the small Elevator. Even when there is no great 
speed to create a current of air relatively to the bird, 
the descent of the ‘body during the upstroke helps to 
lift the wing. In the same way during the down- 
stroke, the work is lightened and the extremity of 
the wing appears to travel a much greater distance 
than it really does. For the raising of the body 
means a relative lowering of the wings, and helps 
them home just as the motion of the boat seems to 
‘help the oar through the water. 
The Tail. 
The tail feathers have their clastic ligament. At 
“their bases they are firmly held by muscles, and ‘are 
arranged like a fan. Some little way out from their 
1 See p..190. 
