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Meteorology   


from the earth after them. Islands near the continent really form part

of it: the intervening sea is not enough to make any difference; but

those in the open sea can only be shaken if the whole of the sea

that surrounds them is shaken too.

We have now explained earthquakes, their nature and cause, and the

most important of the circumstances attendant on their appearance.



9



Let us go on to explain lightning and thunder, and further

whirlwind, fire-wind, and thunderbolts: for the cause of them all is

the same.

As we have said, there are two kinds of exhalation, moist and dry,

and the atmosphere contains them both potentially. It, as we have said

before, condenses into cloud, and the density of the clouds is highest

at their upper limit. (For they must be denser and colder on the

side where the heat escapes to the upper region and leaves them.

This explains why hurricanes and thunderbolts and all analogous

phenomena move downwards in spite of the fact that everything hot

has a natural tendency upwards. Just as the pips that we squeeze

between our fingers are heavy but often jump upwards: so these

things are necessarily squeezed out away from the densest part of

the cloud.) Now the heat that escapes disperses to the up region.

But if any of the dry exhalation is caught in the process as the air

cools, it is squeezed out as the clouds contract, and collides in

its rapid course with the neighbouring clouds, and the sound of this

collision is what we call thunder. This collision is analogous, to

compare small with great, to the sound we hear in a flame which men

call the laughter or the threat of Hephaestus or of Hestia. This

occurs when the wood dries and cracks and the exhalation rushes on the

flame in a body. So in the clouds, the exhalation is projected and its

impact on dense clouds causes thunder: the variety of the sound is due

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