198 



ANIMAL MECHANISM. 



naturally the formation of the descending branch of the 8 

 passed through by the wing. 



The same theory applies to the formation of the ascending 

 branch of this figure. In short, a kind of pendular oscilla- 

 tion executed by the nervure of the wing is sufficient, to- 

 gether with the resistance of the air, to give rise to all the 

 movements revealed to us by our experiments. 



2. Artificial representation of the movements of the insect's wing. 

 — These theoretical deductions require experimental verifica- 

 tion, in order that they may be thoroughly borne out. We 

 have succeeded in obtaining the following results : — 



Let fig. 84 be an instrument, which, by means of a multi- 

 plying wheel and a connecting rod, gives to a flexible shaft 

 rapid to and fro movements in a vertical plane. Let us take 

 a membrane similar to that in the wings of insects, and fix it 

 to this shaft, which will then represent the main rib of the 

 wing ; we shall see this contrivance execute all the move- 

 ments which the wing of the insect describes in space. 



If we illuminate the extremity of this artificial wing, we 

 shall see that its point describes the figure 8, like a real wing ; 

 we shall observe also that the plane of the wing changes 

 twice during each revolution in the same manner as in the 

 insect itself. But in the apparatus which we now employ, 

 the movement communicated to the wing is only upwards 

 and downwards. Were it not for the resistance of the air, 

 the wing would only rise and fall in a vertical plane ; all 

 these complicated movements are due therefore only to the 

 resistance presented by the air. Consequently, it is this 

 which bends the main rib of the wing, turning it in a direc- 

 tion perpendicular to the plane in which its oscillation is 

 effected. 



But if the wing be pushed aside from its main-rib at each 

 of its alternate movements, it is evident that the air, acted 

 upon by this wing, will receive an impulse in an opposite 

 direction ; that is to say, it will escape at the side of the 

 fiexible portion of the wing, and cause in this direction a 

 current of air. It is seen, in figure 84, that a candle placed 

 by the side of the thin edge of the wing, is strongly blown 

 by the current of air which is produced. In front of the wing. 



