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Meteorology   


in Scythia a bitter spring. The water from this makes the whole of the

river into which it flows bitter. These differences are explained by a

knowledge of the particular mixtures that determine different savours.

But these have been explained in another treatise.

We have now given an account of waters and the sea, why they

persist, how they change, what their nature is, and have explained

most of their natural operations and affections.



4



Let us proceed to the theory of winds. Its basis is a distinction we

have already made. We recognize two kinds of evaporation, one moist,

the other dry. The former is called vapour: for the other there is

no general name but we must call it a sort of smoke, applying to the

whole of it a word that is proper to one of its forms. The moist

cannot exist without the dry nor the dry without the moist: whenever

we speak of either we mean that it predominates. Now when the sun in

its circular course approaches, it draws up by its heat the moist

evaporation: when it recedes the cold makes the vapour that had been

raised condense back into water which falls and is distributed through

the earth. (This explains why there is more rain in winter and more by

night than by day: though the fact is not recognized because rain by

night is more apt to escape observation than by day.) But there is a

great quantity of fire and heat in the earth, and the sun not only

draws up the moisture that lies on the surface of it, but warms and

dries the earth itself. Consequently, since there are two kinds of

evaporation, as we have said, one like vapour, the other like smoke,

both of them are necessarily generated. That in which moisture

predominates is the source of rain, as we explained before, while

the dry evaporation is the source and substance of all winds. That

things must necessarily take this course is clear from the resulting

phenomena themselves, for the evaporation that is to produce them must

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