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a little, light, long-drawn cloud is seen, like a long very straight

line. This is because the wind is leaving the air and dying down.

Something analogous to this happens on the sea-shore. When the sea

breaks in great waves the marks left on the sand are very thick and

crooked, but when the sea is calm they are slight and straight

(because the secretion is small). As the sea is to the shore so the

wind is to the cloudy air; so, when the wind drops, this very straight

and thin cloud is left, a sort of wave-mark in the air.

An earthquake sometimes coincides with an eclipse of the moon for

the same reason. When the earth is on the point of being interposed,

but the light and heat of the sun has not quite vanished from the

air but is dying away, the wind which causes the earthquake before the

eclipse, turns off into the earth, and calm ensues. For there often

are winds before eclipses: at nightfall if the eclipse is at midnight,

and at midnight if the eclipse is at dawn. They are caused by the

lessening of the warmth from the moon when its sphere approaches the

point at which the eclipse is going to take place. So the influence

which restrained and quieted the air weakens and the air moves again

and a wind rises, and does so later, the later the eclipse.

A severe earthquake does not stop at once or after a single shock,

but first the shocks go on, often for about forty days; after that,

for one or even two years it gives premonitory indications in the same

place. The severity of the earthquake is determined by the quantity of

wind and the shape of the passages through which it flows. Where it is

beaten back and cannot easily find its way out the shocks are most

violent, and there it must remain in a cramped space like water that

cannot escape. Any throbbing in the body does not cease suddenly or

quickly, but by degrees according as the affection passes off. So here

the agency which created the evaporation and gave it an impulse to

motion clearly does not at once exhaust the whole of the material from

which it forms the wind which we call an earthquake. So until the rest

of this is exhausted the shocks must continue, though more gently, and

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