and first cause is the circle in which the sun moves. For the sun as
it approaches or recedes, obviously causes dissipation and
condensation and so gives rise to generation and destruction. Now
the earth remains but the moisture surrounding it is made to evaporate
by the sun's rays and the other heat from above, and rises. But when
the heat which was raising it leaves it, in part dispersing to the
higher region, in part quenched through rising so far into the upper
air, then the vapour cools because its heat is gone and because the
place is cold, and condenses again and turns from air into water.
And after the water has formed it falls down again to the earth.
The exhalation of water is vapour: air condensing into water is
cloud. Mist is what is left over when a cloud condenses into water,
and is therefore rather a sign of fine weather than of rain; for
mist might be called a barren cloud. So we get a circular process that
follows the course of the sun. For according as the sun moves to
this side or that, the moisture in this process rises or falls. We
must think of it as a river flowing up and down in a circle and made
up partly of air, partly of water. When the sun is near, the stream of
vapour flows upwards; when it recedes, the stream of water flows down:
and the order of sequence, at all events, in this process always
remains the same. So if 'Oceanus' had some secret meaning in early
writers, perhaps they may have meant this river that flows in a circle
about the earth.
So the moisture is always raised by the heat and descends to the
earth again when it gets cold. These processes and, in some cases,
their varieties are distinguished by special names. When the water
falls in small drops it is called a drizzle; when the drops are larger
it is rain.
Some of the vapour that is formed by day does not rise high