A DROP OF WATER. 93 
many things how far this vapour is carried. Some of 
it, chilled by cold blasts, or by striking on cold moun- 
tain tops, as it travels northwards, will fall in rain 
in Europe and Asia, while that which travels south- 
wards 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 moun 
tains in cooler lands, such as the Alps of Switzerland ; 
or is carried to the poles and to such countries as Green- 
land 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 
