4 THE FLIGHT OF BIRDS 



to 10 metres per second (about 22 miles per hour) 

 the increase in resistance is rather less than the 

 square of the velocity, Avhereas for velocities greater 

 than 10 metres per second the resistance increases 

 at a still more rapid rate.* 



But to resist, to give effective support, the air 

 must be in proper condition. If it has just been 

 pounded and battered, it is useless to trust to it. 

 And so perpetual forward movement to ever fresh 

 fields is necessary. A bird cannot mark time in 

 the air. It is true that one often sees a Kestrel 

 Hawk hovering, apparently without any forward 

 motion, his wings, one might imagine, pounding 

 the same air over and over again. But there is 

 good reason to believe that the Kestrel never hovers 

 except when there is a fair breeze to bring fresh 

 unbattered air to his wings. If a long string be 

 tied to a Pigeon's leg, he will fly perfectly well till 

 he reaches the end of his tether. But as soon as he 

 feels the pull of the string he will drop to the 

 ground. When a flock of birds are travelling across 

 the sky, it is easy to see that not one of them puts 

 himself immediately behind any other. Were any 

 individual to do so, he would not have at his disposal 

 the fresh undilapidated columns of air that are 

 essential, and moreover he would feel all the back- 

 wash from the bird in front. At least one aviator 

 has lost his life through getting into the wash of 

 an aeroplane that was a little ahead of him. In 

 building a biplane the question of finding air that 

 has not been tumbled is one that cannot be neglected. 

 A writer who appears to speak with authority says 



* Marey, Vol des Oiseaux, p; 218. 



