Table of Contents
Previous section |
Next section
3
We must now explain why the
sea is salt, and ask whether it
eternally exists as identically the same body, or whether it did not
exist at all once and some day will exist no longer, but will dry up
as some people think.
Every one admits this, that if the whole world originated the sea
did too; for they make them come into being at the same time. It
follows that if the universe is eternal the same must be true of the
sea. Any one who thinks like
Democritus that the sea is diminishing
and will disappear in the end reminds us of Aesop's tales. His story
was that
Charybdis had twice sucked in the sea: the first time she
made the mountains visible; the second time the islands; and when
she sucks it in for the last time she will dry it up entirely. Such
a tale is appropriate enough to Aesop in a rage with the ferryman, but
not to serious inquirers. Whatever made the sea remain at first,
whether it was its weight, as some even of those who hold these
views say (for it is easy to see the cause here), or some other
reason-clearly the same thing must make it persist for ever. They must
either deny that the water raised by the sun will return at all, or,
if it does, they must admit that the sea persists for ever or as
long as this process goes on, and again, that for the same period of
time that sweet water must have been carried up beforehand. So the sea
will never dry up: for before that can happen the water that has
gone up beforehand will return to it: for if you say that this happens
once you must admit its recurrence. If you stop the sun's course there
is no drying agency. If you let it go on it will draw up the sweet
water as we have said whenever it approaches, and let it descend again
when it recedes. This notion about the sea is derived from the fact
that many places are found to be drier now than they once were. Why
this is so we have explained. The phenomenon is due to temporary
excess of
rain and not to any process of becoming in which the
universe or its parts are involved. Some day the opposite will take
place and after that the
will grow dry once again. We must
recognize that this process always goes on thus in a cycle, for that
is more satisfactory than to suppose a change in the whole world in
order to explain these facts. But we have dwelt longer on this point
than it deserves.
To return to the saltness of the sea: those who create the sea
once for all, or indeed generate it at all, cannot account for its
saltness. It makes no difference whether the sea is the residue of all
the moisture that is about the earth and has been drawn up by the sun,
or whether all the flavour existing in the whole mass of sweet water
is due to the admixture of a certain kind of earth. Since the total
volume of the sea is the same once the water that evaporated has
returned, it follows that it must either have been salt at first
too, or, if not at first, then not now either. If it was salt from the
very beginning, then we want to know why that was so; and why, if salt
water was drawn up then, that is not the case now.
Again, if it is maintained that an admixture of earth makes the
sea salt (for they say that earth has many flavours and is washed down
by the rivers and so makes the sea salt by its admixture), it is
strange that rivers should not be salt too. How can the admixture of
this earth have such a striking effect in a great quantity of water
and not in each river singly? For the sea, differing in nothing from
rivers but in being salt, is evidently simply the totality of river
water, and the rivers are the vehicle in which that earth is carried
to their common destination.
It is equally absurd to suppose that anything has been explained
by calling the sea 'the sweat of the earth', like Empedicles.
Metaphors are poetical and so that expression of his may satisfy the
requirements of a poem, but as a scientific theory it is
unsatisfactory. Even in the case of the body it is a question how
the sweet liquid drunk becomes salt sweat whether it is merely by
the departure of some element in it which is sweetest, or by the
admixture of something, as when water is strained through ashes.
Actually the saltness seems to be due to the same cause as in the case
of the residual liquid that gathers in the bladder. That, too, becomes
bitter and salt though the liquid we drink and that contained in our
food is sweet. If then the bitterness is due in these cases (as with
the water strained through lye) to the presence of a certain sort of
stuff that is carried along by the urine (as indeed we actually find a
salt deposit settling in chamber-pots) and is secreted from the
flesh in sweat (as if the departing moisture were washing the stuff
out of the body), then no doubt the admixture of something earthy with
the water is what makes the sea salt.
Now in the body stuff of this kind,
viz. the sediment of food, is
due to failure to digest: but how there came to be any such thing in
the earth requires explanation. Besides, how can the drying and
warming of the earth cause the secretion such a great quantity of
water; especially as that must be a mere fragment of what is left in
the earth? Again, waiving the question of quantity, why does not the
earth sweat now when it happens to be in process of drying? If it
did so then, it ought to do so now. But it does not: on the
contrary, when it is dry it graws moist, but when it is moist it
does not secrete anything at all. How then was it possible for the
earth at the beginning when it was moist to sweat as it grew dry?
