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210
The Fairy-Land of Science
·
Arabella B. Buckley
51
But
the
chief
reason
why
the
air
is
thicker
or
denser
nearer
the
earth,
is
because
the
upper
layers
press
it
down.
If
you
have
a
heap
of
papers
lying
one
on
the
top
of
the
other,
you
know
that
those
at
the
bottom
of
the
heap
will
be
more
closely
pressed
together
than
those
above,
and
just
the
same
is
the
case
with
the
atoms
of
the
air.
Only
there
is
this
difference,
if
the
papers
have
lain
for
some
time,
when
you
take
the
top
ones
off,
the
under
ones
remain
close
together.
But
it
is
not
so
with
the
air,
because
air
is
elastic,
and
the
atoms
are
always
trying
to
fly
apart,
so
that
directly
you
take
away
the
pressure
they
spring
up
again
as
far
as
they
can.
Week
8
I
have
here
an
ordinary
pop-gun.
If
I
push
the
cork
in
very
tight,
and
then
force
the
piston
slowly
inwards,
I
can
compress
the
air
a
good
deal.
Now
I
am
forcing
the
atoms
nearer
and
nearer
together,
but
at
last
they
rebel
so
strongly
against
being
more
crowded
that
the
cork
cannot
resist
their
pressure.
Out
it
flies,
and
the
atoms
spread
themselves
out
comfortably
again
in
the
air
all
around
them.
Now,
just
as
I
pressed
the
air
together
in
the
pop-gun,
so
the
atmosphere
high
up
above
the
earth
presses
on
the
air
below
and
keeps
the
atoms
closely
packed
together.
And
in
this
case
the
atoms
cannot
force
back
the
air
above
them
as
they
did
the
cork
in
the
pop-gun;
they
are
obliged
to
submit
to
be
pressed
together.
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