MOVEMKNTS OF THE WINGS OF BIRDS. 258 



conditions, he compresses the ball, and produces the tracing 

 represented in fig. 105. 



Fig. 105.— Tracing of the movements of a pigeon's wing. The upper line, 

 A P, shows the movements forwards and backwards. The lower line 

 H B, the movements up and down. 



Interpretation of the tracings. — The curves are read from left 

 to right, like ordinary writing. The upper curve is that 

 described by the humerus of a bird in its movements for- 

 wards and backwards ; the direction of these movements is 

 indicated by the letters A and P, which signify that all the 

 tops of the curves, as well as that at A, correspond with the 

 time when the wing has reached the most forward part of its 

 course ; the lower parts of the curves, on the contrary, indicate, 

 as well as that at the point P, the moment when the wing 

 has reached the hinder part of its movement. 



The horizontal line which cuts this curve has been traced 

 in a previous experiment by the point of the lever at the 

 instant when the wings of the bird, kept motionless by an 



