54 o THE POPULAR SCIENCE MONTHLY. 



vation, but they are kites which continually change their surfaces and 

 position in respect to the air, which artificial kites do not. An im- 

 portant difference between them is the rigidity of the one and the 

 wonderful flexibility of the other. The kite rises as its oblique surface 

 is pressed against the elastic air ; the same is true of the wing. But 

 the wing rotates, so that the proper obliquity of its parts is continually 

 maintained ; it rolls on and off the wind ; it rotates not only through- 

 out its length, but in each of its parts. The quills, which are convex, 

 rotate, and present closed or oblique surfaces, which hold or discharge 

 the air. 



We have space for but one more of the numerous diagrams and 

 figures which Dr. Pettigrew has prepared, illustrating the phenomena 

 of flight. Fig. 16 is of the extended wing of a partridge, seen from 

 beneath and from behind. 



Fig. 16. 



Curves in the Wing of a Partridge in Flight. 



The wings, when flexed and extended in flight, assume curved sur- 

 faces, which change at each instant and carve the air, as does the tail 

 of the fish the water, into complex wave-lines ; and such is the struct- 

 ure of the wing that these results are inevitable when it is put in 

 motion. " The muscles, bones, ligaments, and feathers, are so adjusted 

 with reference to each other that, if the wing is moved at all, it must 

 move in the proper direction." The bird no more thinks of its mo- 

 tions in flying than we do of ours in walking ; the actions are mechan- 

 ical and instinctive. An opinion long prevailed that heated air in the 

 hollow bones of the bird gave it buoyancy and power of flight. This 

 is shown to be a fallacy. 



Three principal forces are expressed in flight : muscular and elastic 

 force of the wing, weight of the body, and recoil of the air. By the 

 mechanical structure of the wing, these forces act, react, and combine. 

 Thus birds traverse the aerial ocean ; the wild-goose drives his train 

 along invisible tracks ; the albatross and petrel are at home in the 

 gale, undisturbed by its clamor ; and the condor, with easy motion, 

 treads with his pinions the elastic floors of the upper air. 



The more rapid the strokes of the wing the greater the achieve- 

 ment. Not so with one of the most ingenious of human contrivances 

 for progression. The screw, if urged beyond a certain velocity, holds 

 and carries with it the water, and its propelling power is lost. It 

 wants the flexibility of the wing and the fin — the adaptation is not 

 complete. 



