Thus, by tracing a river backwards, from its end to its real beginning, we come at length to the sun; for it is the sun that produces aqueous vapour, from which, as we have seen, clouds are formed, and it is from clouds that water falls to the earth to become the sources of rivers.
There are, however, rivers which have sources somewhat different from those just mentioned. They do not begin by driblets on a hillside, nor can they be traced to a spring. Go, for example, to the mouth of the river Rhone, and trace it backwards. You come at length to the Lake of Geneva, from which the river rushes, and which you might be disposed to regard as the source of the Rhone. But go to the head of the lake, and you find that the Rhone there enters it; that the lake is, in fact, an expansion of the river. Follow this upwards; you find it joined by smaller rivers from the mountains right and left. Pass these, and push your journey higher still. You come at length to a huge mass of ice—the end of a glacier—which fills the Rhone valley, and from the bottom of the glacier the river rushes. In the glacier of the Rhone you thus find the source of the river Rhone.
But whence come the glaciers? Wherever lofty mountains, like the Alps, rise into the high parts of the atmosphere where the temperature is below the freezing-point, the vapour condensed from the air falls upon them, not as rain, but as snow. In such high mountainous regions, the heat of the summer melts the snow from the lower hills, but the higher parts remain covered, for the heat cannot melt all the snow which falls there in a year. When a considerable depth of snow has accumulated, the pressure upon the lower layers squeezes them into a firm mass, and after a time the snow begins to slide down the slope of the mountain. It passes downward from one slope to another, joined continually by other sliding masses from neighbouring slopes, until they all unite into one long tongue, which creeps slowly down some valley to a point where it melts. This tongue from the snow-fields is called a glacier.
Without solar fire, therefore, we could have no atmospheric vapour, without vapour no clouds, without clouds no snow, and without snow no glaciers. Curious then as the conclusion may be, the cold ice of the Alps has its origin in this heat of the sun.
Tyndall: "The Forms of Water."
(Adapted)
For what are men better than sheep or goats
That nourish a blind life within the brain,
If, knowing God, they lift not hands of prayer
Both for themselves and those who call them friend?
Tennyson