420 Transactions of the South African Philosophical Society. 



the wing bone is at the lower level, and propulsive effect is obtained 

 by the bird thus — 



During the upward stroke the wing bone is at the higher level, thus- 



the bird being supported by the wind striking the inclined plane of 

 the wing. This seems simple enough, but how are we to explain 

 the flight of the albatross and other sea birds which seldom beat the 

 air with their wings? When and where is the work done which 

 keeps them in motion ? 



Long observation of the flight of the albatross and other sea birds 

 has convinced me that they are sustained by upward currents of air 

 due to the wind striking the inclined surfaces of the waves. The 

 wind blowing over the sea, and the land, too, to some extent, is a 

 current undulating in a vertical plane chiefly. The wing of the bird 

 is so constructed that the air always impinges on the underside ; so 

 that, while supported by the upward current, the bird escapes the 

 reverse action due to the downward current. Again, the upward 

 current, when the wind is strong, is much more violent than the 

 downward, while tiie velocity of the bird's motion and the elasticity 

 of the wing muscles enable it to derive support from the air even in 

 a current with a downward inclination. 



This diagram represents two large waves with a surface of wave- 

 lets. The arrows show the direction of the wind glancing upward 

 from the backs of the waves, the larger representing the major cur- 

 rents and the smaller the minor, and the upward currents largely 

 exceeding the downward in force. Two facts seem to me noticeable ; 

 one, that the stronger the wind the easier and more rapid the bird's 

 flight; tlie other, that the bird continually glides downwards as close 



