MOVEMENTS OF THE WINGS OF BIRDS. 235 



The graphic method, with its transmission of signals, which 

 we have hitherto employed, only furnishes the expression of 

 movements which take place in a straight line, such as the 

 contraction or lengthening of a muscle, the vertical and hori- 

 zontal oscillations of the body during the act of walking, &c. 

 It is only by combining this rectilinear movement with the 

 uniform advance of the smoked surface that receives the 

 tracing, that we obtain the expression of the velocity with 

 which the movement at each instant is effected. 



The action of the wing during flight does not consist 

 merely of alternate elevations and depressions. We have only 

 to look at a bird as it flies over our head to ascertain that the 

 wing is carried also forward and backward at each stroke. 

 From this double action must result a curve w^hich it is neces- 

 sary to describe. 



It can be geometrically shown- that every plane figure, 

 that is to say, every figure susceptible of being described upon 

 a plane surface, can be produced by the rectangular combina- 

 tion of two rectilinear movements. The tracings obtained by 

 Koenig by arming with a style Wheatstone's vibrating rods, 

 and the luminous figures of musical chords which Lissajous 

 produced by the reflection of a pencil of light upon two 

 mirrors vibrating perpendicularly to each other, are well- 

 known examples of the formation of a plane figure by means 

 of two rectilinear movements at right angles to each other. 



Thus, if we can transmit at the same time the movements 

 of elevation and depression executed by the wing of the bird, 

 as well as those which the organ makes forwards and back- 

 wards ; then, supposing that a tracing point can receive simul- 

 taneously the impulse of these two movements at right angles 

 to each other, this point will describe on the paper the exact 

 tracing of the movements of the bird's wing. 



We have endeavoured first to construct an instrument which 

 would thus transmit to a distance any movement whatever, 

 and register it on a plane surface, without attending to the 

 method by which this machine, which may be more or less 

 heavy, might be adapted to the body of the bird. Fig. 97 

 represents our first experimental 'instrument, the description 

 of which is indispensable in order to enable our readers to 



