PROGKESSION IN OR THROUGH THE AIR. 123 



be heightened. The swan in flying produces a loud whistling 

 sound, and the pheasant, partridge, and grouse a sharp whirring 

 noise like the stone of a knife-grinder. 



It is a mistake to suppose, as many do, that the tone or 

 note produced by the wing during its vibrations is a true 

 indication of the number of beats made by it in any given 

 time. This will be at once understood when I state, that a 

 long wing will produce a higher note than a shorter one 

 driven at the same speed and having the same superficial 

 area, from the fact that the tip and body of the long wing 

 will move through a greater space in a given time than the 

 tip and body of the shorter wing. This is occasioned by all 

 wings being jointed at their roots, the sweep made by the 

 diff*erent parts of the wing in a given time being longer or 

 shorter in proportion to the length of the pinion. It ought, 

 moreover, not to be overlooked, that in insects the notes pro- 

 duced are not always referable to the action of the wings, 

 these, in many cases, being traceable to movements induced 

 in the legs and other parts of the body. 



It is a curious circumstance, that if portions be removed 

 from the posterior margins of the wings of a buzzing insect, 

 such as the wasp, bee, blue-bottle fly, etc., the note produced 

 by the vibration of the pinions is raised in pitch. This is 

 explained by the fact, that an insect whose wings are curtailed 

 requires to drive them at a much higher speed in order to 

 sustain itsdf in the air. That the velocity at which the wing 

 is urged is instrumental in causing the sound, is proved by 

 the fact, that in slow-flying insects and birds no note is pro- 

 duced ; whereas in those which urge the iving at a high 

 speed, a note is elicited which corresponds within certain 

 limits to the number of vibrations and the form of the wing. 

 It is the posterior or thin flexible margin of the wing which 

 is more especially engaged in producing the sound; and if 

 this be removed, or if this portion of the wing, as is the case 

 in the bat and owl, be constructed of very soft materials, the 

 character of the note is altered. An artificial wing, if pro- 

 perly constructed and impelled at a sufficiently high speed, 

 emits a drumming noise which closely resembles the note 

 produced by the vibration of short-winged, heavy-bodied 



