7 8 THE FAIRY-LAND OF SCIENCE. 
but even in our own country many feet of water are 
drawn up in the summer-time. 
What, then, becomes of all this water ? Let us follow 
it as it struggles upwards to the sky. We see it in 
our imagination first carrying layer after layer of air 
up with it from the sea till it rises far above our heads 
and above the highest mountains. But now, call to 
mind what happens to the air as it recedes from the 
earth. Do you not remember that the air-atoms are 
always trying to fly apart, and are only kept pressed 
together by the weight of air above them ? Well, as 
this water-laden air rises up, its particles, no longer 
so much pressed together, begin to separate, and in 
so doing they use up part of the heat which they 
carried up from the earth, and thus the air becomes 
colder. Then you know at once what must happen 
to the invisible vapour, it will form into tiny water- 
drops, like the steam from the kettle. And so, as the 
air rises and becomes colder, the vapour gathers into 
visible masses, and we can see it hanging in the sky, 
and call it clouds. When these clouds are highest they 
are about ten miles from the earth, but when they are 
made of heavy drops and hang low down, they some- 
times come within a mile of the ground. 
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 
