Look up at the clouds as you go home, and think that
the water of which they are made has all been drawn
up invisibly through the air. Not, however, necessarily
here in London, for we have already seen that air
travels as wind all over the world, rushing in to
fill spaces made by rising air wherever they occur,
and so these clouds may be made of vapour collected
in the Mediterranean, or in the Gulf of Mexico off
the coast of America, or even, if the wind is from
the north, of chilly particles gathered from the surface
of Greenland ice and snow, and brought here by the
moving currents of air. Only, of one thing we
may be sure, that they come from the water of our earth.
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.
There are other ways in which the air may be chilled,
and rain made to fall, as, for example, when a wind
laden with moisture strikes against the cold tops
of mountains. Thus the Khasia Hills in India
which face the Bay of Bengal, chill the air which
crosses them on its way from the Indian Ocean.
The wet winds are driven up the sides of the hills,
the air expands, and the vapour is chilled, and forming
into drops, falls in torrents of rain. Sir J.
Hooker tells us that as much as 500 inches of rain
fell in these hills in nine months. That is to
