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210
The Fairy-Land of Science
·
Arabella B. Buckley
57
And
now
we
are
almost
prepared
to
understand
how
we
can
weigh
the
invisible
air.
One
more
experiment
first.
I
have
here
what
is
called
a
U
tube,
because
it
is
shaped
like
a
large
U.
I
pour
some
water
in
it
till
it
is
about
half
full,
and
you
will
notice
that
the
water
stands
at
the
same
height
in
both
arms
of
the
tube,
because
the
air
presses
on
both
surfaces
alike.
Putting
my
thumb
on
one
end
I
tilt
the
tube
carefully,
so
as
to
make
the
water
run
up
to
the
end
of
one
arm,
and
then
turn
it
back
again.
But
the
water
does
not
now
return
to
its
even
position,
it
remains
up
in
the
arm
on
which
my
thumb
rests.
Why
is
this?
Because
my
thumb
keeps
back
the
air
from
pressing
at
that
end,
and
the
whole
weight
of
the
atmosphere
rests
on
the
water
at
the
other
end.
And
so
we
learn
that
not
only
has
the
atmosphere
real
weight,
but
we
can
see
the
effects
of
this
weight
by
making
it
balance
a
column
of
water
or
any
other
liquid.
In
the
case
of
the
wetted
leather
we
felt
the
weight
of
the
air,
here
we
see
its
effects.
Now
when
we
wish
to
see
the
weight
of
the
air
we
consult
a
barometer,
which
works
really
just
in
the
same
way
as
the
water
in
this
tube.
An
ordinary
upright
barometer
is
simply
a
straight
tube
of
glass
filled
with
mercury
or
quicksilver,
and
turned
upside-down
in
a
small
cup
of
mercury.
The
tube
is
a
little
more
than
30
inches
long,
and
though
it
is
quite
full
of
mercury
before
it
is
turned
up,
yet
directly
it
stands
in
the
cup
the
mercury
falls,
till
there
is
a
height
of
about
30
inches
between
the
surface
of
the
mercury
in
the
cup,
and
that
of
the
mercury
in
the
tube.
As
it
falls
it
leaves
an
empty
space
above
the
mercury
which
is
called
a
vacuum,
because
it
has
no
air
in
it.
Now,
the
mercury
is
under
the
same
conditions
as
the
water
was
in
the
U
tube,
there
is
no
pressure
upon
it
at
the
top
of
the
tube,
while
there
is
a
pressure
of
15
lbs.
upon
it
in
the
bowl,
and
therefore
it
remains
held
up
in
the
tube.
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