ON SOARING FLIGHT. 185 



Stream, but ratlier seem broken into infinitely varied internal^ move- 

 ments like the rapids below Niagara, some of which are often opposed 

 to the movement of the main current which bears them on, and by 

 means of which internal movements it is quite possible in theory that 

 work may be done sufficient to bear a vessel against the main current 

 itself. 



The attention of the reader who is interested in the matter may be 

 again called to the fact that the present writer does not conceive this 

 to be the sole sufficient cause, in the sense that the bird uses it to the 

 rejection of aid from ascending currents and the like, where they 

 present themselves ; but while there is no doubt that the wind's internal 

 horizontal movements are often alone sufficient to sustain a bird, it is 

 difficult to believe that this cause can account for all the flights we see 

 performed, either at great altitudes, where we must suppose the wind 

 relatively uniform, or in wind of such a small velocity that it is hard to 

 suppose that the bird can support its weight with as little work as 

 would then be furnished by the still smaller variations. 



It is indeed possible that as we further study that most marvelous 

 structure, the bird's wing, we may find it capable of utilizing power 

 latent in these internal movements of the wind, in a different way than 

 we now fully understand, and in a degree greater than now seems 

 possible. It remains, nevertheless, true that this hypothesis (of the 

 internal work of the wind), the last which seems to offer itself, is, if 

 trustworthy in theory and able to account for much of what we see, 

 yet apparently insufficient in some instances of the kind we have just 

 noted. 



We seem, then, to have exhausted every suggestion, and yet the 

 soaring bird still soars, and remains sustained in midair almost with- 

 out an effort, as anyone may see in the regions it frequents. Under 

 these circumstances we may feel justified in receiving at least with con- 

 siderate examination, a new hypothesis which does not necessarily 

 violate any mechanical principle, and which, though it may at first 

 have a certain artificial appearance, seems not unsupported by some 

 facts of observation. 



Its author, Mr. Huff'aker, is one of the most acute observers of this 

 class of phenomena whom I have known, and I put trust in the good 

 faith with which he reports his observations, and in the conscientious 

 care with which he has made them. What he has to say about his 

 actual observations is at any rate, then, worth the attention of those 

 interested in the subject. I do not make myself responsible for the 

 validity of his suggested hypothesis, but it seems to me novel, not in 

 contradiction of any mechanical principle, and deserving of consider- 

 ation, if only because it appears to apply to some cases where every 

 other seems to have failed. 



S. P. Langley. 



Smithsonian Institution, January, 1898. 



iThe "Internal Work of the Wind." Smithsonian Contributions to Knowledge, 

 Vol, XXVII, 1893. No. 884. 



