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13
Let us explain the nature of winds, and all windy vapours, also of
rivers and of the sea. But here, too, we must first discuss the
difficulties involved: for, as in other matters, so in this no
theory has been handed down to us that the most ordinary man could not
have thought of.
Some say that what is called air, when it is in motion and flows, is
wind, and that this same air when it condenses again becomes
cloud and
water, implying that the nature of
wind and
water is the same. So they
define wind as a motion of the air. Hence some, wishing to say a
clever thing, assert that all the winds are one wind, because the
air that moves is in fact all of it one and the same; they maintain
that the winds appear to differ owing to the region from which the air
may happen to flow on each occasion, but really do not differ at
all. This is just like thinking that all rivers are one and the same
river, and the ordinary unscientific view is better than a
scientific theory like this. If all rivers flow from one source, and
the same is true in the case of the winds, there might be some truth
in this theory; but if it is no more true in the one case than in
the other, this ingenious idea is plainly false. What requires
investigation is this: the nature of wind and how it originates, its
efficient cause and whence they derive their source; whether one ought
to think of the wind as issuing from a sort of vessel and flowing
until the vessel is empty, as if let out of a wineskin, or, as
painters represent the winds, as drawing their source from themselves.
We find analogous views about the origin of rivers. It is thought
that the water is raised by the sun and descends in rain and gathers
below the earth and so flows from a great
reservoir, all the rivers
from one, or each from a different one. No water at all is
generated, but the volume of the rivers consists of the water that
is gathered into such reservoirs in
winter. Hence rivers are always
fuller in winter than in
summer, and some are
perennial, others not.
Rivers are perennial where the reservoir is large and so enough
water has collected in it to last out and not be used up before the
winter rain returns. Where the reservoirs are smaller there is less
water in the rivers, and they are dried up and their vessel empty
before the fresh rain comes on.
But if any one will picture to himself a reservoir adequate to the
water that is continuously flowing day by day, and consider the amount
of the water, it is obvious that a receptacle that is to contain all
the water that flows in the year would be larger than the earth, or,
at any rate, not much smaller.
Though it is evident that many reservoirs of this kind do exist in
many parts of the earth, yet it is unreasonable for any one to
refuse to admit that air becomes water in the earth for the same
reason as it does above it. If the cold causes the vaporous air to
condense into water above the earth we must suppose the cold in the
earth to produce this same effect, and recognize that there not only
exists in it and flows out of it actually formed water, but that water
is continually forming in it too.
Again, even in the case of the water that is not being formed from
day to day but exists as such, we must not suppose as some do that
rivers have their source in definite
subterranean lakes. On the
contrary, just as above the earth small drops form and these join
others, till finally the water descends in a body as rain, so too we
must suppose that in the earth the water at first trickles together
little by little, and that the sources of the rivers drip, as it were,
out of the earth and then unite. This is proved by facts. When men
construct an aqueduct they collect the water in pipes and trenches, as
if the earth in the higher ground were sweating the water out.
Hence, too, the head-waters of rivers are found to flow from
mountains, and from the greatest mountains there flow the most
numerous and greatest rivers. Again, most springs are in the
neighbourhood of mountains and of high ground, whereas if we except
rivers, water rarely appears in the plains. For mountains and high
ground, suspended over the country like a saturated sponge, make the
water ooze out and trickle together in minute quantities but in many
places. They receive a great deal of water falling as rain (for it
makes no difference whether a spongy receptacle is concave and
turned up or convex and turned down: in either case it will contain
the same volume of matter) and, they also cool the vapour that rises
and condense it back into water.
Hence, as we said, we find that the greatest rivers flow from the
greatest mountains. This can be seen by looking at itineraries: what
is recorded in them consists either of things which the writer has
seen himself or of such as he has compiled after inquiry from those
who have seen them.
