222 THE STRUCTURE AND LIFE OF BIRDS cuHap. 
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 ofa 
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 (2). 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, 
