292 



ANlSrUAL REPORT SMITHSONIAN INSTITUTION, 1954 



HORiZON 



^nny Force 



\,Sinkny Force 



HORtZOIV 



lift. 



^kTotal AeroMpt Force 



'^ ,^ 



HORIZON 



Figure 4.— When gliding steadily in still air the forces (lift and drag) exerted by the air 

 against the bird are exactly equal but opposite to the weight of the animal. The condi- 

 tions are comparable with those which would exist if a biid were sliding down a pair of 

 rigid runners in a vacuum: the reaction from the runners at right angles to their surface 

 would be equivalent to the lift of the wing and the friction of the runners to the drag 

 of the bird's body. 



