1890-91.] Mr R. E. Froude on the Soaring of Birds. 
65 
On the Soaring: of Birds : being a Communication from 
Mr R. E. Froude in continuation of the Extract from a Letter 
by the late Mr William Froude to Sir William Thomson, 
published in these 11 Proceedings,” March 19, 1888. 
(Read January 5, 1891.) 
The object of the present communication is to give the purport of 
the remainder of the letter referred to in the title, as well as that of 
other letters bearing on the same subject written by the late Mr 
Froude shortly afterwards, which were not at hand at the time the 
extract referred to was printed. 
In the extract already printed, Mr Froude expressed the view 
that the continued “ soaring ” (or “ sailing flight,” as it has also been 
called) of birds only took place where there was an ascending 
current of air of sufficient speed. And he noticed as an apparent 
exception, which he had observed one day on the passage to the 
Cape on board H.M.S. “ Boadicea,” that in a very light wind some 
albatrosses were seen soaring (manifestly without wing stroke) 
“ almost ad libitum ,” where there could not possibly be any ascend- 
ing current due to deflection of wind by the ship. He suggested 
as a possible explanation, and one which to all appearance fairly 
accorded with the birds’ visible movements, that they were availing 
themselves of the ascending stratum of air which must have 
extended above the advancing slope of each wave of the well- 
marked ground-swell which was running. From the dimensions of 
this, the maximum upward speed of such air current was estimated 
at about 3 feet per second. 
Thus far the extract already printed. In the original letter there 
followed a mathematical investigation to determine whether this up- 
ward air current of 3 feet per second could suffice for the supposed 
effect. This I now paraphrase and somewhat abbreviate as follows : — 
Suppose a bird soaring with constant speed and direction in still, 
or uniformly moving, air ; and let 
a = the angle (taken downwards from horizontal, in a fore and 
aft vertical plane) of the wing surface ; and, 
a + tr = that (similarly taken) of the direction of motion through 
the air. Hence, 
<r = the angle of the wing surface with the line of motion. 
VOL. XVIII. 1/4/91 e 
