246 MOVEMENT 



the wing at different stages of the trajectory, we pro- 

 ceeded as follows. A small piece of capillary glass 

 tubing was blackened in the smoke of a candle, so that 

 the slightest touch on the glass was sufficient to remove 

 the black coating and show the direction of movement 

 in each limb of the lemniscate. This experiment was 

 arranged as shown in the figure (173). Different 

 points on the path of movement were tested by the 

 smoked rod, and from the track along which the black 

 had been removed the direction of movement was 

 deduced. This direction is represented in the figure 

 by means of arrows. 



For the sake of comparison, we constructed an 

 artificial insect, the wing of which consisted, as in 

 nature, of a rigid nervure anteriorly, and a sort of 

 flexible sail behind. We have watched this little 

 contrivance move like a real insect, give like it a 

 luminous trajectory shaped like a lemniscate, agitate 

 the air behind, and by a kind of suction action aspirate 

 air towards it in front.* 



Theory of Insect Flight. — The theory of insect flight 

 may be completely explained from the preceding 

 experiments. The wing, in its to-and-fro movements, 

 is bent in various directions by the resistance of the 

 air. Its action is always that of an inclined plane, 

 striking against a fluid, and utilizing that part of the 

 resistance which is favourable to its onward progression. 



This mechanism is the same as that of a waterman's 

 scull,| which, as it moves backwards and forwards, is 

 obliquely inclined in opposite directions, each time 

 communicating an impulse to the boat. 



There is, however, a difference between these two 



* La Machine Animale, book iii., chap. ii. 



t This refers to what is generally called "sea-sculling," and of 

 course has no reference to the ordinary river sculling, which requires 

 the use of two sculls. — Translator. 



