J4 THE IXTEKXAI. WORK OF THE WIND. 



direction is opposed to the main current, and to omit those which are not so, and 

 thus without the expenditure of energy fcoconstruct a force which will act against 

 the main current itself. 



Bui we may go materially further, and not only admit that it is not necessary 

 to invoke here, as Maxwell has done in the ease of thermo-dynamics, a being having 

 a power and rapidity of action tar 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 action which now ceases to approach the inconceiv- 

 able, we are not likely, even if we admitted its theoretical possibility, to have thought 

 the mechanical 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 with- 

 out invoking the asserted power of "Aspiration" relative to curved surfaces, or the 

 trend of upward currents, and even to indicate the probability that the mechanical 

 solution of this problem may not be beyond human skill. 



To this conclusion we are invited by the following considerations, among others. 



We will presently examine the means of utilizing this potentiality of internal 

 work, in order to cause an inert body, wholly unrestricted in its motion and wholly 

 immersed in the current, to rise; but first let us consider such a body (a plane) 



* " When the condors in a flock arc wheeling round and round any spot, their flight is beau- 

 tiful. 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 wings ; 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. 

 1 1 i he 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 for. e 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 ere. it, 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 ami gliding over mountain and river." 



Darwin's Journal of Various Countries Visited by II. M.S. Beagle,^. 223, 224. 



