310 SOUND. 



tumbler is held by the hand, so as to prevent vibrations, 

 the sound will be prevented. 



Sound may be produced by one single shock or con- 

 cussion of the sounding body, in which case the vibrations 

 continue but an instant. The blow of a hammer, the 

 crack of a whip, a pistol shot, the stroke of a bell, 

 when the hand is pressed upon the outside of the bell, so 

 as to prevent a continuance of the sound are examples. 



If, however, these vibrations are continued for some 

 time with ^regularity, a continued and equable sound is 

 produced, Called a musical sound or tone. Arnott illus- 

 trates this subject in the following manner. 'If a wheel 

 with teeth be made to turn and to strike a piece of quill 

 with every tooth, it will, when moved slowly, allow every 

 tooth to be seen, and every blow to be separately heard ; 

 but with increasing velocity, the eye will lose sight of the 

 teeth, and the ear will at last hear only a smooth con- 

 tinued sound, called a tone, of which the character will 

 change with the velocity of the wheel.' 



In like manner, the vibrations of a long harp-string, 

 while it is very slack, are separately visible, and the 

 pulses, produced by it in the air, are separately audible ; 

 but as it is gradually tightened, its vibrations quicken, 

 and the eye soon sees that it is moving only by a broad 

 shadowy line; the distinct sounds which the ear lately 

 perceived, run together, owing to the shortness of inter- 

 vals, and are felt as one uniform, continued tone, which 

 constitutes the note or sound proper to the string. It is 

 the elasticity of such a string which causes the repetition 

 of the percussions, and therefore the continuance of the 

 sound. Thus the string having been pulled at its 

 middle to one side, and then let go, its elasticity carries 

 it back quickly to the straight position ; but by the time 

 it has reached this, it has acquired a momentum which, 

 like the momentum of a vibrating pendulum, carries it 

 nearly as far beyond the middle station as the place from 

 whence it came. It has to return therefore from this 

 second deviation, by its elasticity, in the same way ; but 

 still passing the middle as before, it has again to return, 

 and thus continues vibrating as a pendulum does, until 



