62 JOURNAL OF THE 



which would be sufficient to carry it to a poiut higher than that 

 from which it came in stratum b: i. e., to c. From c it would 

 descend into b and then rise into d and so on. 



Lord Rayleigh says he would not have supposed a 'priori that 

 the increment in the velocity of wind at different heights was 

 sufficient, but "soirie explanation is badly wanted." 



Hubert Airy* suggests the possibility of vortices of air cur- 

 rents that are constantly receding from the earth, and that the 

 bird may possibly keep in the rear of one of these. 



R. Courtenayt states that the Black Vulture of Jamaica in 

 France utilizes currents of different velocities, and may even 

 make use of descending currents to acquire an increase of velocity. 

 In all of this discussion the bird's wing was treated of as if it were 

 a smooth plane. All who have carefully examined a bird's wing 

 know how well adapted it is to produce forward motion of the 

 bird by striking the air perpendicularly. This peculiar adapta- 

 tion of the wing has been described before quite frequently, but 

 I repeat it here briefly because of the important bearing it has 

 upon the subject. The work of the wing, either flapping or 

 motionless, is to compress air. The work of the elastic air, as 

 it tends to assume its normal condition, works on the wing and 

 produces forward motion. The uuder surface of the wing is so 

 constructed that air passing to the ulnar (rear) edge meets with 

 little or no resistance, but the air passing to the anterior, or 

 radial, edge meets with great resistance. The radial edge, also, 

 of soaring birds projects downward by the enlargements of the 

 bones and muscles of the brachium, manus, etc. This also catches 

 some of the air and impedes its movement. The ulnar edge 

 of the wing is made up of the tips of the feathers, called the 

 secondaries and tertiaries of the wing. These are bent upward 

 by the air which passes this edge. As there is a partial vacuum 

 above the wing, the air pushes forward on this upturned edge 

 as it flows past to fill it. 



-Nature, Vol. XXVII, p. 590. 

 jlbid., Vol. XXVIII, p. 28. 



