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 produced 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. Now
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 half 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.
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