These two notes, C and G, are a fifth
of an octave apart; if we had two forks, of which one went twice
as fast as the other, making four waves while the other made two,
then that note would be an octave higher.
So we see that all the sounds we hear, - the warning noises which
keep us from harm, the beautiful musical notes with all the tunes
and harmonies that delight us, even the power of hearing the
voices of those we love, and learning from one another that which
each can tell, - all these depend upon the invisible waves of
air, even as the pleasures of light depend on the waves of ether.
It is by these sound-waves that nature speaks to us, and in all
her movements there is a reason why her boice is sharp or tender,
loud or gentle, awful or loving. Take for instance the brook we
spoke of at the beginning of the lecture. Why does it sing so
sweetly, while the wide deep river makes no noise? Because the
little brook eddies and purls round the stones, hitting them as
it passes; sometimes the water falls down a large stone, and
strikes against the water below; or sometimes it grates the
little pebbles together as they lie in its bed.
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