1888.] Mr William Froude on the Soaring of Birds. 
257 
statement is true for “ fish-formed ” bodies moving wholly and 
deeply immersed in water. Of course the bird’s angle of actual 
descent is greater than that of the quasi-inclined plane, owing to 
the equivalent of “ slip ” in the wings. Under these simultaneously 
acting and correlated conditions there is of course — or probably — 
some total angle of descent which enables the bird to minimise his 
rate of approach to the earth in still air. If when there is a wind 
the configuration of the ground or any other circumstances can 
produce a local ascent of air more rapid than the bird’s minimum 
rate of descent when soaring in still air, he may continue to soar 
indefinitely by keeping in the region where the air is thus 
ascending. 
Now in most cases where one sees birds “soaring,” it is easy to 
see that they have plainly selected such a region, and for a long 
time I felt confident that the only two even apparent exceptions I 
had encountered were such as to prove not to invalidate the rule. 
One of these exceptions was that once, when the sea in Torbay was 
in a state of glassy calm, I noticed a large gull thus soaring at some 
distance from the shore, — watching it with a pair of binoculars, so 
that I was sure of the quiescence of the wings. But here the riddle 
was at once solved by the observation of what I had not at first 
noticed, — the dark trace of the front line of a fresh sea breeze 
advancing all across the bay. Such an advance with a definitely 
marked front, encountering an extended body of quiescent air, 
involved of course an ascent of air in the region of the encounter, 
and this was where the bird was soaring. The other exception was 
that when at sea I had often noticed birds thus soaring near the 
ship. The solution was that, so far as I had then noticed, the birds 
always selected a region to leeward of the ship, where the eddies 
created by the rush of air past her hull, &c., might readily have 
created local ascending currents. 
The new exceptions we have seen since we have approached the 
Cape entirely sets these two solutions at defiance. 
The first exception we noticed was in the flight of some albatrosses. 
We were sailing, and steaming (at low speed, being short of coal), 
nearly due east in the latitude of the Cape, with the wind light and 
variable abaft the beam, and with a well-marked S.W. swell of 
about 8” to 9” period, and varying from 3 or 4 feet to 8 or 9 feet 
