THE FAIRY-LAND OF SCIENCE. 
cause the waves of air in the jar are exactly the right 
length to answer to the note of the fork. If I now 
blow across the mouth of the jar you hear the same 
note, showing that a cavity of a particular length will 
only sound to the waves which fit it. Do you see now 
the reason why pan-pipes give different sounds, or 
even the hole at the end of a common key when you 
blow across it? Here 
is a subject you will 
find very interesting if 
you will read about it, 
for I can only just sug- 
gest it here. But now 
you will see that the 
canal of your ear also 
answers only to certain 
waves, and so the wind 
sings in your ear with 
a real if not a musical 
note. 
Again, on a windy night have you not heard the 
wind sounding a wild, sad note down a valley ? Why 
do you think it sounds so much louder and more mu- 
sical here than when it is blowing across the plain? 
Because the air in the valley will only answer to a 
certain set of waves, and, like the pan-pipe, gives a 
particular note as the wind blows across it, and these 
waves go up and down the valley in regular pulses, 
making a wild howl. You may hear the same in the 
chimney, or in the keyhole; all these are waves set up 
in the hole across which the wind blows. Even the 
music in the shell which you hold to your ear is made 
FIG. 38. 
