MOVEMENTS OF THE WINGS OF BIRDS. 255 



millimetres. We have traced in full line one of these curves, 

 that of the movements in the antero -posterior direction, the 

 course of which is indicated by the letters A and P ; then we 

 have represented, by a dotted line, the curve of the upward 

 and downward motions with the letters H and B. We have 

 placed these two tracings over each other, so as to make the 

 zero-lines of each coincide. We have also taken care to 

 preserve the vertical superposition of the corresponding points 

 of each of these curves ; we may therefore be certain that, 

 wherever any vertical line cuts the two curves, the inter- 

 sections correspond with the position which the humerus of 

 the bird occupies, at that instant, with reference to two planes 

 at right angles to each other. The intersection with the dotted 

 line will express, by the length of the ordinate drawn from 

 this point to the axis of the abscissae, the position which the 

 wing then occupies with reference to an horizontal plane ; the 

 intersection with the full line will express the position of the 

 wing as referred to a vertical plane. 



This determination is realised in fig. 107 for the trajectory 

 of the wing, which has been constructed by successive points 

 in the following manner: — 



I——— — ■■■■■■■ wm 



■HBHani 



'SSSSSSSSSSSSSSSSSSHSi 

 SS88 SBSSBSSSSSSSSSS: 



■■■■Hi 



■■■■in 



aaBBsaaBigaa 



iSSSSBB 



11 



SSSS8SSS 



■■■HI 



NHBHS 





iSisiSi 



SBSSgU 



■■■anil 

 ■■■■■■■■ 







Fig. 107.— Constructed from the preceding curves. An arrow indicates the 

 direction of the movement. The separation of the dots expresses the 

 rapidity of the movements of the wing at the different parts of its 

 course. 



