MOVEMENTS OF THE WINGS OF BIKDS. 235 



The graphic method, with its transmission of signals, which 

 we have hitherto employed, only furnishes llio 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 Ihe act of walking, &c. 

 It is only Ly 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 docs not consist 

 merely of alternate elevations and depressions. We have oidy 

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

 wing is carried also for\vard and backward at each stroke. 

 From this double action must result a curve which it is neces- 

 sary to describe. 



It can be geometrically shown that every plums figure, 

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

 a plane surface, c.ni be produced by the rectangular combina- 

 tion of two rectilinear movements. The tracings obtained by 

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

 and the luminous figures of musical chords which Lissajous 

 produced by the retlection of a pencil of light upon two 

 mirrors vibrating perpendicularly to each oilier, are well- 

 known examples of the formation of a plane figure by means 

 of two rectilinear movements at light 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 



