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210
The Fairy-Land of Science
·
Arabella B. Buckley
118
You
may
see
an
excellent
example
of
this
in
a
luggage
train
in
a
railway
station,
when
the
trucks
are
left
to
bump
each
other
till
they
stop.
You
will
see
three
or
four
trucks
knock
together,
then
they
will
pass
the
shock
on
to
the
four
in
front,
while
they
themselves
bound
back
and
separate
as
far
as
their
chains
will
let
them:
the
next
four
trucks
will
do
the
same,
and
so
a
kind
of
wave
of
crowded
trucks
passes
on
to
the
end
of
the
train,
and
they
bump
to
and
fro
till
the
whole
comes
to
a
standstill.
Try
to
imagine
a
movement
like
this
going
on
in
the
line
of
air-
atoms,
the
drum
of
your
ear
being
at
the
end.
Those
which
are
crowded
together
at
that
end
will
hit
on
the
drum
of
your
ear
and
drive
the
membrane
which
covers
it
inwards;
then
instantly
the
wave
will
change,
these
atoms
will
bound
back,
and
the
membrane
will
recover
itself
again,
but
only
to
receive
a
second
blow
as
the
atoms
are
driven
forwards
again,
and
so
the
membrane
will
be
driven
in
and
out
till
the
air
has
settled
down.
This
you
see
is
quite
different
to
the
waves
of
light
which
moves
in
crests
and
hollows.
Indeed,
it
is
not
what
we
usually
understand
by
a
wave
at
all,
but
a
set
of
crowdings
and
partings
of
atoms
of
air
which
follow
each
other
rapidly
across
the
air.
A
crowding
of
atoms
is
called
a
condensation,
and
a
parting
is
called
a
rarefaction,
and
when
we
speak
of
the
length
of
a
wave
of
sound,
we
mean
the
distance
between
two
condensations,
or
between
two
rarefactions.
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