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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

antarctic region.

There are two inhabitable sections of the earth: one near our upper,

or nothern pole, the other near the other or southern pole; and

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