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We must go on to discuss earthquakes next, for their cause is akin

to our last subject.

The theories that have been put forward up to the present date are

three, and their authors three men, Anaxagoras of Clazomenae, and

before him Anaximenes of Miletus, and later Democritus of Abdera.

Anaxagoras says that the ether, which naturally moves upwards, is

caught in hollows below the earth and so shakes it, for though the

earth is really all of it equally porous, its surface is clogged up by

rain. This implies that part of the whole sphere is 'above' and part

'below': 'above' being the part on which we live, 'below' the other.

This theory is perhaps too primitive to require refutation. It is

absurd to think of up and down otherwise than as meaning that heavy

bodies move to the earth from every quarter, and light ones, such as

fire, away from it; especially as we see that, as far as our knowledge

of the earth goes, the horizon always changes with a change in our

position, which proves that the earth is convex and spherical. It is

absurd, too, to maintain that the earth rests on the air because of

its size, and then to say that impact upwards from below shakes it

right through. Besides he gives no account of the circumstances

attendant on earthquakes: for not every country or every season is

subject to them.

Democritus says that the earth is full of water and that when a

quantity of rain-water is added to this an earthquake is the result.

The hollows in the earth being unable to admit the excess of water

it forces its way in and so causes an earthquake. Or again, the

earth as it dries draws the water from the fuller to the emptier

parts, and the inrush of the water as it changes its place causes

the earthquake.

Anaximenes says that the earth breaks up when it grows wet or dry,

and earthquakes are due to the fall of these masses as they break

away. Hence earthquakes take place in times of drought and again of

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