vii FLIGHT 



123 



meanwhile, is moving rapidly onward, so that the 

 line of the moon's course never meets or crosses itself. 

 When the moon moves backward and circles round 

 us, its backward movement is only relative to the 

 earth. In reality it is moving onward, only with less 

 velocity. To apply this to a bird's wing : suppose 

 that a pigeon is flying at the rate of thirty miles an 

 hour — i.e. at a fairly easy-going pace if the weather be 

 good, and with a stroke of 300 per minute. This 

 gives an advance of 2 -J-t yards per stroke. More than 

 half of this advance will be made during the down- 

 stroke. The downward curve, therefore, will be 

 altogether out of reach when the time comes for 

 putting in the upward one to complete the ellipse, 

 and when a velocity of fifty or more miles an hour 

 is attained, the ellipse becomes still more theoretical. 

 But though it is desirable to point this out it hardly 

 diminishes the interest of what Professor Marey has 

 proved, that the trajectory of the humerus and of the 

 wing tip, when the figure is not destroyed by the 

 rapidity with which the bird travels, is an ellipse 

 with, in the latter case sometimes, a small loop at 

 the lower end. 



The Bird's Trajectory. 



Meanwhile the bird's trajectory is an undulating 

 one. It rises with every downstroke, and sinks with 

 every upstroke. Though in the course of the latter 

 there seems to be in some species a slight momentary 

 rise, caused by the wind of the bird's velocity catch- 

 ing the wings and his whole surface, yet the main 



