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.
The sun both checks the formation of winds and stimulates it. When
the evaporation is small in amount and faint the sun wastes it and
dissipates by its greater heat the lesser heat contained in the
evaporation. It also dries up the earth, the source of the
evaporation, before the latter has appeared in bulk: just as, when you
throw a little fuel into a great fire, it is often burnt up before
giving off any smoke. In these ways the sun checks winds and
prevents them from rising at all: it checks them by wasting the
evaporation, and prevents their rising by drying up the earth quickly.
Hence calm is very apt to prevail about the rising of Orion and
lasts until the coming of the Etesiae and their 'forerunners'.
Calm is due to two causes. Either cold quenches the evaporation, for
instance a sharp frost: or excessive heat wastes it. In the
intermediate periods, too, the causes are generally either that the
evaporation has not had time to develop or that it has passed away and
there is none as yet to replace it.
Both the setting and the rising of Orion are considered to be
treacherous and stormy, because they place at a change of season
(namely of summer or winter; and because the size of the constellation
makes its rise last over many days) and a state of change is always
indefinite and therefore liable to disturbance.
The Etesiae blow after the summer solstice and the rising of the
dog-star: not at the time when the sun is closest nor when it is
distant; and they blow by day and cease at night. The reason is that
when the sun is near it dries up the earth before evaporation has