I have here a simple apparatus which I have had made
to show you that rapid and regular shocks produce
a natural musical note. This wheel (Fig. 34)
is milled at the edge like a shilling, and when I
turn it rapidly so that it strikes against the edge
of the card fixed behind it, the notches strike in
rapid succession, and produce a musical sound.
We can also prove by this experiment that the quicker
the blows are, the higher the note will be. I
pull the string gently at first, and then quicker and
quicker, and you will notice that the note grows sharper
and sharper, till the movement begins to slacken,
when the note goes down again. This is because
the more rapidly the air is hit, the shorter are the
waves it makes, and short waves give a high note.
Let us examine this with two tuning-forks. I
strike one, and it sounds D, the third space in the
treble; I strike the other, and it sounds G, the first
leger line, five notes above the C. I have drawn
on this diagram (Fig. 35), an imaginary picture of
these two sets of waves. You see that the G fork
makes three waves, while the C fork makes only two.
Why is this? Because the prong of the G fork
moves three times backwards and forwards while the
prong of the C fork only moves twice; therefore the
G fork does not crowd so many atoms together before
it draws back, and the waves are shorter. 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. Each of these blows makes a small
globe of sound-waves, which spread and spread till
they fall on your ear, and because they fall quickly
and regularly, they make a low, musical note.
We might almost fancy that the brook wished to show
how joyfully it flows along, recalling Shelley’s
beautiful lines:-
“Sometimes it fell
Among the moss with hollow harmony,
Dark and profound; now on the polished
stones
It danced; like childhood laughing as
it went.”
