272 FLIGHT 



the lowest point of the ' circle ' it will have a greater horizontal 

 velocity relative to the air at that level than if the whole air 

 through which it had fallen had been still. . . . Suppose the bird 

 as it rises wheels gradually round and faces the wind. Then in 

 rising it will enter successive strata of air, having successively 

 greater and gi-eater velocity relative to itself (the bird) than if the 

 air had no internal movement, and therefore the air-resistance, 

 which is the lifting force, will ever be greater than that due to 

 (or corresponding with) the height gained by the bird if in still 

 air." As Rayleigh noted, the gain to the bird corresponds, not 

 with the increments in velocity, but Avith the increments in the 

 square of velocity. 



This theory fits in very well with most of the facts noted by 

 different observers in soaring birds. It especially agrees with the 

 fact that soaring birds appear always in making their horizontal 

 curves to alternately rise and descend in the manner above de- 

 scribed. Against this theory, on the other hand, is the fact that 

 birds may be seen to soar a great height above the surface of the 

 globe — too high to justify us in assuming that different strata of 

 the wind travel wdth such differences in velocity as have been 

 observed nearer the earth. This is a matter, however, on which 

 we have as yet insufficient information ; and it is worth noting 

 that any local variations in the velocity of neighbouring currents 

 of air will be capable of being of use to a soaring bird in a manner 

 analogous to that described by Airy. 



In the meantime we are inclined to accept Rayleigh's theory ; 

 but the facts, it appears to us, Avant investigation with more exact 

 methods than have hitherto been employed. What is needed is to 

 learn exactly the course of a soaring bird both in the vertical and 

 horizontal dimensions in each part of its course, as well as the 

 exact velocity and direction of the wind in the strata of air through 

 which the bird progresses. 



Much has been written on size as inttuencing the power of flight 

 in different birds, but this is a subject into Avhich Ave cannot here 

 enter. Tavo opposite AdeAvs regarding it Avill l^e found stated at 

 length by Miillenhoft" {o'p. cif.) and Strasser.^ 



Finally, it is important to note that frictional resistance, Avhich, 

 as the late Mr. W. Froude has sheAvn, plays so great a i)art in 

 obstructing the movement of a ship through Avater, seems, from 

 recent observations by Langley - and Maxim,^ to have very slight 

 influence in hindering the passage of a bird through the air. 



Charles S. Roy. 



•' Ueher den Plug der Vdgel, pp. 404-417 (Jena : 1885). 

 ^ Comptcs Rcndus de I'Acad. Sc. cxiii. pp. 59-63 (1891). 

 3 Century Magazine, xlii. pp. 829-836 (Oct. 1891). 



