that which travels southwards may fall in South America,
Australia, or New Zealand, or be carried over the sea to the South
Pole. Wherever it falls on the land as rain, and is not used by
plants, it will do one of two things; either it will run down in
streams and form brooks and rivers, and so at last find its way
back to the sea, or it will sink deep in the earth till it comes
upon some hard rock through which it cannot get, and then, being
hard pressed by the water coming on behind, it will rise up again
through cracks, and come to the surface as a spring. These
springs, again, feed rivers, sometimes above- ground, sometimes
for long distances under-ground; but one way or another at last
the whole drains back into the sea.

But if the vapour travels on till it reaches high mountains in
cooler lands, such as the Alps of Switzerland; or is carried to
the poles and to such countries as Greenland or the Antarctic
Continent, then it will come down as snow, forming immense snow-
fields. And here a curious change takes place in it. If you make
an ordinary snowball and work it firmly together, it becomes very
hard, and if you then press it forcibly into a mould you can turn
it into transparent ice. And in the same way the snow which falls
in Greenland and on the high mountains of Switzerland becomes
very firmly pressed together, as it slides down into the valleys.
It is like a crowd of people passing from a broad thoroughfare
into a narrow street. As the valley grows narrower and
narrower the great mass of snow in front cannot move down
quickly, while more and more is piled up by the snowfall behind,
and the crowd and crush grow denser and denser. In this way the
snow is pressed together till the air that was hidden in its
crystals, and which gave it its beautiful whiteness, is all