THE VOICES OF NATURE. 141 
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 
Fig- 34- 
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 C, the third space in the 
treble ; I strike the other, and it sounds A, the first 
leger line, five notes above the C. I have drawn on this 
