I 2 8 Summer Studies of Birds and Books CHAP. 



sound produced by it. But in order to alter the 

 pitch, holes have been punched in the tube, by 

 stopping which with the fingers you can make the 

 vibrating column of air in the tube longer or shorter 

 at will, and thereby alter the pitch of the sound ; 

 just as in the trombone the same thing is effected by 

 moving one tube up and down within the other. 



But in B, the bird's instrument, the tube is not 

 hard or stiff; it is as elastic as the body of a worm, 

 and is indeed not unlike that curious compound of 

 rings. It can be lengthened and shortened, squeezed 

 and relaxed, by the many strong muscles which are 

 attached to it; and these give the bird a capacity, 

 whether or no he choose to take advantage of it, of 

 producing an almost endless variety of pitch. Some 

 birds when in full song, the Nightingale for example, 

 will draw their throats in and out, while at the same 

 time the feathers are set quivering with the vibration 

 going on within. These motions are caused by the 

 lengthening and shortening of the tube, while 'at the 

 same time the whole instrument is in strong vibra- 

 tion. No wonder then that the sounds produced by 

 such an instrument are, or can be, endless in variety, 

 and often beautiful in tone. 



But this instrument is a natural and wild one, 

 tmtempered and unfettered by human science or art ; 

 and the sounds it produces are not all of them musical 

 sounds in the true sense, even in the very best bird- 



