BOOK THE THIRD. 



AERIAL LOCOMOTION. 



CHAPTER I. 



OF THE FLIGHT OF INSECTS. 



Frequency of the strokes of the wing of insects during flight; acoustic 

 determination ; graphic determination Influences which modify the 

 frequency of the movements of the wing Synchronism of the action 

 of the two wings Optical determination of the movements of the 

 wing ; its trajectory ; changes in the plane of the wing ; direction 

 of the movement of the wing. 



IN terrestrial locomotion we have been able to measure by 

 experiment the pressure of the feet on the ground, and hence 

 we have deduced the intensity of the re-actions on the body 

 of the animal. These two forces were easily ascertained by 

 direct measurement. In the problem which is now to occupy 

 us, the conditions are very different. The air gives a certain 

 resistance to the wings which strike upon it, but it is a resis- 

 tance every instant yielding, for it is only in proportion to the 

 rapidity with which it is displaced, that the air resists the 

 impulse of the wing. When we study the phenomena of flight, 

 it is therefore necessary to know the movement of the wing in 

 all the phases of its speed, in order to estimate the resistance 

 which the air presents to that organ. We will propound in 

 the following order the questions which must be resolved. 



1. What is the frequency of the movements of the wing of 

 insects ? 



2. What are the successive positions which the wing occu- 

 pies during its complete revolution ? 



