50 



A BOOK OF INSECTS 



impels the boat." With equal aptness we may liken the 

 movement to the screw-propeller of a ship or an aero- 

 plane, for the screw may be considered as an inclined 

 plane whose movement is continuous, and always in the 

 same direction. 



The frequency of wing- vibration may be roughly 

 estimated from the musical note made by the insect as 

 it flies — that is, if the wings vibrate rapidly enough to 

 produce an audible sound ; but it may be graphically 

 determined, and with greater accuracy, by means of an 

 instrument called the kymograph. This consists of a 

 cylinder, covered with smoked paper, and revolved by 

 clockwork at a uniform rate. The insect is held, in a 

 delicate pair of forceps, in such a way that one of its 

 wings brushes against the blackened paper at every move- 

 ment. Each of these contacts scrapes off a small part 

 of the sooty deposit, and exposes the white paper below ; 

 and, as the cylinder revolves, new points continually 

 present themselves to the wing-tip. Professor Marey's 

 kymograph revolved once in a second and a half; and by 

 comparing the record made by the insect with one made 

 by a tuning-fork of known vibration period, the frequency 

 of the wing movement was determined with great 

 accuracy. Some of his results were as follows : — 



Common fly 



Drone-fly .... 



Bee . 



Wasp .... 



Humming-bird hawk-moth 



Dragon-fly 



Small white butterfly 



Wing 



beats per second. 



330 



240 



190 



110 



72 



28 



9 



Obviously, the movements of the captive insects must 

 have been retarded by friction ; but these figures clearly 



