Geoscience Reference
In-Depth Information
Fig. 14.2 Sketch of the rainfall percolation theory of the Presocratic philosophers in ancient Greece on
the origin of rivers. The concept appears to have evolved from rough but seminal ideas by
Anaximander, followed by more complete formulations by Xenophanes and Anaxagoras.
caused by the streams which flow into it carrying different salty admixtures picked up along
the way. Clearly, the views of Xenophanes are a further development of Anaximander's.
Anaxagoras of Clazomenae (
c
. 500-428 BCE), who came some 70 years after Xeno-
phanes, appears to have been even more explicit on the matter. It is again from Hippolytus
(Diels, 1879, p. 562, 8, 4-5) that Anaxagoras is known to have said
. . . that the sea began to exist from the moist parts on the earth, that it originated this way as the waters
in it were being evaporated or settled down, and also from the downflowing rivers; that the rivers take
their substance from the rains and out of the waters that are in the earth; for this is hollow and that it
has water in the caves.
But the most solid proof that Anaximander, Xenophanes, Anaxagoras and perhaps other
Presocratics developed the notion, that the origin of streams and rivers can be accounted
for by rain (see Figure 14.2), is found in its attempted refutation by Aristotle in his
Meteorologica
, some one to two centuries later. Evidently, at the time of Aristotle, the
rainfall percolation theory was well enough established, that he considered it necessary to
mount a head-on attack against it. Aristotle (1952, I 349 b,2) summarized the theory as
follows.
Some people hold similar views about the origin of rivers. They suppose that the water drawn up by
the sun when it falls again as rain is collected beneath the earth into a great hollow from which the
rivers flow, either all from the same one or each from a different one: no additional water is formed
in the process, and the rivers are supplied by the water collected during the winter in these reservoirs.
This explains why rivers always run higher in winter than in summer, and why some are perennial,
some are not. When the hollow is large and the amount of water collected therefore great enough to
last out and not be exhausted before the return of the winter rains, then rivers are perennial and flow
continuously: when the reservoirs are smaller, then, because the supply of water is small, rivers dry up
before the rainy weather returns to replenish the empty container.
The statement, that “no additional water is formed in the process,” is a clear indication that
the rainfall percolation theory had eventually led to the concepts of a water cycle and of
water mass conservation. From the space devoted to it in Aristotle's
Meteorologica
, the
rainfall percolation theory was undoubtedly the more widely accepted at the time.
Both Anaxagoras and Aristotle refer to caves and hollows as the main underground
storage spaces of water. This should not be surprising. About 65% of the terrain of Greece
is limestone; this is easily eroded, resulting in a karst landscape (Higgins and Higgins,
