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.