186 ON SOARING FLIGHT. 



Hypothesis of Unstable Equilibrium in the Lower 



Atmosphere. 



I wish to offer au explanation of some of the seemingly inexplicable 

 phenomena referred to by Mr. Langley in his introduction to this paper 

 under the name of soaring flight, by which term may be included all 

 sustained flight which is accomplished without flapping. This is the 

 characteristic flight of the eagles, hawks, vultures, albatrosses, and the 

 frigate bird, which may daily be seen sweeping through the air on 

 nearly motionless wings, making long journeys, ascending to great 

 heights, and in some undiscovered way keeping aloft in apparent defi- 

 ance of the laws of gravity, for they so seldom flap their wings, and 

 move them so little, that the power visibly expended seems wholly inad- 

 equate to the purpose of keeping their heavy bodies, as it were, afloat 

 in a fluid so subtle and yielding as the air. No hypothesis has as yet 

 been suggested which oflers a complete and satisfactory explanation of 

 the problem. The theory of the internal work of the wind, which in 

 the present state of our knowledge seems to offer the only solution of 

 the problem of flight in high winds, apparently fails to account for 

 soaring flight in very light winds, and the theory of ascending currents 

 as hitherto advocated, while offering a doubtful solution of flight in 

 light winds, fails com^fletely to explain how the bird soars in high ones. 

 It does not seem to me likely that any single explanation can be 

 found which will adequately account for flight in both light and heavy 

 winds; by which I mean that we may be forced to recognize that one 

 method must be employed for light winds and another and totally dif- 

 ferent method for high winds. Our theories must be formed in accord- 

 ance with facts, but I believe there are two important but not obvious 

 facts which have hitherto escaped observation and which I shall 

 endeavor to establish. The first is, as I have just said, that the bird 

 employs a distinctively different method for soaring in light winds 

 from that used in heavy ones; and the second, and the more important 

 fact of the two, is that this method reposes on a widely acting cause, 

 which has never yet been connected with the observations in question. 



Incredible as it may seem to those who have been denied the oppor- 

 tunity for observing the soaring birds, the stronger the wind blows, 

 within certain limits, the more readily can the bird penetrate it. Thus 

 the soaring bird can maintain a steady advance in a direct line into a 

 wind having a velocity of 40 miles an hour, without flapping and with- 

 out loss of elevation, and this it may continue to do indefinitely. 

 Such paradoxical cases however have been considered and explained 

 by Mr. Langley in his treatise on the " Internal work of the wind." 



I wish here to point out the fact that if the wind have a velocity of 

 only 4 or 5 miles au hour it lies wholly beyond the power of the bird to 

 penetrate the wind at all in the manner just described. When he 

 attempts to do so — that is, to sail straight ahead — he invariably 

 descends, and if he continues to do so without flapping he must 



