vii 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 

 uniinc it forward. When the end of the descent has 

 been reached, muscular action and the air acting 

 mainly on the front margin, 1 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 elastic ligament. At 

 their bases they arc firmly held by muscles, and arc 

 arranged like a fan. Some little way out from their 

 1 See p. 190. 



