438 Prof. S. P. Langley on the 



But we may go materially further, and not only admit that 

 it is not necessary to invoke here, as Maxwell has done in the 

 case of thermodynamics, a being having power and rapidity 

 of action far above ours, but that, in actual fact, a being of a 

 lower order than ourselves, guided only by instinct, may so 

 utilize these internal motions. 



We might not, indeed, have conceived this possible, were 

 it not that nature has already, to a large extent, exhibited it 

 before our eyes in the soaring bird*, which sustains itself 

 endlessly in the air with nearly motionless wings, for without 

 this evidence of the possibility of an action which now ceases 

 to approach the inconceivable, we are not likely, even if ad- 

 mitting its theoretical possibility, to have thought the mecha- 

 nical solution of this problem possible. 



But although to show how this physical miracle of nature 

 is to be imitated, completely and in detail, may be found to 

 transcend any power of analysis, I hope to show that this may 

 be possible without invoking the asserted power of " aspira- 

 tion " relative to curved surfaces, or the trend of upward 

 currents, and even to indicate the probability that the me- 

 chanical solution of this problem may not be beyond human 

 skill. 



To this conclusion we are invited by the follow ing consider- 

 ation, among others. 



* " When the condors in a flock are wheeling 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 once flapping. As they glided close over my head, 

 I intently watched, from an oblique position, the outlines of the separate 

 and terminal feathers of the wing ; and if there had been the least vibra- 

 tory movement these would have blended together, but they were seen 

 distinct against the blue sky. The head and neck were moved frequently 

 and apparently with force, and it appeared as if the extended wings 

 formed the fulcrum on which the movements of the neck, body, and tail 

 acted. If the bird wished to descend, the wings for a moment collapsed ', 

 and then, 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 movement of a paper kite. In the case of any bird 

 soaring, its motion must be sufficiently rapid so that the action of the 

 inclined surface of its body on the atmosphere may counterbalance its 

 gravity. The force to keep up the momentum of a body moving in a 

 horizontal plane in that fluid (in which there is so little friction) cannot 

 be great, and this force is all that is wanted. The movement 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." — Darwin's Journal of the Various Countries Visited 

 by H.M.S. < Beagle,' pp. 223-224. 



