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210
The Fairy-Land of Science
·
Arabella B. Buckley
71
Sometimes,
if
the
air
is
warm,
these
water-particles
may
travel
a
long
way
without
ever
forming
into
clouds;
and
on
a
hot,
cloudless
day
the
air
is
often
very
full
of
invisible
vapour.
Then,
if
a
cold
wind
comes
sweeping
along,
high
up
in
the
sky,
and
chills
this
vapour,
it
forms
into
great
bodies
of
water-dust
clouds,
and
the
sky
is
overcast.
At
other
times
clouds
hang
lazily
in
a
bright
sky,
and
these
show
us
that
just
where
they
are
(as
in
Fig.
19)
the
air
is
cold
and
turns
the
invisible
vapour
rising
from
the
ground
into
visible
water-dust,
so
that
exactly
in
those
spaces
we
see
it
as
clouds.
Such
clouds
form
often
on
warm,
still
summer's
day,
and
they
are
shaped
like
masses
of
wool,
ending
in
a
straight
line
below.
They
are
not
merely
hanging
in
the
sky,
they
are
really
resting
upon
a
tall
column
of
invisible
vapour
which
stretches
right
up
from
the
earth;
and
that
straight
line
under
the
clouds
marks
the
place
where
the
air
becomes
cold
enough
to
turn
this
invisible
vapour
into
visible
drops
of
water.
Week
11
And
now,
suppose
that
while
these
or
any
other
kind
of
clouds
are
overhead,
there
comes
along
either
a
very
cold
wind,
or
a
wind
full
of
vapour.
As
it
passes
through
the
clouds,
it
makes
them
very
full
of
water,
for,
if
it
chills
them,
it
makes
the
water-
dust
draw
more
closely
together;
or,
if
it
brings
a
new
load
of
water-dust,
the
air
is
fuller
than
it
can
hold.
In
either
case
a
number
of
water-particles
are
set
free,
and
our
fairy
force
"cohesion"
seizes
upon
them
at
once
and
forms
them
into
large
water-drops.
Then
they
are
much
heavier
than
the
air,
and
so
they
can
float
no
longer,
but
down
they
come
to
the
earth
in
a
shower
of
rain.
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