inguish the difference. If however the
reflecting body should be placed at such a distance, that the
reflected sound should enter the ear some considerable or sensible
time after the original, an echo or distinct sound would be heard.

It appears from experiment that the ear of an experienced musician
can only distinguish such sounds as follow each other at the rate of
nine or ten in a second, or any lower rate; and therefore that we may
have a distinct perception of the direct and reflected sound, there
should at least be an interval of 1/9 of a second; but in this time
sound passes over one hundred and twenty seven feet, and
consequently, unless the space between the sounding body and the
reflecting surface, added to that between the reflecting surface and
the ear, be greater than one hundred and twenty seven feet, no echo
will be heard, because the reflected sound will enter the ear so soon
after the original, that the difference cannot be distinguished; and
therefore it will only serve to augment the original sound.

From what has been said, it is evident, in order that a person may
hear the echo of his own voice, that he should stand at least sixty
three, or sixty four feet from the reflecting obstacle, so that the
sound may have time to move over at least one hundred and twenty
seven feet before it come to his ear, otherwise he could not
distinguish it from the original sound.

But though the first reflected pulses may produce no echo, both on
account of their being too few in number, and too rapid in their
return to the ear; yet it must be evident that the reflecting surface
may be so formed, that the pulses, which come to the ear after two or
more reflections, may, after having passed over one hundred and
twenty seven feet or more, arrive at the ear in sufficient numbers to
produce an echo, though the distance of the reflecting surface from
the ear be less than the limit of echoes. This is instanced by the
echoes that we hear in several caves or caverns.

The sense of hearing is more apt to be vitiated or diseased than any
of the other senses, which indeed is not surprising, when we consider
that its organ is complex, consisting of many minute parts, which are
apt to be deranged.

It sometimes becomes too acute, and this may arise either from too
great an irritability of the whole nervous system, which often occurs
in hysteria, also in phrenitis, and some fevers; or from an inflamed
state of the ear itself