38 LIFE ON THE EAKTil. 



the other; (2) by the bending of the jointed wing to 

 extend or reduce the surface, and increase or dimi- 

 nish the flexibility; and (3) by the construction of 

 each effective wing-feather. For each of these is 

 so made, both in regard to the axis of the plume, 

 and in regard to the smaller elements of the feather, 

 as to be strong in the direction in which the whole 

 wing-arrangement is strong, and yielding in the di- 

 rection in which the whole wing is yielding. 



This remarkable concurrence of the individual 

 strength of the feather in all its parts with the me- 

 chanical conditions of the problem is secured by the 

 general shape and also by a change of the substance 

 of the feather on the opposite faces ; the upper con- 

 vex face of the wing-feather having the compact 

 quill-sheath extended along it to the very extremity 

 and giving origin to the plume, while below it is a 

 thick mass of cellular substance, not so covered, in 

 which extension and contraction take place advan- 

 tageously. This is exactly the arrangement indicated 

 by the experiments of engineers, and the theories of 

 mechanicians, for the employment of the least weight 

 of such elastic materials in the production of the 

 required result 1 ; but no human hands could make 

 an apparatus embodying so perfectly the abstract 



1 Hodgkinson and Fairbairn, in Reports of the British Asso- 

 ciation* Barlow, On Strength of Materials. 



