THE VOICES OF NA TURE. 143 
speaks to us, and in all her movements there is a 
reason why her voice is sharp or tender, loud or 
gentle, awful or loving. Take for instance the brook 
we spoke of at the beginning of the lecture. Why 
does it sing so sweetly, while the wide deep river 
makes no noise ? Because the little brook eddies and 
purls round the stones, hitting them as it passes ; 
sometimes the water falls down a large stone, and 
strikes against the water below ; or sometimes it 
grates the little pebbles together as they lie in its bed. 
Each of these blows makes a small globe of sound- 
waves, which spread and spread till they fall on your 
ear, and because they fall quickly and regularly, they 
make a low, musical note. We might almost fancy 
that the brook wished to show how joyfully it flows 
along, recalling Shelley's beautiful lines : 
" Sometimes it fell 
Among the moss with hollow harmony, 
Dark and profound ; now on the polished stones 
It danced ; like childhood laughing as it went." 
The broad deep river, on the contrary, makes none 
of these cascades and commotions. The only places 
against which it rubs are the banks and the bottom ; 
and here you can sometimes hear it grating the par- 
ticles of sand against each other if you listen very 
carefully. But there is another reason why falling 
water makes a sound, and often even a loud roaring 
noise in the cataract and in the breaking waves of the 
sea. You do not only hear the water dashing against 
the rocky ledges or on the beach, you also hear the 
bursting of innumerable little bladders of air which 
are contained in the water. As each of these bladders 
