taken place, but when it has receded a little its heat and the
evaporation are present in the right proportion; so the ice melts
and the earth, dried by its own heat and that of the sun, smokes and
vapours. They abate at night because the cold pf the nights checks the
melting of the ice. What is frozen gives off no evaporation, nor
does that which contains no dryness at all: it is only where something
dry contains moisture that it gives off evaporation under the
influence of heat.
The question is sometimes asked: why do the north winds which we
call the Etesiae blow continuously after the summer solstice, when
there are no corresponding south winds after the winter solstice?
The facts are reasonable enough: for the so-called 'white south winds'
do blow at the corresponding season, though they are not equally
continuous and so escape observation and give rise to this inquiry.
The reason for this is that the north wind I from the arctic regions
which are full of water and snow. The sun thaws them and so the
Etesiae blow: after rather than at the summer solstice. (For the
greatest heat is developed not when the sun is nearest to the north,
but when its heat has been felt for a considerable period and it has
not yet receded far. The 'bird winds' blow in the same way after the
winter solstice. They, too, are weak Etesiae, but they blow less and
later than the Etesiae. They begin to blow only on the seventieth
day because the sun is distant and therefore weaker. They do not
blow so continuously because only things on the surface of the earth
and offering little resistance evaporate then, the thoroughly frozen
parts requiring greater heat to melt them. So they blow intermittently
till the true Etesiae come on again at the summer solstice: for from
that time onwards the wind tends to blow continuously.) But the
south wind blows from the tropic of Cancer and not from the
There are two inhabitable sections of the earth: one near our upper,
or nothern pole, the other near the other or southern pole; and