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All these streams, then, and many others, add their waters to swell
the flood of the Ister, which thus increased becomes the mightiest of
rivers; for undoubtedly if we compare the stream of the Nile with the
single stream of the Ister, we must give the preference to the Nile,
of which no tributary river, nor even rivulet, augments the volume.
The Ister remains at the same level both summer and winter - owing to
the following reasons, as I believe. During the winter it runs at
its natural height, or a very little higher, because in those
countries there is scarcely any rain in winter, but constant snow.
When summer comes, this snow, which is of great depth, begins to
melt, and flows into the Ister, which is swelled at that season, not
only by this cause but also by the rains, which are heavy and frequent
at that part of the year. Thus the various streams which go to form
the Ister are higher in summer than in winter, and just so much higher
as the sun's power and attraction are greater; so that these two
causes counteract each other, and the effect is to produce a balance,
whereby the Ister remains always at the same level.
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