NATURE OP CONTINUOUS TONES. 411 



for the interval between them not to be distinguished. Thus, 

 if we cause a tight string to vibrate by pulling or striking it, 

 we occasion, not one vibration only, but a long succession of 

 vibrations (MEGHAN. PHILOS. 187) ; every one of which 

 gives a new impulse to the air, and produces a new impression 

 on the organ of hearing. These vibrations we can see, when 

 they are sufficiently extensive ; and we can always feel them, 

 by placing the finger on the string. In the same manner, 

 the vibrations of a bell or of a tuning-fork continue long after 

 the first blow ; and these, though we cannot see them, may 

 be readily felt with the finger. It is, in fact, in their power 

 of continuing to vibrate after they have been struck, that the 

 peculiarity of these resonant bodies consists. In other instances 

 in which continuous tones are produced, the vibrations are 

 kept-up by the continued application of the original cause, 

 and cease as soon as it is withdrawn : this is the case, for 

 instance, in the string of the violin when set in vibration by 

 the bow, and in the flute and organ-pipe when caused to 

 sound by the passage of air through them. 



523. In all these cases, then, the continuous tones are due 

 to a succession of impulses given by the sounding body to the 

 air ; and according to the rapidity with which the impulses 

 succeed one another, will be the pitch of the sound. It is 

 not difficult to ascertain by experiment the number of such 

 impulses required to produce particular tones. The lowest 

 note (C) given by any musical instrument (that which is 

 produced by an open organ-pipe of 32 feet long, or by a 

 stopped pipe of 16 feet) requires 16 impulses per second; 1 

 but continuous tones have been produced by impulses occur- 

 ring at the rate of only 7 or 8 per second; so that the 

 impression produced upon the ear by each must have lasted 

 at least l-7th or l-8th of a second. On the other hand, it 

 has been ascertained that the ear can appreciate tones pro- 

 duced by 24,000 impulses in a second; so that the limit 

 already adverted to ( 517) must be above this tone, the 

 pitch of which is about 4 octaves above the highest F of the 

 pianoforte. 



524. The strength or loudness of musical tones depends 



1 A backward as well as forward vibration must take place with 

 every impulse ; consequently the number of the vibrations is twice that 

 of the impulses. 