Indeed, the theory that maintains that most of the moisture departed
and was drawn up by the sun and that what was left over is the sea
is more reasonable; but for the earth to sweat when it is moist is
impossible.
Since all the attempts to account for the saltness of the sea seem
unsuccessful let us explain it by the help of the principle we have
used already.
Since we recognize two kinds of
evaporation, one moist, the other
dry, it is clear that the latter must be recognized as the source of
phenomena like those we are concerned with.
But there is a question which we must discuss first. Does the sea
always remain numerically one and consisting of the same parts, or
is it, too, one in form and volume while its parts are in continual
change, like
air and sweet water and
fire? All of these are in a
constant state of change, but the form and the quantity of each of
them are fixed, just as they are in the case of a flowing river or a
burning flame. The answer is clear, and there is no doubt that the
same account holds good of all these things alike. They differ in that
some of them change more rapidly or more slowly than others; and
they all are involved in a process of perishing and becoming which yet
affects them all in a regular course.
This being so we must go on to try to explain why the sea is salt.
There are many facts which make it clear that this taste is due to the
admixture of something. First, in
animal bodies what is least
digested, the residue of liquid food, is salt and bitter, as we said
before. All animal excreta are undigested, but especially that which
gathers in the bladder (its extreme lightness proves this; for
everything that is digested is condensed), and also sweat; in these
then is excreted (along with other matter) an identical substance to
which this flavour is due. The case of things burnt is analogous. What
heat fails to assimilate becomes the excrementary residue in animal
bodies, and, in things burnt, ashes. That is why some people say
that it was burnt earth that made the sea salt. To say that it was
burnt earth is absurd; but to say that it was something like burnt
earth is true. We must suppose that just as in the cases we have
described, so in the world as a whole, everything that grows and is
naturally generated always leaves an undigested residue, like that
of things burnt, consisting of this sort of earth. All the earthy
stuff in the dry exhalation is of this
nature, and it is the dry
exhalation which accounts for its great quantity. Now since, as we
have said, the moist and the dry evaporations are mixed, some quantity
of this stuff must always be included in the
clouds and the water that
are formed by condensation, and must redescend to the earth in rain.
This process must always go on with such regularity as the sublunary
world admits of. and it is the answer to the question how the sea
comes to be salt.
It also explains why rain that comes from the south, and the first
rains of
autumn, are brackish. The south is the warmest of
winds and
it blows from dry and hot regions. Hence it carries little moist
vapour and that is why it is hot. (It makes no difference even if this
is not its true character and it is originally a cold
wind, for it
becomes warm on its way by incorporating with itself a great
quantity of dry evaporation from the places it passes over.) The north
wind, on the other hand, comb ing from moist regions, is full of
vapour and therefore cold. It is dry in our part of the world
because it drives the clouds away before it, but in the south it is
rainy; just as the south is a dry wind in Libya. So the south wind
charges the rain that falls with a great quantity of this stuff.
Autumn rain is brackish because the heaviest water must fall first; so
that that which contains the greatest quantity of this kind of earth
descends quickest.
This, too, is why the sea is warm. Everything that has been
exposed to fire contains heat potentially, as we see in the case of
lye and ashes and the dry and liquid excreta of animals. Indeed
those animals which are hottest in the belly have the hottest excreta.
The action of this cause is continually making the sea more salt,
but some part of its saltness is always being drawn up with the
sweet water. This is less than the sweet water in the same ratio in
which the salt and brackish element in rain is less than the sweet,
and so the saltness of the sea remains constant on the whole. Salt
water when it turns into vapour becomes sweet, and the vapour does not
form salt water when it condenses again. This I know by experiment.
The same thing is true in every case of the kind: wine and all
fluids that evaporate and condense back into a liquid state become
water. They all are water modified by a certain admixture, the
nature of which determines their flavour. But this subject must be
considered on another more suitable occasion.
For the present let us say this. The sea is there and some of it
is continually being drawn up and becoming sweet; this returns from
above with the rain. But it is now different from what it was when
it was drawn up, and its weight makes it sink below the sweet water.
This process prevents the sea, as it does rivers, from drying up
except from local causes (this must happen to sea and rivers alike).
On the other hand the parts neither of the earth nor of the sea remain
constant but only their whole bulk. For the same thing is true of
the earth as of the sea: some of it is carried up and some comes
down with the rain, and both that which remains on the surface and
that which comes down again change their situations.
There is more evidence to prove that saltness is due to the
admixture of some substance, besides that which we have adduced.
