284 SOUND 



wheel is mounted and slowly rotated, and a stiff card is held 

 against the teeth of the wheel, a distinct rap is heard every 

 time the card strikes the wheel. But if the wheel is rotated 

 rapidly, the ear ceases to hear the various taps and recognizes 

 a deep continuous musical tone. The blending of the indivi- 

 dual taps, occurring at regular intervals, has produced a sus- 

 tained musical tone. A similar result is obtained if a card is 

 drawn slowly and then rapidly over the teeth of a comb. 



MUSICAL INSTRUMENTS 



Musical instruments may be divided into three groups accord- 

 ing to the different ways in which their tones are produced : 



First. The stringed instruments in which sound is produced 

 by the vibration of stretched strings, as in the piano, violin, 

 guitar, mandolin. 



Second. The wind instruments in which sound is produced 

 by the vibrations of definite columns of air, as in the organ, 

 flute, cornet, trombone. 



Third. The percussion instruments, in which sound is pro- 

 duced by the motion of stretched membranes, as in the drum, 

 or by the motion of metal disks, as in the tambourines and 

 cymbals. 



Stringed instruments. When the lid of a piano is open, 

 numerous wires are seen within : some long, some short, some 

 coarse, some fine. Beneath each wire is a small felt hammer 

 connected with the keys in such a way that when a key is pressed, 

 a string is struck by a hammer and is thrown into vibration, 

 thereby producing a tone. 



If we press the lowest key, that is, the key giving forth the 

 lowest pitch, we see that the longest wire is struck and set 

 into vibration ; if we press the highest key, that is, the key 

 giving the highest pitch, we see that the shortest wire is struck. 

 In addition, it is seen that the short wires which produce the 



