VIL FLIGHT 223 
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—ze. at a fairly easy-going pace if the weather be 
good, and with a stroke of 300 per minute. This 
gives an advance of 234 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 
