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 say, if you could measure
off all the ground over which the rain fell, and spread the whole
nine months' rain over it, it would make a lake 500 inches, or
more than 40 feet deep! You will not be surprised that the
country on the other side of these hills gets hardly any rain,
for all the water has been taken out of the air before
it comes there. Again for example in England, the wind comes to
Cumberland and Westmorland over the Atlantic, full of vapour, and
as it strikes against the Pennine Hills it shakes off its watery
load; so that the lake district is the most rainy in England,