would make a desert region of all that portion of Egypt now
so productive.

The great rivers of China, the Yang-tse-Kiang and the Hwangho, are also
tropical rivers and have an annual flood. Sometimes the rise is as much
as fifty-six feet. These annual floods are also caused by the monsoon
winds that carry moisture from the ocean, which is condensed and
precipitated in the mountains of central Asia. The conditions are
substantially the same as those which prevail at the sources of the Nile
in Africa.

Rivers are produced from all sorts of causes, some of them flowing only
during the rainy season, while others are fed by melting snow from the
higher mountains, and as the snow is rarely melted away entirely during
the summer, in the high mountains, there is a continual flow from this
source. The snow forms a system of storage, so that the water is held
back and is gradually given up as it melts. If this were not true
mountainous regions would be subjected to disastrous floods. If the
precipitation were always in the form of rain it would immediately run
off instead of being distributed over a whole season. The Platte is an
instance of a river largely fed by the melting snows--of the Rocky
Mountains.

In the region of glaciers in the mountains of Alaska and Switzerland
rivers are fed by the melting ice. These rivers are usually of a milky
color occasioned by the pulverization of rock caused by the grinding of
the great glaciers as they flow down the gulches in the mountain side.
In some regions these glacial rivers have a diurnal variation. This is
caused by the fact that the glacier is so situated that it freezes at
night, which checks the flow, and thaws in the daytime, which increases
it.

Rivers are to the globe what the veins are to the animal organization.
They pick up the surplus moisture not needed in the growth of vegetation
and for the sustenance of animal life, and carry it on, together with
the debris that it gathers in its course, to the great reservoirs, the
seas and oceans, where it is redistilled and purified by the action of
the sun's rays. From here it is carried back in the form of invisible
moisture and again precipitated in the purified state, to help carry on
the great operations of growth--animal and vegetable. The vaporized
moisture that is carried back by the winds and redistributed corresponds
to the blood, after it has been purified and is carried back through the
arteries to the extremities