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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
t