Provide two glass bottles, and tune them by pouring water into them,
so that each corresponds to the sound of a different tuning-fork. Then
apply both tuning-forks to the mouth of each bottle alternately, when
that sound only will be heard, in each case, which is reciprocated by
the unisonant bottle, or, in other words, by that bottle which contains
a column of air susceptible of vibrating in unison with the fork.