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they must go on until there is too little of the evaporation left to

have any perceptible effect on the earth at all.

Subterranean noises, too, are due to the wind; sometimes they

portend earthquakes but sometimes they have been heard without any

earthquake following. Just as the air gives off various sounds when it

is struck, so it does when it strikes other things; for striking

involves being struck and so the two cases are the same. The sound

precedes the shock because sound is thinner and passes through

things more readily than wind. But when the wind is too weak by reason

of thinness to cause an earthquake the absence of a shock is due to

its filtering through readily, though by striking hard and hollow

masses of different shapes it makes various noises, so that the

earth sometimes seems to 'bellow' as the portentmongers say.

Water has been known to burst out during an earthquake. But that

does not make water the cause of the earthquake. The wind is the

efficient cause whether it drives the water along the surface or up

from below: just as winds are the causes of waves and not waves of

winds. Else we might as well say that earth was the cause; for it is

upset in an earthquake, just like water (for effusion is a form of

upsetting). No, earth and water are material causes (being patients,

not agents): the true cause is the wind.

The combination of a tidal wave with an earthquake is due to the

presence of contrary winds. It occurs when the wind which is shaking

the earth does not entirely succeed in driving off the sea which

another wind is bringing on, but pushes it back and heaps it up in a

great mass in one place. Given this situation it follows that when

this wind gives way the whole body of the sea, driven on by the

other wind, will burst out and overwhelm the land. This is what

happened in Achaea. There a south wind was blowing, but outside a

north wind; then there was a calm and the wind entered the earth,

and then the tidal wave came on and simultaneously there was an

earthquake. This was the more violent as the sea allowed no exit to

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