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Meteorology   


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.

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