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Meteorology   


necessarily differ; and the sun and the warmth in the earth not only

can but must produce these evaporations.

Since the two evaporations are specifically distinct, wind and

rain obviously differ and their substance is not the same, as those

say who maintain that one and the same air when in motion is wind, but

when it condenses again is water. Air, as we have explained in an

earlier book, is made up of these as constituents. Vapour is moist

and cold (for its fluidity is due to its moistness, and because it

derives from water it is naturally cold, like water that has not

been warmed): whereas the smoky evaporation is hot and dry. Hence each

contributes a part, and air is moist and hot. It is absurd that this

air that surrounds us should become wind when in motion, whatever be

the source of its motion on the contrary the case of winds is like

that of rivers. We do not call water that flows anyhow a river, even

if there is a great quantity of it, but only if the flow comes from

a spring. So too with the winds; a great quantity of air might be

moved by the fall of some large object without flowing from any source

or spring.

The facts bear out our theory. It is because the evaporation takes

place uninterruptedly but differs in degree and quantity that clouds

and winds appear in their natural proportion according to the

season; and it is because there is now a great excess of the vaporous,

now of the dry and smoky exhalation, that some years are rainy and

wet, others windy and dry. Sometimes there is much drought or rain,

and it prevails over a great and continuous stretch of country. At

other times it is local; the surrounding country often getting

seasonable or even excessive rains while there is drought in a certain

part; or, contrariwise, all the surrounding country gets little or

even no rain while a certain part gets rain in abundance. The reason

for all this is that while the same affection is generally apt to

prevail over a considerable district because adjacent places (unless

there is something special to differentiate them) stand in the same

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