In
Asia we find that the most numerous and greatest rivers flow from
the mountain called
Parnassus, admittedly the greatest of all
mountains towards the south-east. When you have crossed it you see the
outer ocean, the further limit of which is unknown to the dwellers
in our world. Besides other rivers there flow from it the
Bactrus, the
Choaspes, the
Araxes: from the last a branch separates off and flows
into lake
Maeotis as the
Tanais. From it, too, flows the
Indus, the
volume of whose stream is greatest of all rivers. From the
Caucasus
flows the
Phasis, and very many other great rivers besides. Now the
Caucasus is the greatest of the mountains that lie to the northeast,
both as regards its extent and its height. A proof of its height is
the fact that it can be seen from the so-called 'deeps' and from the
entrance to the lake. Again, the sun shines on its peaks for a third
part of the night before sunrise and again after sunset. Its extent is
proved by the fact that thought contains many inhabitable regions
which are occupied by many nations and in which there are said to be
great lakes, yet they say that all these regions are visible up to the
last peak. From
Pyrene (this is a mountain towards the west in
Celtice) there flow the
Istrus and the
Tartessus. The latter flows
outside the pillars, while the Istrus flows through all Europe into
the Euxine. Most of the remaining rivers flow northwards from the
Hercynian mountains, which are the greatest in height and extent about
that region. In the extreme north, beyond furthest
Scythia, are the
mountains called
Rhipae. The stories about their size are altogether
too fabulous: however, they say that the most and (after the Istrus)
the greatest rivers flow from them. So, too, in
Libya there flow
from the
Aethiopian mountains the
Aegon and the
Nyses; and from the
so-called
Silver Mountain the two greatest of named rivers, the
river called
Chremetes that flows into the outer ocean, and the main
source of the
Nile. Of the rivers in the
Greek world, the
Achelous
flows from
Pindus, the
Inachus from the same mountain; the
Strymon,
the
Nestus, and the
Hebrus all three from
Scombrus; many rivers,
too, flow from
Rhodope.
All other rivers would be found to flow in the same way, but we have
mentioned these as examples. Even where rivers flow from marshes,
the marshes in almost every case are found to lie below mountains or
gradually rising ground.
It is clear then that we must not suppose rivers to originate from
definite reservoirs: for the whole earth, we might almost say, would
not be sufficient (any more than the region of the clouds would be) if
we were to suppose that they were fed by actually existing water
only and it were not the case that as some water passed out of
existence some more came into existence, but rivers always drew
their stream from an existing store. Secondly, the fact that rivers
rise at the foot of mountains proves that a place transmits the
water it contains by gradual percolation of many drops, little by
little, and that this is how the sources of rivers originate. However,
there is nothing impossible about the existence of such places
containing a quantity of water like lakes: only they cannot be big
enough to produce the supposed effect. To think that they are is
just as absurd as if one were to suppose that rivers drew all their
water from the sources we see (for most rivers do flow from
springs). So it is no more reasonable to suppose those lakes to
contain the whole volume of water than these springs.
That there exist such chasms and cavities in the earth we are taught
by the rivers that are swallowed up. They are found in many parts of
the earth: in the
Peloponnesus, for instance, there are many such
rivers in Arcadia. The reason is that
Arcadia is mountainous and there
are no channels from its valleys to the sea. So these places get
full of water, and this, having no outlet, under the pressure of the
water that is added above, finds a way out for itself underground.
In Greece this kind of thing happens on quite a small scale, but the
lake at the foot of the
Caucasus, which the inhabitants of these parts
call a sea, is considerable. Many great rivers fall into it and it has
no visible outlet but issues below the earth off the land of the
Coraxi about the so-called 'deeps of Pontus'. This is a place of
unfathomable depth in the sea: at any rate no one has yet been able to
find bottom there by sounding. At this spot, about three hundred
stadia from land, there comes up sweet water over a large area, not
all of it together but in three places. And in Liguria a river equal
in size to the
Rhodanus is swallowed up and appears again elsewhere:
the Rhodanus being a navigable river.
