ELISHA MITCHELL SCIENTIFIC SOCIETY. 61 



spiral, the spire leaning in the direction of the current of air. 

 Lord Rayleigh, F. R. S.,.was the first to demonstrate, mathe- 

 matically, how this elevation might be attained under such cir- 

 cumstances without resort to flapping of the wings.* In mid 

 air the bird starts to soar with the momentum acquired by flap- 

 ping its wings in rising. Say, with outstretched wings, it faces 

 the wind, and gradually rises until the momentum it had acquired 

 is overcome by gravity; it then turns and gradually descends, 

 on a plain oblique to the horizontal, for a short distance. In 

 doing this its velocity is increased from the operation of two 

 causes. The first of these causes is the ever present action of 

 gravity. The second and more important cause requires some 

 introductory remarks. For the sake of clearness let us divide 

 the air into a number of strata parallel with the surface of the 

 earth. During a wind, the different strata of air, starting with 

 the lowest, move with successively increasing velocities. When 

 the bird, facing the wind, has used up the momentum it had 

 acquired, it turns with the wind and passes into a lower stratum 

 of air the velocity of which is less than that of the stratum from 

 which the bird came. In this position the simple act of trans- 

 position to a lower stratum of less velocity gives the bird a rela- 

 tively increased velocity. With this increment of velocity it 

 sails along in the lower stratum, and turning rises into the 

 stratum above. Here another increment of relative velocity is 

 acquired. This enables the bird to rise into a still higher stratum, 

 which moves with greater velocity, and another increment of 

 relative velocity is added. 



Suppose the bird was in stratum b when it first turned, and 

 that 6 moves at the rate of 10 miles per hour. As the bird has 

 used up its acquired momentum, relatively to the air it is not 

 moving forward. Now as it passes to stratum a which has a 

 velocity of 5 miles per hour, the bird acquires a relative velocity 

 of five miles per hour. Now turning and facing the wind it 

 rises into b and has a relative velocity of 15 miles per hour, 



*Xature, Vol. XXVII, p. 534. 



