222 THE STRUCTURE AND LIFE OF BIRDS chap. 



of the 8 either does not exist, or at any rate is reduced 

 to very small dimensions. Applying machinery to 

 the bird's wing, Professor Marey succeeded in tracing 

 the figure described by the outer end of the humerus. 

 It is an ellipse with the long axis inclining down- 

 wards. The difficulties were found to be great in 

 obtaining by the aid of similar machinery a tracing 

 of the course followed by the wing tip. Professor 

 Marey at length hit upon the following plan. He 

 fastened a small piece of white paper to the tip of a 

 crow's wing, and as the bird flew in front of a perfectly 

 black screen, he took a photograph of this moving 

 speck of white, while of course no image of the crow 

 appeared upon the plate. In the figure the long 

 forward sweep of the downstroke comes out very 

 clearly ; at last the line curves backward, the wind 

 of the bird's velocity making the wing retreat : the 

 muscles arrest this backward movement, and we have 

 an apparent forward twitch (due perhaps to the onward 

 momentum which the wing-tip shares with the whole 

 body), forming the small loop at the bottom. Soon 

 the wing hunches up at the wrist and for some distance 

 the tip moves upward and backward (fl). It will be 

 noticed that as the bird moves from right to left with 

 increasing speed, the tracings alter, and at last the 

 smaller circle disappears altogether. The figure, in 

 fact, varies considerably even when the same bird is 

 experimented on, especially when it begins to get up 

 speed. When the velocity is at all considerable, no 

 ellipse is actually formed. It only exists in imagina- 

 tion, like the ellipse which we say the moon forms 

 in revolving round the earth. The earth itself, 



