MECHANISM OF INSECT FLIGHT 



201 



tions of this trajectory. These arrows are in the same 

 direction, and this first fact is opposed to the experiment 

 described in page 195, where we have investigated the direc- 

 tion of the movement of the wing, and have found it pass in 

 opposite directions in the two branches of the 8. In order to 

 explain the form which he assigns to this trajectory, Dr. Pet- 

 tigrew admits that in its passage from right to left, the wing 

 describes by its thicker edge the thick branch of the 8, and the 



Fig. 86. — Trajectory of the wing. 



thin branch by its narrow edge. The crossing of the 8 there- 

 fore would be formed by a complete reversal of the plane of 

 the wing during one of the phases of its revolution. In fact, 

 the author seems to perceive in this reversal of the plane, an 

 action similar to that of a screw, of which the air would form 

 the nut. We will not dwell any longer on this theory, but 

 we have deemed it necessary to bring it forward, in con- 

 sequence of the appeal which has been made to us. 



4. Artificial representation of an insect's flight. — In order to 

 render the action of the wing and the effects of the resistance 

 of the air more intelligible, we have made use of the following 

 apparatus : — 



Let fig. 87 represent two artificial wings composed of a rigid 

 main-rib connected with a flexible membrane, composed of 

 gold-beater's skin, strengthened by fine nervures of steel ; the 

 plane of these wings is horizontal ; a system of bent levers 

 raises or lowers them without giving them any lateral mo- 

 tion. 



The movement of the wings is caused by a little cop;^er 

 drum, in which the air is alternately condensed and rarefied 

 by the action of a pump. The circular surfaces of this drum 

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