446 Prof. S. P. Langley on the 



observed in nature are as sharp as those here represented, 

 which are intentionally exaggerated, while in all which has 

 just preceded, by an equally intentional exaggeration of the 

 normal action, the wind-pulsations have been supposed to 

 alternate with absolute calm. This being understood, it is 

 scarcely necessary to point out that if the calm is not absolute, 

 but if there are simply frequent successive winds or pulsa- 

 tions of wind of considerably differing velocity (such as the 

 anemometer observations show are realized in nature), the 

 same general effect will obtain*, though we are not entitled 

 to assume from any demonstration thus far given that the 

 total advance will be necessarily greater than that of the 

 whole distance the mean wind has travelled. It may also be 

 observed that the actual actions of the soaring bird may be, 

 and doubtless are, more complex in detail than those of this 

 diagram, while yet in their entirety depending on the prin- 

 ciples it sets forth. 



The theoretical possibility at least will now, it is hoped, be 

 granted, not only of the body's rising indefinitely, or of its 

 descending in the interval of calm to a higher level C than it 

 rose from at A, but of its advancing against the calm or light 

 wind through a distance BC, greater than that of AB, and so 

 on. The writer, however, repeats that he has reason to suppose, 

 from the data obtained by him, that this is not only a theor- 

 etical possibility, but a mechanical probability under the con- 

 ditions stated, although he does not here offer a quantitative 

 demonstration of the fact, other than by pointing to the 

 movements of the soaring bird and inviting their recon- 

 sideration in the light of the preceding statements. 



The bird, by some tactile sensibility to the pressure and 

 direction of the air, is able, in nautical phrase, to " see the 

 wind " t an d to time its movements so that, without any 

 reference to its height from the ground, it reaches the lowest 

 portion of its descent near the end of the more rapid wind- 

 pulsation ; but the writer believes that to cause these adaptive 



* The rotation of the body about a vertical axis so as to change the 

 aspect of the inclination, as in the first figure, may be illustrated by the 

 well-known habit of many soaring birds, of moving in small closed curves 

 or spirals, but it may also be observed, in view of the fact that even in 

 intervals of relative calm during which the body descends, there is always 

 some wind, — that in making the descents, if the body, animate or in- 

 animate, maintain its direct advance, this wind tends to strike on the 

 upper side of the plane or pinion. Mr. G. E. Curtis offers the suggestion 

 that the soaring bird avoids such a position when possible, and therefore 

 turns at right angles to, or with, the wind, and that this may be an 

 additional reason for its well-known habit of moving in spirals. 



t Mouillard. 



