272 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



method is here inapplicable. The motion of a bird's wing, while too 

 rapid to be followed by the eye, is not sufficiently rapid to form a 

 persistent impression of its entire trajectory upon the retina. The 

 graphic method, which I have hitherto employed, only furnishes 

 impressions of motions which happen to follow a straight line, and it is 

 only by combining this rectilinear movement with the revolving cylinder 

 with a smoked surface that the expression of the rapidity with which 

 the motion is effected at each instant is obtained. 



The problem is to find the means of registering on an immovable 

 plane all the motions which the point of a bird's wing makes in space, 

 as if a style had been placed at the end of the winjr, and this style 

 traced or rubbed on a piece of paper by its side. It is still further 

 necessary to have a figure of the same nature as the luminous figure of 

 the gilded wing of an insect, that the piece of paper on which the trace 

 is to be made shall remain motionless in regard to the center of motion 

 of the wing of the flying bird, or in effect that it shall follow the bird in 

 all its phases of impulsion through space. 



Now, physics teach us that all motion susceptible of registration in 

 one plane can be generated by the rectangular combination of two 

 rectilinear motions. The tracings obtained by Koenig by arming a 

 vibrating Wheatstone's rod with a style, the luminous figures of musical 

 chords which M. Lissajous has produced by the reflection of a ray of 

 light from two vibrating mirrors perpendicular to one another, are well 

 known examples of the formation of a plane figure by means of two 

 rectilinear movements. Thus, admitting that the motions of elevation 

 and depression of the wing can be transmitted at one time, as well as 

 the back and forward motions of this organ, by supposing that a writing 

 style can simultaneously receive the impulse of these two motions, per- 

 pendicular to each other, this point will write on the cylinder the exact 

 figure of the motions of the bird's wing. I tried at first to construct an 

 apparatus which would thus transmit such a motion to a distance and 

 register it, without concerning myself with the way in which I might 

 apply this rather weighty mechanism to the bird. 



Fig. 23 represents this provisional apparatus, the description of 

 which is indisi)ensable for the comprehension of the second mechanism, 

 which I shall describe hereafter. Upon two solid feet, carrying vertical 

 supports, are seen two horizontal arms parallel to each other. These are 

 two aluminium levers which, by the transmitting apparatus to be 

 described, should both execute the same motions. Each of these levers 

 is mounted ou a ball-and-socket joint, or double articulation, which 

 permits all kinds of motion ; thus each lever can be carried above, 

 below, to the right or to the left. It can by its point describe the base 

 of a cone of which the joint will be the apex. In fact, it will execute 

 any kind of motion which the experi mentor may choose to impart to it. 

 It is also necessary to establish the transmission of motion from one 

 lever to the other at a distance of ten or fifteen meters. This is done 

 by means of a process with which the reader is already fiimiliar — the use 

 of drums and air tubes. 



The lever, which is seen at the left in the figure, is fastened by a 

 metallic arm articulated at one of its extremities to the membrane of a 

 drum placed below it. In the vertical motions of the lever the mem- 

 brane of the drum rises or falls by turns, i)roducing a throbbing motion 

 of the air in another drum through a long tube, which establishes a 

 communication between them. In the apparatus to the right in the 

 figure, the second drum is placed above the corresponding lever articu- 

 lated -with it, and faithfully transmits all the motions which have been 



