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210
The Fairy-Land of Science
·
Arabella B. Buckley
122
Sometimes,
in
the
mountains,
walls
of
rock
rise
at
some
distance
one
behind
another,
and
then
each
one
will
send
back
its
echo
a
little
later
than
the
rock
before
it,
so
that
the
"Ha"
which
you
give
will
come
back
as
a
peal
of
laughter.
There
is
an
echo
in
Woodstock
Park
which
repeats
the
word
twenty
times.
Again
sometimes,
as
in
the
Alps,
the
sound-waves
coming
back
rebound
from
mountain
to
mountain
and
are
driven
backwards
and
forwards,
becoming
fainter
and
fainter
till
they
die
away;
these
echoes
are
very
beautiful.
If
you
are
now
able
to
picture
to
yourselves
one
set
of
waves
going
to
the
wall,
and
another
set
returning
and
crossing
them,
you
will
be
ready
to
understand
something
of
that
very
difficult
question,
How
is
it
that
we
can
hear
many
different
sounds
at
one
time
and
tell
them
apart?
Have
you
ever
watched
the
sea
when
its
surface
is
much
ruffled,
and
noticed
how,
besides
the
big
waves
of
the
tide,
there
are
numberless
smaller
ripples
made
by
the
wind
blowing
the
surface
of
the
water,
or
the
oars
of
a
boat
dipping
in
it,
or
even
rain-
drops
falling?
If
you
have
done
this
you
will
have
seen
that
all
these
waves
and
ripples
cross
each
other,
and
you
can
follow
any
one
ripple
with
you
eye
as
it
goes
on
its
way
undisturbed
by
the
rest.
Or
you
may
make
beautiful
crossing
and
recrossing
ripples
on
a
pond
by
throwing
in
two
stones
at
a
little
distance
from
each
other,
and
here
too
you
can
follow
any
one
wave
on
to
the
edge
of
the
pond.
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