NATURE OF SOUND. 275 



motion imparted to the auditory nerve by each individual 

 pulse of the series continue until the arrival of its suc- 

 cessor, the sound will not cease at all. TJiat the sound 

 may be mere noise, the pulses must be irregular in 

 their recurrence. > 



(a.) Momentary noises may be produced by pounding with a 

 hammer, stamping with the foot, clapping the hands, or drawing a 

 stick slowly along the pickets of a fence. Continuous noises may 

 be produced by sawing boards or filing saws. They are more or 

 less familiar in the rattling of wheels over a stony pavement, the 

 roar of waves, or the crackling of a large fire. 



429. Music. / musical sound consists of a 

 regular and rapid succession of pulses. The regu- 

 larity of the succession renders the sound smooth and 

 agreeable ; the rapidity renders it continuous. To secure 

 this smoothness the pulses must be perfectly periodic ; the 

 sounding body must vibrate with the unerring regularity 

 of the pendulum, but impart much sharper and quicker 

 shocks to the air. Every musical sound has a well-defined 

 period and wave length. 



430. Elements of Musical Sounds. Musical sounds or 

 tones have three elements intensity or loudness, pitch, and timbre 

 or quality. The first two of these we shall consider at once, the 

 third, a little further on. 



431. Intensity and Amplitude. Intensity or 

 loudness of sound depends upon the amplitude of 

 vibration. The greater the amplitude, the louder the 

 sound. 



(a.) If the middle of a tightly-stretched cord or wire, as a guitar 

 string, b3 drawn aside from its position of rest and then set free, it 

 will vibrate to and fro across its place of rest, striking the air and 

 sending sound waves to the ear. If the middle of the string be 

 drawn aside to a greater distance and then set free, the swing to 

 and fro will be increased, harder blows will be struck upon the air. 



