4 THE FLIGHT OF BIRDS 
to 10 metres per ssh tatoalh 22 miles per hour) 
the increase in resistance is rather less than the 
square of the velocity, whereas 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. 
