1890 - 91 .] Mr H. E. Froude on the Soaring of Birds. 
69 
ceivably be derived from the curvature of the wing surface;* also 
r is taken as 1*5, instead of over 2’0 as it prima facie should be, 
on the score of eddies conceivably annulling in part the friction 
on the upper surfaces of the wings. These values put into 
equation (8) give no less than 4 *7 feet 'per second as the value for 
V^cq + oq). Thus, apparently — 
(1) The formula as interpreted by these constants requires no less 
than 4 '7 ft. per second rate of descent. 
(2) The suggested explanation of the soaring admits of no more 
than 3 ft. per second. 
(3) The fact that birds can sustain flight in still air admits of very 
much less still , unless we can suppose that in birds the relative power 
capacity is many times greater than it is in horses and men. 
It is not my purpose here to attempt to clear up this paradox. 
Mr Froude appears to have considered the formula unimpeachable in 
structure, at least as a fair approximation (and so I think it evidently 
is), but the constants probably in error. At any rate, he seems to 
have treated the argument from the power capacity of animals as 
sufficient prima facie evidence that the updraught of the advancing 
wave slopes would suffice for soaring ; because in subsequent letters 
he describes further observations made^with the object of identifying 
the occasions of soaring in a calm with position of the bird over the 
advancing wave slopes. But first he had an opportunity of observ- 
ing soaring in a strong wind, under circumstances which appeared 
to defy the idea that advantage was being taken of local ascending 
currents. This must be described in his own words, in a letter to 
myself dated 14th February 1879 : — 
“ But since I have been here we have had a lot of S.E. gales ; and 
though the sea surface has been like that at Torquay pierhead in a 
S.S.W. gale, and thus without any big waves, we have seen a lot of 
whale birds , as they are called, playing the skim trick in the most 
marvellous and fascinating way. 
“The albatrosses did occasionally flap, but these birds went high 
and went low, went fast and went slow, with the wind or against 
* As a justification for this, Mr Froude suggests the circumstance that in 
the cup anemometer the circumferential speed of the cups is accounted to be 
§ the speed of the wind ; hence the relative speed of the wind facing the con- 
cave surfaces is only one-half that facing the convex surfaces, yet the wind 
presumably exerts the same force on both. 
