FLIGHT OF INSECTS. 



191 



is adapted to other experiments intended to verify those which 

 we have already made by other means. By varying the inci- 

 dence of the wing on the revolving cylinder, we can foretell 

 what will be the figure traced, if it be true that the wing 

 really describes the form of an 8 . Thus, if we obtain a figure 



Fig. 75. — Tracings of the wing of a wasp ; several of the lower loops are 

 distinctly seen. This tracing was obtained by holding the insect so as 

 to rub the cylinder by the hinder point of the wing, which gives very ex- 

 tended curves. 



conformable to that which we have foreseen, it will be an 

 evident proof of the reality of these movements. 



Fig. 76.— Tracing of a Wheatstone's kaleidophone rod, tuned to the octave, 

 that is to say. vibrating twice transversely for each longitudinal vibration. 

 (This figure is taken from R. Koenig.) The slackening speed of the 

 cylinder produces an approximation of the curves towards the end of the 

 figure. 



Let us suppose that the wing of the insect, instead of 

 touching the cylinder with its point, as we have seen just 

 now, brushes it with one of its edges ; and let us admit for 

 an instant that the 8 described by the wing is so lengthened 

 that it departs but slightly from the plane passing through 

 the vertical axis of this figure. If we press the wing slightly 

 against the cylinder the contact will be continuous, and the 

 tracing uninterrupted ; but the figure obtained will no 

 longer be an 8 ; if the cylinder be immovable it will be an 

 arc of a circle, whose concavity will be turned towards the 

 point of insertion of the wing, a point which will occupy pre- 

 cisely the centre of the curve described. 



