1890 - 91 .] Mr R. E. Froude on the Soaring of Birds. 
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speed must be greater on the ascending side than on the descending 
side. The particles of spray passing across and through the vortices 
must be subjected alternately to the upward and downward forces 
due to the ascending and descending speeds. True, the ascending 
streams, being thinner in proportion as their speed is greater, will 
presumably act on the particles for a proportionately smaller share 
of the total time; but the resistance being as speed squared, the 
aggregate upward momentum imparted to the particles will never- 
theless exceed the aggregate downward momentum. 
This suggestion is interesting, as a plausible explanation of the 
phenomenon of the rising spray ; at the same time I hardly think 
the suggested operation can favour the soaring of birds except by a 
second order quantity. For, in proportion as the bird’s speed is 
high (as I think it must be), relatively to the speed of the eddies, 
the effect of the local contrarities of the eddy speeds becomes to the 
bird one simply of small differences in angle of impact on the wings ; 
and, since the pressure on an obliquely moving plane varies simply 
as the angle (for small angles), the consequent differences in upward 
pressure would be proportional to the times for which those pres- 
sures act, so that the aggregate upward momentum and mean upward 
force would be the same as in still air. 
I imagine that the soaring witnessed by Mr Froude on the occa- 
sion which he describes, is to be ascribed to an operation which, 
so far as I know, was first suggested by Lord Rayleigh in a com- 
munication to Nature of 5th April 1883, viz., a utilisation by the 
birds of the difference of wind-speed at different levels. But this 
explanation evidently did not occur to Mr Froude at the time, and I 
need make no further reference to it here. 
Mr Froude’s next letter bearing on the subject was dated 
Saldanha Bay, 24th February 1879, and in it he says : — 
“ The voyage up from Simons Bay was delightful ; for 
it was a glassy calm ; and as there was also a tolerably pronounced 
swell, especially the latter part of the way, I was able (and Tower 
helped me) to watch the albatross’s flight in a calm, with the 
following results : — When flying high they had to flap their wings 
continuously, except when descending. When near the surface they 
c skimmed ’ occasionally, and, as far as we could distinguish, they did 
this only when traversing a region over an ascending wave slope. 
