182 



ANIMAL MECHANISM. 



interpretation; Pisko, the German writer on acoustics, has 

 perfectly explained it. There is no doubt that the vibrations 

 always follow each other after the same interval of time ; when 

 a vibrating plate remains at the same distance from the ear, 

 the vibrations require the same time to reach us, and the 

 phenomenon, uniform for the instrument, is uniform also for 

 our organ. On the contrary, if the instrument be brought 

 rapidly nearer, the vibration which is produced every instant 

 has less space to traverse before it reaches the tympanum ; it 

 thus approximates to that which preceded it, and the sound 

 grows sharper. If the instrument be removed to a greater 

 distance the vibrations are more extended, and the tone be- 

 comes more grave. Every one has remarked, when travelling 

 on a railroad, that if a locomotive passes us while the driver 

 is sounding the whistle, the sharpness of the tone increases as 

 the engine comes nearer, and becomes graver when it has 

 passed by us, and the whistle is rapidly carried to a greater 

 distance. 



From these considerations we must be convinced that it is 

 very difficult to estimate from the musical tone produced by 

 a flying insect, the absolute frequency of the strokes of its 

 wings. This depends to some extent on the variation of the 

 tone thus produced, which passes at each instant from grave to 

 sharp, according to the rapidity and the direction of the flight. 

 Besides this, it is not easy to assign to each wing the part 

 which it plays in the production of the sound. We have also 

 to take into consideration that the wing of an insect may, by 

 brushing through the air as it flies, be subjected to sonorous 

 vibrations much more numerous than the complete revolutions 

 which it accomplishes. 



The graphic method furnishes a simple and precise solution 

 of the question ; it enables us to ascertain almost to a single 

 beat the number of movemects made per second by aninsect^s 

 wing. 



Experiment, — A sheet of paper blackened by the smoke 

 of a wax-candle, is stretched upon a cylinder. This cylinder 

 turns uniformly on itself at the rate of a turn in a second 

 and a-half. 



The insect, the frequency of the movement of whose wings 



