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210
The Fairy-Land of Science
·
Arabella B. Buckley
121
Sometimes,
when
the
sound
comes
from
a
great
explosion,
these
reflected
waves
are
so
strong
that
they
are
able
to
break
glass.
In
the
explosion
of
gunpowder
in
St.
John's
Wood,
many
houses
in
the
back
streets
had
their
windows
broken;
for
the
sound-waves
bounded
off
at
angles
from
the
walls
and
struck
back
upon
them.
Now
suppose
the
wall
were
so
far
behind
you
that
the
reflected
sound-waves
only
hit
upon
your
ear
after
those
coming
straight
from
me
had
died
away;
then
you
would
hear
the
sound
twice,
"Ha"
from
me
and
"Ha"
from
the
wall,
and
here
you
have
an
echo,
"Ha,
ha."
In
order
for
this
to
happen
in
ordinary
air,
you
must
be
standing
at
least
56
feet
away
from
the
point
from
which
the
waves
are
reflected,
for
then
the
second
blow
will
come
one-tenth
of
a
second
after
the
first
one,
and
that
is
long
enough
for
you
to
feel
them
separately.*
Miss
C.
A.
Martineau
tells
a
story
of
a
dog
which
was
terribly
frightened
by
an
echo.
Thinking
another
dog
was
barking,
he
ran
forward
to
meet
him,
and
was
very
much
astonished,
when,
as
he
came
nearer
the
wall,
the
echo
ceased.
I
myself
once
knew
a
case
of
this
kind,
and
my
dog,
when
he
could
find
no
enemy,
ran
back
barking,
till
he
was
a
certain
distance
off,
and
then
the
echo
of
course
began
again.
He
grew
so
furious
at
last
that
we
had
great
difficulty
in
preventing
him
from
flying
at
a
strange
man
who
happened
to
be
passing
at
the
time.
(*Sound
travels
1120
feet
in
a
second,
in
air
of
ordinary
temperature,
and
therefore
112
feet
in
the
tenth
of
a
second.
Therefore
the
journey
of
56
feet
beyond
you
to
reach
the
wall
and
56
feet
to
return,
will
occupy
the
sound-wave
one-tenth
of
a
second
and
separate
the
two
sounds.)
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