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210
The Fairy-Land of Science
·
Arabella B. Buckley
131
Again,
on
a
windy
night
have
you
not
heard
the
wind
sounding
a
wild,
sad
note
down
a
valley?
Why
do
you
think
it
sounds
so
much
louder
and
more
musical
here
than
when
it
is
blowing
across
the
plain?
Because
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
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.
Sometimes
the
roll
is
made
even
longer
by
the
echo,
as
the
sound-waves
are
reflected
to
and
fro
by
the
clouds
on
their
road;
and
in
the
mountains
we
know
how
the
peals
echo
and
re-echo
till
they
die
away.
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