FLIGHT 



73 



The arched curvature of the under-surface of the wing 

 is not to be thought of as hke the interior of a dome. It is 

 a subtle curvature, changing in different areas of the wing ; 

 and it differs considerably in different birds. Further 

 inquiry will show that it is of fundamental importance in 

 flight. It is largely because of the anterior arching of the 

 anterior under-surface of the wing that the down-stroke 

 makes the bird speed onwards, forcing the air out back- 

 wards. 



It seems certain that the secondary feathers are of 



Fig. 17. — Wing of a bird, showing feathers and bones. From a 

 specimen, h., humerus ; E., elbow-joint ; r., radius ; u., ulna, r., radiale ; 

 II,, ulnare ; th., thumb ; a.s., ala spuria ; cmc, carpometacarpus ; 2, second 

 digit; 3, third digit ; pr., primary feathers ; s., secondary feathers. 



major importance in the stroke ; it is by them that the air 

 is most effectively gripped. The primary feathers are of 

 great use for lateral steering, hence their exaggerated 

 length in insect-catching swallows and swifts. They often 

 project separately at their tips, so that the air passes between 

 them, and it is pointed out by Hilzheimer that this allows 

 the air-waves caused by the down-stroke to pass gradually 

 outwards without producing a disadvantageous vortex. 



Some steering is effected by the tail feathers, which also 

 serve for balancing and as a brake or drag in flight. If the 

 tail be spread out horizontally, the air driven backwards by 

 the down-stroke of the wings strikes it from above and the 

 bird rises — it may be avoiding a telegraph wire. If the 



