THE VOICES OF NA TURE. 
145 
Fig. 36. 
all kinds of sound-waves must be surging about in the 
disturbed air ? 
This glass jar will answer our question roughly. If 
I strike my tuning-fork and hold it over the jar, you 
cannot hear it, because the sound is feeble, but if I 
fill the jar gently with water, when the water rises 
to a certain point you will hear a loud clear note, 
because the waves of 
air in the jar are ex- 
actly 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 par- 
ticular 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 suggest 
it to you 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 
musical here than when it is blowing across the plain ? 