Make a vessel of wax and put it in the sea, fastening its mouth in
such a way as to prevent any water getting in. Then the water that
percolates through the wax sides of the vessel is sweet, the earthy
stuff, the admixture of which makes the water salt, being separated
off as it were by a filter. It is this stuff which make salt water
heavy (it weighs more than fresh water) and thick. The difference in
consistency is such that ships with the same cargo very nearly sink in
a river when they are quite fit to navigate in the sea. This
circumstance has before now caused loss to shippers freighting their
ships in a river. That the thicker consistency is due to an
admixture of something is proved by the fact that if you make strong
brine by the admixture of salt, eggs, even when they are full, float
in it. It almost becomes like mud; such a quantity of earthy matter is
there in the sea. The same thing is done in salting fish.
Again if, as is fabled, there is a lake in
Palestine, such that if
you bind a man or beast and throw it in it floats and does not sink,
this would bear out what we have said. They say that this lake is so
bitter and salt that no fish live in it and that if you soak clothes
in it and shake them it cleans them. The following facts all of them
support our theory that it is some earthy stuff in the water which
makes it salt. In Chaonia there is a spring of brackish water that
flows into a neighbouring river which is sweet but contains no fish.
The local story is that when
Heracles came from Erytheia driving the
oxen and gave the inhabitants the choice, they chose salt in
preference to fish. They get the salt from the spring. They boil off
some of the water and let the rest stand; when it has cooled and the
heat and moisture have evaporated together it gives them salt, not
in lumps but loose and light like
snow. It is weaker than ordinary
salt and added freely gives a sweet taste, and it is not as white as
salt generally is. Another instance of this is found in Umbria.
There is a place there where reeds and rushes grow. They burn some
of these, put the ashes into water and boil it off. When a little
water is left and has cooled it gives a quantity of salt.
Most salt rivers and springs must once have been hot. Then the
original fire in them was extinguished but the earth through which
they percolate preserves the character of lye or ashes. Springs and
rivers with all kinds of flavours are found in many places. These
flavours must in every case be due to the fire that is or was in them,
for if you expose earth to different degrees of heat it assumes
various kinds and shades of flavour. It becomes full of alum and lye
and other things of the kind, and the fresh water percolates through
these and changes its character. Sometimes it becomes acid as in
Sicania, a part of Sicily. There they get a salt and acid water
which they use as vinegar to season some of their dishes. In the
neighbourhood of Lyncus, too, there is a spring of acid water, and
in Scythia a bitter spring. The water from this makes the whole of the
river into which it flows bitter. These differences are explained by a
knowledge of the particular mixtures that determine different savours.
But these have been explained in another treatise.
We have now given an account of waters and the sea, why they
persist, how they change, what their nature is, and have explained
most of their natural operations and affections.
4
Let us proceed to the theory of winds. Its basis is a distinction we
have already made. We recognize two kinds of evaporation, one moist,
the other dry. The former is called vapour: for the other there is
no general name but we must call it a sort of
smoke, applying to the
whole of it a word that is proper to one of its forms. The moist
cannot exist without the dry nor the dry without the moist: whenever
we speak of either we mean that it predominates. Now when the sun in
its circular course approaches, it draws up by its heat the moist
evaporation: when it recedes the cold makes the vapour that had been
raised condense back into water which falls and is distributed through
the earth. (This explains why there is more rain in winter and more by
night than by day: though the fact is not recognized because rain by
night is more apt to escape observation than by day.) But there is a
great quantity of fire and heat in the earth, and the sun not only
draws up the moisture that lies on the surface of it, but warms and
dries the earth itself. Consequently, since there are two kinds of
evaporation, as we have said, one like vapour, the other like smoke,
both of them are necessarily generated. That in which moisture
predominates is the source of rain, as we explained before, while
the dry evaporation is the source and substance of all winds. That
things must necessarily take this course is clear from the resulting
phenomena themselves, for the evaporation that is to produce them must
necessarily differ; and the sun and the warmth in the earth not only
can but must produce these evaporations.
Since the two evaporations are specifically distinct, wind and
rain obviously differ and their substance is not the same, as those
say who maintain that one and the same air when in
motion is wind, but
when it condenses again is water. Air, as we have explained in an
earlier book, is made up of these as constituents. Vapour is moist
and cold (for its fluidity is due to its moistness, and because it
derives from water it is naturally cold, like water that has not
been warmed): whereas the smoky evaporation is hot and dry. Hence each
contributes a part, and air is moist and hot. It is absurd that this
air that surrounds us should become wind when in motion, whatever be
the source of its motion on the contrary the case of winds is like
that of rivers. We do not call water that flows anyhow a river, even
if there is a great quantity of it, but only if the flow comes from
a spring. So too with the winds; a great quantity of air might be
moved by the fall of some large object without flowing from any source
or spring.