14
The same parts of the earth are not always moist or dry, but they
change according as rivers come into existence and dry up. And so
the relation of land to sea changes too and a place does not always
remain land or sea throughout all time, but where there was dry land
there comes to be sea, and where there is now sea, there one day comes
to be dry land. But we must suppose these changes to follow some order
and cycle. The principle and cause of these changes is that the
interior of the earth grows and decays, like the bodies of plants
and animals. Only in the case of these latter the process does not
go on by parts, but each of them necessarily grows or decays as a
whole, whereas it does go on by parts in the case of the earth. Here
the causes are cold and heat, which increase and diminish on account
of the sun and its course. It is owing to them that the parts of the
earth come to have a different character, that some parts remain moist
for a certain time, and then dry up and grow old, while other parts in
their turn are filled with life and moisture. Now when places become
drier the springs necessarily give out, and when this happens the
rivers first decrease in size and then finally become dry; and when
rivers change and disappear in one part and come into existence
correspondingly in another, the sea must needs be affected.
If the sea was once pushed out by rivers and encroached upon the
land anywhere, it necessarily leaves that place dry when it recedes;
again, if the dry land has encroached on the sea at all by a process
of silting set up by the rivers when at their full, the time must come
when this place will be flooded again.
But the whole vital process of the earth takes place so gradually
and in periods of time which are so immense compared with the length
of our life, that these changes are not observed, and before their
course can be recorded from beginning to end whole nations perish
and are destroyed. Of such destructions the most utter and sudden
are due to wars; but pestilence or famine cause them too. Famines,
again, are either sudden and severe or else gradual. In the latter
case the disappearance of a nation is not noticed because some leave
the country while others remain; and this goes on until the land is
unable to maintain any inhabitants at all. So a long period of time is
likely to elapse from the first departure to the last, and no one
remembers and the lapse of time destroys all record even before the
last inhabitants have disappeared. In the same way a nation must be
supposed to lose account of the time when it first settled in a land
that was changing from a marshy and watery state and becoming dry.
Here, too, the change is gradual and lasts a long time and men do
not remember who came first, or when, or what the land was like when
they came. This has been the case with Egypt. Here it is obvious
that the land is continually getting drier and that the whole
country is a deposit of the river Nile. But because the neighbouring
peoples settled in the land gradually as the marshes dried, the
lapse of time has hidden the beginning of the process. However, all
the mouths of the Nile, with the single exception of that at
Canopus, are obviously artificial and not natural. And Egypt was
nothing more than what is called
Thebes, as
Homer, too, shows,
modern though he is in relation to such changes. For Thebes is the
place that he mentions; which implies that Memphis did not yet
exist, or at any rate was not as important as it is now. That this
should be so is natural, since the lower land came to be inhabited
later than that which lay higher. For the parts that lie nearer to the
place where the river is depositing the silt are necessarily marshy
for a longer time since the water always lies most in the newly formed
land. But in time this land changes its character, and in its turn
enjoys a period of prosperity. For these places dry up and come to
be in good condition while the places that were formerly well-tempered
some day grow excessively dry and deteriorate. This happened to the
land of
Argos and
Mycenae in
Greece. In the time of the
Trojan wars
the
Argive land was marshy and could only support a small
population, whereas the land of Mycenae was in good condition (and for
this reason Mycenae was the superior). But now the opposite is the
case, for the reason we have mentioned: the land of Mycenae has become
completely dry and barren, while the Argive land that was formerly
barren owing to the water has now become fruitful. Now the same
process that has taken place in this small district must be supposed
to be going on over whole countries and on a large scale.
