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Meteorology   


particles and then the wind is beaten about and so catches fire.

A phenomenon in these islands affords further evidence of the fact

that winds move below the surface of the earth. When a south wind is

going to blow there is a premonitory indication: a sound is heard in

the places from which the eruptions issue. This is because the sea

is being pushed on from a distance and its advance thrusts back into

the earth the wind that was issuing from it. The reason why there is a

noise and no earthquake is that the underground spaces are so

extensive in proportion to the quantity of the air that is being

driven on that the wind slips away into the void beyond.

Again, our theory is supported by the facts that the sun appears

hazy and is darkened in the absence of clouds, and that there is

sometimes calm and sharp frost before earthquakes at sunrise. The

sun is necessarily obscured and darkened when the evaporation which

dissolves and rarefies the air begins to withdraw into the earth.

The calm, too, and the cold towards sunrise and dawn follow from the

theory. The calm we have already explained. There must as a rule be

calm because the wind flows back into the earth: again, it must be

most marked before the more violent earthquakes, for when the wind

is not part outside earth, part inside, but moves in a single body,

its strength must be greater. The cold comes because the evaporation

which is naturally and essentially hot enters the earth. (Wind is

not recognized to be hot, because it sets the air in motion, and

that is full of a quantity of cold vapour. It is the same with the

breath we blow from our mouth: close by it is warm, as it is when we

breathe out through the mouth, but there is so little of it that it is

scarcely noticed, whereas at a distance it is cold for the same reason

as wind.) Well, when this evaporation disappears into the earth the

vaporous exhalation concentrates and causes cold in any place in which

this disappearance occurs.

A sign which sometimes precedes earthquakes can be explained in

the same way. Either by day or a little after sunset, in fine weather,

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