PROGRESSION IN OR THROUGH THE AIR. 



149 



at right angles to each other, the vertical wave-track being 

 the more distinct. 



Compoimd Eotatmi of the Wing. — To work the tip and 

 posterior margin of the wing independently and yet simul- 

 taneously, two axes are necessary, one axis (the short axis) 

 corresponding to the root of the wing and running across 

 it ; the second (the long axis) corresponding to the anterior 

 margin of the wing, and running in the direction of its length. 

 The long and short axes render the movements of the wing 

 eccentric in character. In the wing of the bird the movements 

 of the primary or rowing feathers are also eccentric, the shaft 

 of each feather being placed nearer the anterior than the pos- 

 terior margin ; an arrangement which enables the feathers to 

 open up and separate during flexion and the up stroke, and 

 approximate and close during extension and the down one. 



These points are illustrated at fig, 80, where a b represents 



a 



L 



Fig. 80. 



the short axis (root of wing) with a radius e f; c d represent- 

 ing the long axis (anterior margin of wing) with a radius g 7;. 



Fig. 80 also shows that, in the wing of the bird, the indi- 

 vidual, primary, secondary, and tertiary feathers have each 

 what is equivalent to a long and a short axis. Thus the 

 primary, secondary, and tertiary feathers marked A, ij\ k, I are 

 capable of rotating on their long axes {r s), and upon their 

 short axes (m n). The feathers rotate upon their long axes 

 in a direction from below upwards during the down stroke, 

 to make the wing impervious to air ; and from above down- 



