CHANGES IN THE PLANE OF THE BIRD's WING. 261 



horizontal movements by two systems of cords. For the 

 vertical ones, a cord v goes to the lever of the experimental 

 drum. The cord h transmits to another apparatus the 

 movements in the horizontal, that is, in the antero -posterior 

 direction. 



Experiment, — A buzzard to which this apparatus has been 

 adapted is harnessed to the instrument and allowed to fly : we 

 obtain at the same time the three curves represented in 

 fig. 110. With these three data, we can construct, not only the 

 trajectory of the wing, but the series of inclinations of its plane 

 at the different points of its course. 



The curve traced with a full line corresponds with the 

 movements of the wing in an antero-posterior direction. The 

 point A, and those homologous with it, correspond with the 

 extreme anterior position of the wing ; the point P with the 

 extreme posterior position. The curve formed of interrupted 

 strokes indicates the relative height of the wing in space ; the 

 point H corresponds with the maximum elevation of the wing, 

 and the point B with its greatest depression. 



These two first curves enable us to construct, by means of 

 points, the closed curve* (fig. Ill) representing the trajectory 

 of the buzzard^s wing. It is by this trajectory that we shall 

 determine the inclination of the plane of the wing at every 

 part of its elliptical course. 



For this purpose, we must return (fig. 110) to the dotted 

 curve S, which is the expression of the torsions of the wing at 

 different instants. The positive and negative ordinates of this 

 curve correspond with the trigonometrical tangents of the 

 anglesf which the wing makes with the axis of the body.;}: 



* This curve is not always closed ; this is the case only when the flight 

 is extremely regular. 



+ We must subtract algebraically from the angle found, a constant 

 quantity, the angle of 30° which the wing, during repose, makes with the 

 horizon. 



t We cannot positively affirm that this axis is horizontal ; it seems 

 rather that it is inclined so that the beak of the bird turns slightly 

 upwards. This inclination of the axis would necessitate a correction in 

 the absolute inclinations of the wing at the different points of its 

 revolution. 



