330 OUR PHYSICAL WORLD 



strings can be made more or less taut, and so tuned to play in any 

 desired key (Fig. 168). 



The sound produced by a vibrating string is weak. It does 

 not hit enough air particles to start vigorous waves. If, however, 

 it is mounted on a thin-walled box so that the vibrations of the 

 string are imparted to the box, which presents a broad area 

 to the air and in turn imparts its vibrations to many air particles, 

 then the volume of sound given out is greatly increased. A 

 watch held in the hand is scarcely heard, but place it on an 

 empty cigar box and it sounds quite loudly. Strike an ordinary 

 table fork on the edge of the table so as to set its tines in vibration, 

 and the sound it gives out is scarcely audible; but press the end 







FIG. 169. Mouth end of a clarinet, showing reed 



of its handle on the table, and its note is loud and clear. Not 

 only does the wood vibrate in the violin and similar instruments, 

 but the contained air is thrown into vibration and contributes 

 to the volume and character of the sound. The shape of the 

 instrument affects the quality of its notes and hence it must 

 be skilfully made. That is one reason why the "old masters" 

 are such costly instruments; they were made with rare skill 

 and some luck, which even their skilful makers could seldom 

 duplicate. 



In wind instruments, it is the contained column of air that 

 is thrown into vibration, and, pulsing back and forth, imparts 

 its motion to the surrounding air to start the sound we hear. 

 This column of air may be thrown into vibration by blowing 

 across a hole in the instrument as in the flute, or by a vibrating 



