mingles with the air in contact with the earth. The
vapour-charged air, being heated by the warm earth, expands, becomes
lighter, and rises. It expands also, as it rises, because the pressure
of the air above it becomes less and less with the height it attains.
But an expanding body always becomes colder as the result of its
expansion. Thus the vapour-laden air is chilled by its expansion. It is
also chilled by coming in contact with the colder, higher air. The
consequence is that the invisible vapour which it contains is chilled,
and forms into tiny water-drops, like the steam from a kettle or the
funnel of the locomotive. And so, as the air rises and becomes colder,
the vapour gathers into visible masses, which we call clouds.

This ascending moist air might become chilled, too, by meeting with a
current of cold, dry air, and then clouds would be formed; and should
this chilling process continue in either case until the water-drops
become heavier than the surrounding air, they would fall to the earth as
raindrops. Rain is, therefore, but a further stage in the condensation
of aqueous vapour caused by the chilling of the air.

Mountains also assist in the formation of clouds. When a wind laden with
moisture strikes against a mountain, it is tilted and flows up its side.
The air expands as it rises, the vapour is chilled and becomes visible
in the form of clouds, and if sufficiently chilled, it comes down to the
earth in the form of rain, hail, or snow.

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