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too. Wells, for instance, give off more vapour in a north than in a

south wind. Only the north winds quench the heat before any

considerable quantity of vapour has gathered, while in a south wind

the evaporation is allowed to accumulate.

Water, once formed, does not freeze on the surface of the earth,

in the way that it does in the region of the clouds.



11



From the latter there fall three bodies condensed by cold, namely

rain, snow, hail. Two of these correspond to the phenomena on the

lower level and are due to the same causes, differing from them only

in degree and quantity.

Snow and hoar-frost are one and the same thing, and so are rain

and dew: only there is a great deal of the former and little of the

latter. For rain is due to the cooling of a great amount of vapour,

for the region from which and the time during which the vapour is

collected are considerable. But of dew there is little: for the vapour

collects for it in a single day and from a small area, as its quick

formation and scanty quantity show.

The relation of hoar-frost and snow is the same: when cloud

freezes there is snow, when vapour freezes there is hoar-frost.

Hence snow is a sign of a cold season or country. For a great deal

of heat is still present and unless the cold were overpowering it

the cloud would not freeze. For there still survives in it a great

deal of the heat which caused the moisture to rise as vapour from

the earth.

Hail on the other hand is found in the upper region, but the

corresponding phenomenon in the vaporous region near the earth is

lacking. For, as we said, to snow in the upper region corresponds

hoar-frost in the lower, and to rain in the upper region, dew in the

lower. But there is nothing here to correspond to hail in the upper

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