A DROP OF WATER. 
97 
ever 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 mountains in 
Alaska ; 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 to- 
gether, 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 Alaska be- 
comes 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 be- 
hind, 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 
