MOVEMENTS OF THE WINGS OF BIRDS. 255 



millimetres. We have tniced 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 \ve 

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

 and downward motions with the letters H and 13. 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 inny 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 rig - ] it 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 abscissa?, 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. 



Tli is determination is realised in fig. 107 for the trajectory 

 ol the wing, which has been constructed by successive points 

 in the following manner: 



!sii:::::U:::i:HH;::ss sis:::;::;:::: I:::;::::: 



Fir. 107 Constructed from the preceding- curves. An arrow indicates the 

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

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

 course. 



