THK EAR. 495 



moving together, but it is a molecular motion throughout 

 its substance. Touched with an object, the heat vibrations 

 do not cease, and a small piece of metal placed in contact 

 with it is not violently thrown off. In other words, sound 

 is a vibratory motion of the mass of an object, heat and 

 light of its component molecules. 



THE PRODUCTION OF SOUND. 



From the vibratory nature of sound it follows that 

 sounds may be produced in an endless variety of ways, the 

 requirement in each case being simply that some kind of a 

 body be set in rapid motion and that this motion be trans- 

 mitted to a suitable medium. A familiar object for the pro- 

 duction of sound is the tuning-fork, the vibrations of which 

 are very apparent when sounding; in the case of the organ 

 the metal tongue is set in vibration by the air thrown against 

 it; in the piano it is the strings made to vibrate by the 

 stroke of the key against them, while in wind-instruments 

 the air itself within is set into definite oscillations. 



THE RANGE OF THE NUMBER OF VIBRATIONS IN THE PRODUC- 

 TION OF SOUND. 



It is not every vibration that produces a sound. It is not 

 until the vibrations reach a certain frequency that they be- 

 come perceptible to the ear. The minimum number of vi- 

 brations to be so perceptible varies a little for different ears, 

 but is in the neighborhood of sixteen per second. A string 

 vibrating fewer times than sixteen per second produces no 

 perceptible sound, although its vibrations may be distinctly 

 visible to the eye. Vibrations above this lower limit are 

 audible. There is, however, an upper limit beyond which 

 the ear is not able to go. This upper limit also varies for 

 different ears, but seems to be in the neighborhood of 60,- 

 000 vibrations to the second. These limits between which 

 the human ear is able to appreciate sounds comprise a range 

 of about thirteen octaves. A high-grade piano has usually 

 no more than seven octaves, or seven and one- third. The 



