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.”