294 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



-4- Movenimt'^ (ird ret^xUve i^-'^ec^rth/ 



, (Bird stationary ) 

 * ' A.B.C -^ 



Figures S (upper) and 6. — When gliding in still air a bird is constantly losing height 

 relative to the ground, but if the air is rising from the ground as fast as the bird is falling, 

 it is able to glide along a horizontal path for an indefinite period (fig. 5). If the air is 

 moving backward and upward from the ground at the same speed as the bird is traveling 

 forward and downward, the bird can hover with motionless wings, quite stationary to 

 an observer on the ground (fig. 6). 



