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210
The Fairy-Land of Science
·
Arabella B. Buckley
58
Week
9
But
why
will
it
not
remain
more
than
30
inches
high
in
the
tube?
You
must
remember
it
is
only
kept
up
in
the
tube
at
all
by
the
air
which
presses
on
the
mercury
in
the
cup.
And
that
column
of
mercury
now
balances
the
pressure
of
the
air
outside,
and
presses
down
on
the
mercury
in
the
cup
at
its
mouth
just
as
much
as
the
air
does
on
the
rest.
So
this
cup
and
tube
act
exactly
like
a
pair
of
scales.
The
air
outside
is
the
thing
to
be
weighed
at
one
end
as
it
presses
on
the
mercury,
the
column
answers
to
the
leaden
weight
at
the
other
end
which
tells
you
how
heavy
the
air
is.
Now
if
the
bore
of
this
tube
is
made
an
inch
square,
then
the
30
inches
of
mercury
in
it
weigh
exactly
15
lbs,
and
so
we
know
that
the
weight
of
the
air
is
15
lbs.
upon
every
square
inch,
but
if
the
bore
of
the
tube
is
only
half
a
square
inch,
and
therefore
the
30
inches
of
mercury
only
weigh
7
1/2
lbs.
instead
of
15
lbs.,
the
pressure
of
the
atmosphere
will
also
be
halved,
because
it
will
only
act
upon
half
a
square
inch
of
surface,
and
for
this
reason
it
will
make
no
difference
to
the
height
of
the
mercury
whether
the
tube
be
broad
or
narrow.
But
now
suppose
the
atmosphere
grows
lighter,
as
it
does
when
it
has
much
damp
in
it.
The
barometer
will
show
this
at
once,
because
there
will
be
less
weight
on
the
mercury
in
the
cup,
therefore
it
will
not
keep
the
mercury
pushed
so
high
up
in
the
tube.
In
other
words,
the
mercury
in
the
tube
will
fall.
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