142 
THE FAIRY-LAND OF SCIENCE. 
diagram (Fig. 35), an imaginary picture of these two 
sets of waves. You see that the A fork makes three 
waves, while the C fork makes only two. Why is 
this ? Because the prong of the A fork moves three 
times backwards and forwards while the prong of the 
C fork only moves twice ; therefore the A fork does 
not crowd so many atoms together before it draws 
back, and the waves are shorter. These two notes, C 
and A, are a fifth apart ; if we had two forks, of which 
Fig- 35- 
one went twice as fast as the other, making four 
waves while the other made two, then the notes of 
these forks 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 
