that if three seconds elapse between the
time when we see the flash and hear the report of the gun, it must be
distant 1142 yards.

From experiments that have been made at different times, by various
philosophers, we may collect the following results. First, That the
mean velocity of sound is a mile in about 4 3/4 seconds, or 1142 feet
in a second of time. Secondly, That all sounds, whether they be weak
or strong, have the same velocity. Thirdly, That sound moves over
equal spaces in equal times, from the beginning to the end.

The tone of a musical string, or a bell, appears continuous. This
depends upon a law of sensation, formerly mentioned, namely, that
impressions made upon any of the organs of sense do not immediately
vanish, but remain some time; and we hear sound continuous from these
vibrations, for the same reason that we hear it continuous when we
draw a stick quickly along a rail, or a quill along the teeth of a
comb; the vibrations succeed each other so quickly that we hear the
succeeding before the effect of the preceding is worn off; though it
must be evident that the impression produced by each pulse or wave of
the air is perfectly distinct and insulated.

The act of combining sounds in such a manner as to be agreeable to
the ear, is called music. This art is usually divided into melody and
harmony. An agreeable succession of sounds is called melody; but when
two or more sounds are produced together, and afford an agreeable
sensation, the effect is called harmony. When two sounds are produced
together, and afford pleasure to the sense of hearing, the effect is
called a concord; but when the sensation produced is harsh or
disagreeable, it is called a discord. These different effects seem to
depend upon the coincidence of the vibrations of the two strings, and
consequently on the coincidence of the pulses which they excite in
the air. When the strings are equally stretched, and of the same
length and thickness, their vibrations will always coincide, and they
produce a sound so similar to each other, that it is called unison,
which is the most perfect concord. When one string is only half the
length of the other, the vibrations coincide at every second
vibration of the shorter string: this produces a compound sound,
which is more agreeable to the ear than any other, except the unison;
this note, when compared with the tone produced by the longer string,
is called the octave to it, because the interval be