I 4 8 THE FAIRY-LAND OF SCIENCE. 
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 peb- 
bles 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 beauti- 
ful 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 
is dashed on the ground, it explodes and sends sound- 
waves to your ear. Listen to the sea some day when 
the waves are high and stormy, and you cannot fail 
to be struck by the irregular bursts of sound. 
The waves, however, do not only roar as they dash 
