THE VOICES OF NA TURE. 
149 
on the ground; have you ever noticed how they seem 
to scream as they draw back down the beach? Tenny- 
son calls it, 
" The scream of the madden'd beach dragged down by the wave ; " 
and it is caused by the stones grating against each 
other as the waves drag them down. Dr. Tyndall 
tells us that it is possible to know the size of the stones 
by the kind of noise they make. If they are large, it 
is a confused noise; when smaller, a kind of scream; 
while a gravelly beach will produce a mere hiss. 
Who could be dull by the side of a brook, a water- 
fall, or the sea, while he can listen for sounds like these, 
and picture to himself how they are being made? You 
may discover a number of other causes of sound made 
by water, if you once pay attention to them. 
Nor is it only water that sings to us. Listen to 
the wind, how sweetly it sighs among the leaves. 
There we hear it, because it rubs the leaves together, 
and they produce the sound-waves. But walk against 
the wind some day and you can hear it whistling in 
your own ear, striking against the curved cup, and 
then setting up a succession of waves in the hearing 
canal of the ear itself. 
Why should it sound in one particular tone when 
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 can not 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, be- 
