other words, the terms, "vibration" and "sound-wave," are synonymous.

If two strings, tuned to give forth the same number of vibrations per
second, are struck at the same time, the tone produced will appear to
come from a single source; one sweet, continuous, smooth, musical
tone. The reason is this: The condensations sent forth from each of
the two strings occur exactly together; the rarefactions, which, of
course, alternate with the condensations, are also simultaneous. It
necessarily follows, therefore, that the condensations from each of
the two strings travel with the same velocity. Now, while this
condition prevails, it is evident that the two strings assist each
other, making the condensations more condensed, and, consequently, the
rarefactions more rarefied, the result of which is, the two allied
forces combine to strengthen the tone.

In opposition to the above, if two strings, tuned to produce the same
tone, could be so struck that the condensation of one would occur at
the same instant with the rarefaction of the other, it is readily seen
that the two forces would oppose, or counteract each other, which, if
equal, would result in absolute silence.[G]

[G] When the bushing of the center-pin of the hammer butt becomes
badly worn or the hammer-flange becomes loose, or the condition of
the hammer or flange becomes so impaired that the hammer has too
much play, it may so strike the strings as to tend to produce the
phenomenon described in the above paragraph. When in such a
condition, one side of the hammer may strike in advance of the other
just enough to throw the vibrations in opposition. Once you may get
a strong tone, and again you strike with the same force and hear but
a faint, almost inaudible sound. For this reason, as well as that of