80 THE FAIRY-LAND OF SCIENCE. 
line under the clouds marks the place where the air 
becomes cold enough to turn this invisible vapour into 
visible drops of water. 
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 
