MUSICAL SOUNDS. 195 



and- intensity, and must recur after exactly equal inter- 

 vals of time. 



What is meant ^20. If the souud impulscs be repeated at 

 pUchhTsound'? '^^U short intervals, the ear is unable to at- 

 tend to them individually, but hears them as 

 a continued sound, which is uniform, or has what is called 

 a tone or pitch, if the impulses be similar and at equal 

 intervals. 



. 421. The nature of musical sounds, and indeed of all sounds, 



nient illustrates may be illustrated by the following experiment: If wo take 



the nature of a ^ ^jjj^ elastic plate of metal, a few inches in lensrtb, firmly 



musical sound 7 '■ ' o i j 



fixed at one end, and free at the other, and cause it to 



vibrate, it will be found to emit a clear, musical sound, having a certain 



tone. 



If the plate be gradually lengthened, it yields tones, or notes, of different 



characters, until finally the vibrations become so slow that the eye can follow 



them without difficulty, and all sound ceases. 



422. When the impulses, or vibrations, are few in number 



When 18 a tone . • .• ., 4. ■ • , ^ , 1 ^i 



grave or sharp f in a given time, the tone is said to be grave; when they are 



many, the tone is said to be sharp. Musical sounds are spoken 

 of as notes, or as high and low. Of two notes, the higher isthat wliich arises 

 from more rapid, and the lower from slower vibrations. 



Beside this, sounds differ in their quality. The same musical note, pro- 

 duced with the same degree of loudness, and by the same number of vibra- 

 tions in the flute, the clarionet, the piano, and the human voice, is in each 

 instance peculiar and wholly different. "Why this is we are unable to say. 

 The French call this property, by which one sound is distinguished from an^ 

 other, the timbre. 



, ^, To produce any sound whatever it is necessary that a cer» 



Is there any . . 



limit to the tam number of vibrations should be made in a certain time. 



number of vi- jf ^j^g number produced in a second falls below a certain rate, 

 Dratioiis requi- '^ '■ ' 



Bite to produce no sound sensation will be made upon the ear. It is believed 

 that the ear can distinguish a sound caused by fifteen vibra- 

 tions in a second, and can also continue to hear though the number reaches 

 4 .000 per second. Trained and sensitive ears are said to be able to exceed 

 these limits. 



When are t^ro 423. Two musical uotcs are said to be in 

 S'u^son?"'"^ iinison when the vibrations which cause them 



are performed in equal times. 

 What is an 424. When one note makes twice the number 



octavo ? r, ., . , . . , 



ot Vibrations in a fi^iven time that another makes, 

 it is said to be its octave. The relation, or interval which 



