The Flight of Birds and the Mechanics of Flight. 423 



I have observed these birds when disturbed run flapping violently 

 up wind until lifted from the ground, and continuing to beat the air 

 until they got an elevation of from twenty to thirty feet, when they 

 began to wheel, beating their wings scarcely at all. The bird ascends 

 by an inclined spiral. The work is done by the wind, which lifts the 

 bird, partly by the slight upward currents, but chiefly by the inertia 

 of the bird, which drifts with the wind, the horizontal distance 

 drifted being the mechanical equivalent of the same distance advanced 

 in still air. The diagram shows the bird's course in air in side 

 elevation and plan. 



The small expenditure of power required to carry the albatross 

 for hours over the sea seems to prove that mechanical flight is 

 practicable, at any rate over the surface of the sea. The important 

 point is to imitate the elasticity of the bird's wing. This seems to 

 have been overlooked hitherto. Many machines have been made 

 with supporting planes, l)ut, so far as I know, they were all too 

 rigid. With thoroughly elastic planes the greater part of the work 

 would be done by the wind, and the motive power would chiefly be 

 necessary for steering purposes. 



