

1891.] Problem of the Soaring Birds. 787 
THE PROBLEM OF THE SOARING BIRDS. 
_ BY I. LANCASTER. 
> Darwin, in the naturalist’s voyage around the world. 
“ When the condors are wheeling in a flock round and round 
any spot, their flight is beautiful. Except when rising from the 
ground, I do not recollect ever_having seen one of these birds 
flap its wings. Near Lima I watched several for nearly half an 
hour, without once taking off my eyes: they moved in large 
curves, sweeping in circles, descending and ascending without 
giving a single flap. As they glided close over my head, I 
intently watched from an oblique position the outlines of the 
separate and great terminal feathers of each wing; and these 
separate feathers, if there had been the least vibratory movement, 
would have appeared.as if blended together ; -but they were seen 
distinct against the blue sky. The head and neck were moved 
frequently, and apparently with force; and the extended wings 
seemed to form the fulcrum on which the movements of the neck, 
body, and tail acted. If the bird wished to descend, the wings 
were for a moment collapsed; and when again expanded with an 
altered inclination, the momentum gained by the rapid descent 
seemed to urge the bird upwards with the even and steady move- 
ment of a paper kite. In the case of any bird soaring, its motion 
must be sufficiently rapid so that the action of the inclined sur- 
face of its body on the atmosphere may counterbalance its gravity. 
The force to keep up the momentum of a body moving ina 
horizontal plane in the air (in which there is so little friction) 
cannot be great, and this force is all that is wanted. The move- 
ment of the neck and body of the condor, we must suppose, is 
sufficient for this. However this may be, it is truly wonderful 
and beautiful to see so great a bird, hour,after hour, without any 
apparent exertion, wheeling and gliding over mountain and river.” 
The above remarkable words for many years have served to 
sustain my interest in the endeavor to substitute the actual 
for his provisional solution of soaring flight. 

