284 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



the registering lever. Experiments upon the kinds of birds previously 

 mentioned furnish tracings analogous to those of the vertical oscilla- 

 tions. If it is true, as I suppose, that the vertical oscillation of the 

 bird at the moment of raising the wing be due to the upward trans- 

 formation of velocity, by obtaining, simultaneously, the tracing of the 

 vertical oscillations and those of the variations of velocity, we shall 

 have the means of confirming this theory. When obtaining at one 

 time the two kinds of oscillations in the flight of a harrier, I have seen 

 that the phase of descent of the wing resulted both in the elevation ot 

 the bird and the acceleration of its speed. This effect is the necessary 

 consequence of the inclination of the plaue of the wing at the moment 

 of its descent, as we have previously shown in the flight of insects. As 

 for the phase of elevation of the wing, it is proved that during the 

 slight ascension which it produces the speed of the bird is diminished. 

 In fact, the curve of the variations of rapidity falls as soon as the bird 

 begins to rise. This i«, then, a confirmation of the previously suggested 

 theory of the upward transformation of the speed of birds. Thus by 

 this mechanism the descending stroke of the wing creates the force 

 which produces the two oscillations of the bird in the vertical plane. 

 The downward stroke directly produces the ascent which is synchro- 

 nous with it, and indirectly by creating the velocity which prepares for 

 the second vertical oscillation. 



Simultaneous tracing of the tico Mnds of oscillation of the bird. — Instead 

 of representing each kind of oscillation separately, I have thought that 

 it would be more instructive to obtain a single line which, by its curves, 

 should represent both of the movements which the body of the bird 

 executes in its course through space. The method which has been used 

 to obtain the curve of the point of the wing, with some modifications can 

 be made to furnish a simultaneous tracing of both kinds of motion. 

 For this both drums must be connected with the same inert mass, and 

 placed at right angles to each other. Turning back to Fig. 23, which 

 shows the two levers connected by tubes which transmit to the one all 

 the motions executed by the other, when any motion is imparted to the 

 first lever, the second lever reproduces the same motion in the same 

 direction. Now, let us charge one of the levers with a mass of lead, 

 and, taking the support of the apparatus in the hand, make it describe 

 some motion in a ]ilane perpendicular to the direction of the lever. We 

 see that the lever No. 2 executes directly opposite movements. In 

 fact, since the motive force which acts on the membranes of the drums 

 is simply the inertia of the mass of lead, and since this mass is always 

 behind the motion given to the apparatus, it is clear that if the whole 

 be raised the mass will keep the lever down ; if the whole be lowered, 

 the mass will raise the lever ; if it be carried forward, the mass will hold 

 back the lever, &g. Now, the second lever, executing the same motions 

 as the first, will give curves which are directly the opposite of the mo- 

 tion which has been given to the support of the apparatus. This being 

 settled, now for the experiment : For this I take the apparatus repre- 

 sented on the back of the harrier in Fig. 25 ; I remove the rod which 

 receives the motion of the wing, and the parallelogram which transmits 

 it to the lever. I keep only the lever connected with the two drums and 

 the mounting which attaches it to the bird's back. I fix a mass of lead 

 on this lever and let the animal fly. The tracing obtained is repre- 

 sented by Fig. 32. 



The analysis of this curve is at fir^^^sight extremely difficult. I hope, 

 aowever, to suceecd i" showing its signification. It is traced on the 

 cylinder under the same conditions as Fig. 2G, showing the different 



