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
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