146 
THE FAIRY-LAND OF SCIENCE. 
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 
FIG. 36. 
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 C, 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. 37) an imaginary picture of these 
two sets of waves. You see that the G fork makes 
three waves, while the C iork makes only two. Why 
is this? Because the prong of the G fork moves three 
times backward and forward while the prong of the 
