NATURE OF 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 (Mecnan. Patios. § 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 mstances 
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and cease as soon as it is withdrawn: this is the case, for 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 fiute 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 ;} 
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 1-7th or 1-8th 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, 
t 
| a 
4 
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