The facts bear out our theory. It is because the evaporation takes
place uninterruptedly but differs in degree and quantity that clouds
and winds appear in their natural proportion according to the
season; and it is because there is now a great excess of the vaporous,
now of the dry and smoky exhalation, that some years are rainy and
wet, others windy and dry. Sometimes there is much
drought or rain,
and it prevails over a great and continuous stretch of country. At
other times it is local; the surrounding country often getting
seasonable or even excessive rains while there is drought in a certain
part; or, contrariwise, all the surrounding country gets little or
even no rain while a certain part gets rain in abundance. The reason
for all this is that while the same affection is generally apt to
prevail over a considerable district because adjacent places (unless
there is something special to differentiate them) stand in the same
relation to the sun, yet on occasion the dry evaporation will
prevail in one part and the moist in another, or conversely. Again the
reason for this latter is that each evaporation goes over to that of
the neighbouring district: for instance, the dry evaporation
circulates in its own place while the moist migrates to the next
district or is even driven by winds to some distant place: or else the
moist evaporation remains and the dry moves away. Just as in the
case of the body when the stomach is dry the lower belly is often in
the contrary state, and when it is dry the stomach is moist and
cold, so it often happens that the evaporations reciprocally take
one another's place and interchange.
Further, after rain wind generally rises in those places where the
rain fell, and when rain has come on the wind ceases. These are
necessary effects of the principles we have explained. After rain
the earth is being dried by its own heat and that from above and gives
off the evaporation which we saw to be the material cause of. wind.
Again, suppose this secretion is present and wind prevails; the heat
is continually being thrown off, rising to the upper region, and so
the wind ceases; then the fall in temperature makes vapour form and
condense into water. Water also forms and cools the dry evaporation
when the clouds are driven together and the cold concentrated in them.
These are the causes that make wind cease on the advent of rain, and
rain fall on the cessation of wind.
The cause of the predominance of winds from the north and from the
south is the same. (Most winds, as a matter of fact, are north winds
or south winds.) These are the only regions which the sun does not
visit: it approaches them and recedes from them, but its course is
always over the-west and the east. Hence clouds collect on either
side, and when the sun approaches it provokes the moist evaporation,
and when it recedes to the opposite side there are storms and rain. So
summer and winter are due to the sun's motion to and from the
solstices, and water ascends and falls again for the same reason.
Now since most rain falls in those regions towards which and from
which the sun turns and these are the north and the south, and since
most evaporation must take place where there is the greatest rainfall,
just as green wood gives most smoke, and since this evaporation is
wind, it is natural that the most and most important winds should come
from these quarters. (The winds from the north are called Boreae,
those from the south
Noti.)
The course of winds is oblique: for though the evaporation rises
straight up from the earth, they blow round it because all the
surrounding air follows the motion of the
heavens. Hence the
question might be asked whether winds originate from above or from
below. The motion comes from above: before we feel the wind blowing
the air betrays its presence if there are clouds or a mist, for
their motion shows that the wind has begun to blow before it has
actually reached us; and this implies that the source of winds is
above. But since wind is defined as 'a quantity of dry evaporation
from the earth moving round the earth', it is clear that while the
origin of the motion is from above, the matter and the generation of
wind come from below. The oblique movement of the rising evaporation
is caused from above: for the motion of the heavens determines the
processes that are at a distance from the earth, and the motion from
below is vertical and every cause is more active where it is nearest
to the effect; but in its generation and origin wind plainly derives
from the earth.
The facts bear out the view that winds are formed by the gradual
union of many evaporations just as rivers derive their sources from
the water that oozes from the earth. Every wind is weakest in the spot
from which it blows; as they proceed and leave their source at a
distance they gather
strength. Thus the winter in the north is
windless and calm: that is, in the north itself; but, the breeze
that blows from there so gently as to escape observation becomes a
great wind as it passes on.
We have explained the nature and origin of wind, the occurrence of
drought and rains, the reason why rain stops wind and wind rises after
rain, the prevalence of north and south winds and also why wind
moves in the way it does.
Table of Contents
Previous section |
Next section