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
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