ELISHA MITCHELL SCIENTIFIC SOCIETY. 65 



compared with the extent of compressing surface of the entire 

 wing, will be sufficient to convince one of the truth of the propo- 

 sition. 



Take a single primary. The rachis (the portion of the quill 

 extending through the length of the feather) is quite stiff, rect- 

 angular in cross-section, and projects downward below the vanes 

 of the feather for their entire length. It is also near the anterior 

 edge of the feather and offers effective resistance to all air mov- 

 ing forward on the under surface. The posterior edge of the 

 vaue is easily bent upward by the passing air and forms a resist- 

 ance to the air passing forward over it, in the same manner as 

 the posterior edge of the entire wing presents resistance to the 

 air flowing in to fill the partial vacuum spoken of. About mid- 

 way of the feather both the anterior and posterior vanes are 

 suddenly narrowed; the anterior one is narrowed close down to 

 the rachis, so that the rachis forms the anterior edge of the 

 feather; the posterior vane is narrowed down to about one-half its 

 width. When the wing is outstretched the primaries are sepa- 

 rated from the point of narrowing of the vanes to their tips. 

 The development of the peculiar shape of the primaries, as de- 

 scribed, was for the purpose of admitting their separation at this 

 point. The distance from this point of separation to the tips of 

 the primaries is about eight inches; from the same point to the 

 body of the bird is about two feet. The depth of the wing from 

 anterior to posterior edge is about one foot. When the bird is 

 soaring the compact portion of the wing, a surface two feet by 

 one foot, compresses the air. The compressed air tends to rush 

 out in three directions, cephalad (anteriorly), eaudad (posteriorly), 

 and distad, i. e., toward the outer end of the wing, at a right angle 

 to the direction the bird is moving. All of the air which rushes 

 out at the distal end of the wing passes upward between the 

 separated primaries, and each one utilizes the air next to it in 

 forward motion. Did these feathers forma compact surface, the 

 only portion of air utilized would be the small amount passing by 

 the anterior primary and the posterior outstretched primary. All 

 the remainder would pass out at the tips of the primaries, and 



