146 THE FAIRY-LAND OF SCIENCE. 
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 
by the air in the shell pulsating to and fro. And how 
do 'you think it is set going ? By the throbbing of the 
veins in your own ear, which causes the air in the shell 
to vibrate. 
Another grand voice of nature is the thunder 
People often have a vague idea that thunder is pro- 
duced by the clouds knocking together, which is very 
absurd, if you remember that clouds are but water- 
dust. The most probable explanation of thunder is 
much more beautiful than this. You will remember 
from Lecture III. that heat forces the air-atoms apart. 
Now, when a flash of lightning crosses the sky it 
suddenly expands the air all round it as it passes, so 
that globe after globe of sound-waves is formed at 
every point across which the lightning travels. New 
light, you remember, travels so wonderfully rapidly 
(192,000 miles in a second) that a flash of lightning is 
seen by us and is over in a second, even when it is two 
or three miles long. But sound comes slowly, taking 
five seconds to travel a mile, and so all the sound- 
waves at each point of the two or three miles fall on 
our ear one after the other, and make the rolling 
thunder. Sometimes the roll is made even longer 
by the echo, as the sound-waves are reflected to and 
