The isotherm which touches the southern limits of France passes midway
between the source and mouth of the river. In the northern half, it has
the mean annual temperature of France, England, and Germany; in the
southern half, of the Mediterranean coasts.
From the gulf into which it empties, a river (that is, an ocean river, or
current) runs through the ocean to the western coasts of Europe; another
runs out along the northeastern coast of South America, and, still another
is in waiting at the western terminus of the Panama Canal to assist the
ships across the Pacific.
A fair regularity and reliability of rainfall have made the rich soil of
the valley tillable and productive without irrigation, except in the far
western stretches; and these blessings are likely to continue, as one
authority puts it, "so long as the earth continues to revolve toward the
east and the present relationship of ocean and continent continues."
Including Texas and Alabama (which lie between the same ranges of
mountains with this valley, though their rivers run into the gulf and not
into the Mississippi), this valley has perhaps one hundred and forty
thousand miles of railway, or about sixty per cent of the total mileage of
the country, or twenty-five per cent of the mileage of the entire globe.
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