184 ON SOARING FLIGHT. 



must come down," aud there must be areas where tlie currents are 

 descending to supply the void. But perhaps a better answer lies in 

 the citation of the simple and most familiar fact of observation ; that 

 the great soaring birds (which are chiefly of the vulture class) are 

 found uniformly suspended, frequently in large numbers, above the 

 carrion or other object of their interest, wherever it lies, and it is too 

 grotesque a supposition that an ascending current capable of sustain- 

 ing them should always emanate from such a source. The hypothesis 

 of such ascending currents is not, then, irrational as a partial explana- 

 tion, but wholly insufficient as a complete one. 



The next which arises, and which to an observer untrained in mechan- 

 ics seems extremely plausible, is that the wind holds the bird up as it 

 bears up a kite. The sight, familiar from childhood, of kites sustained 

 at great heights in the air without any power emanating from them- 

 selves is perhaps responsible largely for this delusion; for it is one to 

 suppose that the bird, not upheld by any string, visible or invisible, 

 actual or virtual, can sustain itself in a wind, at least if the wind be 

 what it has until lately been treated as being, a nearly homogeneous 

 moving mass of air, with occasional little eddies or disturbances which 

 do not affect its fundamental quality of a current flowing altogether, 

 like a river or a tide. 



It is absolutely contradictory to mechanical principles, however, that 

 in such a uniformly moving mass of air a kite or any other body with- 

 out internal power or external support, or any bird on rigidly extended 

 pinions, can sustain itself except momentarily, any more than in an 

 absolute calm. The fact, however, that the soaring birds very rarely 

 indeed perform their special evolutions except in a wind, and do have 

 to resort to flapping their wings in a calm, is so obvious that many 

 writers have tried to persuade themselves that in some way or other 

 well-known laws can be evaded, and that the birds can continuously 

 soar in such a wind by a power derived in some way from it. I think 

 it superfluous to do here more than repeat that such action is mechan- 

 ically impossible. 



Next, it is indeed true that if there be two winds, or two strata of a 

 wind, moving at different velocities, it is in this case mechanically pos- 

 sible that the evolution can be x^erformed, and this Lord Eayleigh has 

 pointed out. Though this is a true cause as far as it goes, it seems 

 hardly necessary to say that it can account but for a very limited por- 

 tion of the actual phenomena. 



Another hypothesis, in accord with mechanical principles, and by 

 which the work of supporting the bird can be derived from the wind in 

 which it moves, has been iiut forth by the writer, after a study of the 

 internal movements of the wind, which he has shown by much experi- 

 ment, are incomparably more complex than had been supi^osed before 

 attention had been brought to them ; movements whose possible effect 

 may be illustrated in this untechnical article, by saying that if we could 

 see the wind, it would not appear a smooth- flowing tide like the Gulf 