Men whose outlook is narrow suppose the cause of such events to be
change in the universe, in the sense of a coming to be of the world as
a whole. Hence they say that the sea being dried up and is growing
less, because this is observed to have happened in more places now
than formerly. But this is only partially true. It is true that many
places are now dry, that formerly were covered with water. But the
opposite is true too: for if they look they will find that there are
many places where the sea has invaded the land. But we must not
suppose that the cause of this is that the world is in process of
becoming. For it is absurd to make the universe to be in process
because of small and trifling changes, when the bulk and size of the
earth are surely as nothing in comparison with the whole world. Rather
we must take the cause of all these changes to be that, just as winter
occurs in the seasons of the year, so in determined periods there
comes a great winter of a great year and with it excess of rain. But
this excess does not always occur in the same place. The deluge in the
time of Deucalion, for instance, took place chiefly in the Greek world
and in it especially about ancient Hellas, the country about Dodona
and the Achelous, a river which has often changed its course. Here the
Selli dwelt and those who were formerly called Graeci and now
Hellenes. When, therefore, such an excess of rain occurs we must
suppose that it suffices for a long time. We have seen that some say
that the size of the subterranean cavities is what makes some rivers
perennial and others not, whereas we maintain that the size of the
mountains is the cause, and their density and coldness; for great,
dense, and cold mountains catch and keep and create most water:
whereas if the mountains that overhang the sources of rivers are small
or porous and stony and clayey, these rivers run dry earlier. We
must recognize the same kind of thing in this case too. Where such
abundance of rain falls in the great winter it tends to make the
moisture of those places almost everlasting. But as time goes on
places of the latter type dry up more, while those of the former,
moist type, do so less: until at last the beginning of the same
cycle returns.
Since there is necessarily some change in the whole world, but not
in the way of coming into existence or perishing (for the universe
is permanent), it must be, as we say, that the same places are not for
ever moist through the presence of sea and rivers, nor for ever dry.
And the facts prove this. The whole land of the Egyptians, whom we
take to be the most ancient of men, has evidently gradually come
into existence and been produced by the river. This is clear from an
observation of the country, and the facts about the
Red Sea suffice to
prove it too. One of their kings tried to make a canal to it (for it
would have been of no little advantage to them for the whole region to
have become navigable; Sesostris is said to have been the first of the
ancient kings to try), but he found that the sea was higher than the
land. So he first, and Darius afterwards, stopped making the canal,
lest the sea should mix with the river water and spoil it. So it is
clear that all this part was once unbroken sea. For the same reason
Libya-the country of
Ammon-is, strangely enough, lower and hollower
than the land to the seaward of it. For it is clear that a barrier
of silt was formed and after it lakes and dry land, but in course of
time the water that was left behind in the lakes dried up and is now
all gone. Again the silting up of the lake Maeotis by the rivers has
advanced so much that the limit to the size of the ships which can now
sail into it to trade is much lower than it was sixty years ago. Hence
it is easy to infer that it, too, like most lakes, was originally
produced by the rivers and that it must end by drying up entirely.
Again, this process of silting up causes a continuous current
through the Bosporus; and in this case we can directly observe the
nature of the process. Whenever the current from the Asiatic shore
threw up a sandbank, there first formed a small lake behind it.
Later it dried up and a second sandbank formed in front of the first
and a second lake. This process went on uniformly and without
interruption. Now when this has been repeated often enough, in the
course of time the strait must become like a river, and in the end the
river itself must dry up.
So it is clear, since there will be no end to time and the world
is eternal, that neither the Tanais nor the Nile has always been
flowing, but that the region whence they flow was once dry: for
their effect may be fulfilled, but time cannot. And this will be
equally true of all other rivers. But if rivers come into existence
and perish and the same parts of the earth were not always moist,
the
sea must needs change correspondingly. And if the sea is always
advancing in one place and receding in another it is clear that the
same parts of the whole earth are not always either sea or land, but
that all this changes in course of time.
So we have explained that the same parts of the earth are not always
land or sea and why that is so: and also why some rivers are
perennial
and others not.
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