A DROP OF WATER. 8 1 
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 fol- 
low it as it struggles upward 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 as all 
work requires an expenditure of heat, the air becomes 
colder, and 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, or even lower. 
When they rest upon its surface we call them fog and 
mist. 
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 where we live, 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 
