FLIGHT OF SEA GULLS — MILLER 403 



sible that such currents may be in operation, due to convection or 

 other causes of atmospheric disturbance with which aeronauts are 

 unpleasantly familiar. The wing of a bird, particularly of a large 

 bird, is, as we have shown above, an extremely efficient instrument, 

 capable of immediate adjustment to derive the maximum advantage 

 from every movement of the air, so that a very slight upward draft 

 may yield it considerable lift. 



In any case, it seems wiser to go as far as we can with explana- 

 tions in terms of known physical laws, rather than to postulate 

 forces of which we know nothing, and which, if they exist, we have 

 little chance of discovering. 



LITERATURE CITED 

 Beetham, B. 



1911. On positions assumed by birds in flight. Smithsonian Ann. Rept, 



1911, pp. 433-439, 8 pis. 

 Hankin, E. H. 



1913. Animal flight. (London, Iliffe & Sons, Ltd.), 405 pp., 98 text figs. 

 Headley, F. W. 



1895. The structure and life of birds. (London, Macmillan), xx 4-412 

 pp., 78 illus. 



1912. The flight of birds. (London, Witherby & Co.), x+163 pp., 16 



pis., 27 text figs. 

 Hendricks, J. E. 



1886. The mechanics of soaring. Am. Nat., 20, pp. 532-534. 

 Lancaster, I. 



1886. The mechanics of soaring. Am. Nat., 20, pp. 326-333. 



LlLIENTHAL, O. 



1911. Birdflight as the basis of aviation. (Isenthal trans., London, Long- 

 mans, Green & Co.), xxiv+142 pp., 8 pis., 94 text illus. (German 

 ed., 1889). 

 Mabey, E. J. 



1890. Le vol de oiseaux. (Paris, Libraire de l'Acadeniie de Medecine), 

 xvi+394 pp., 1 pi., 164 figs, in text. 



1895. Movement. (Pritchard trans., New York, D. Appleton Co.), 

 xv+323 pp., 200 illus. 

 Pettigrew, J. B. 



1874. Animal locomotion. (New York. D. Appleton Co.), xiii+264 pp., 

 130 illus. 



