S. P. Langley — Internal Work of the Wind. 53 



wind is an approximately homogeneously moving body, 

 because moving as a whole in one direction. It is, on the 

 contrary, always, as we see here, filled (even if we consider 

 only movments in some one horizontal plane), with amazingly 

 complex motions, some of which if not in direct opposition to 

 the main movement, are relatively so, that is, are slower, while 

 others are faster than this main movement, so that a portion is 

 always opposed to it. 



From this, then, we may now at least see that it is plainly 

 within the capacity of an intelligence like that suggested by 

 Maxwell, and which Lord Kelvin has called the "Sorting 

 Demon," to pick out from the internal motions, those whose 

 direction is opposed to the main current, and to omit those 

 which are not so, and thus without the expenditure of energy to 

 construct a force which will act against the main current itself. 



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 thermo-dynamics, 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 end- 

 lessly 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 admit- 

 ting its theoretical possibility to have thought the mechanical 

 solution of this problem possible. 



* " When the condors in a flock are wheeling round and round any spot, their 

 flight is beautiful. Kxcept 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 vibratory 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 ex- 

 panded 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 counter- 

 balance 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. 



