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 greater 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 with 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 with 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, want 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 influencing the power of flight 
in different birds, but this is a subject into which we cannot here 
enter. ‘Two opposite views regarding it will be found stated at 
length by Miillenhoff (op. cit.) and Strasser.+ 
Finally, it is important to note that frictional resistance, which, 
as the late Mr. W. Froude has shewn, plays so great a part in 
obstructing the movement of a ship through water, 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. 
' Ueber den Flug der Vogel, pp. 404-417 (Jena; 1885), 
2 Comptes Rendus de l’ Acad. Se. exiii. pp. 59-63 (1891). 
3 Century Magazine, xlii. pp. 829-836 (Oct. 1891). 
