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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
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
the wind that had entered the earth； but shut it in。 So in their
struggle with one another the wind caused the earthquake； and the wave
by its settling down the inundation。
Earthquakes are local and often affect a small district only；
whereas winds are not local。 Such phenomena are local when the
evaporations at a given place are joined by those from the next and
unite； this； as we explained； is what happens when there is drought or
excessive rain locally。 Now earthquakes do come about in this way
but winds do not。 For earthquakes； rains； and droughts have their
source and origin inside the earth； so that the sun is not equally
able to direct all the evaporations in one direction。 But on the
evaporations in the air the sun has more influence so that； when
once they have been given an impulse by its motion； which is
determined by its various positions； they flow in one direction。
When the wind is present in sufficient quantity there is an
earthquake。 The shocks are horizontal like a tremor； except
occasionally； in a few places； where they act vertically； upwards from
below； like a throbbing。 It is the vertical direction which makes this
kind of earthquake so rare。 The motive force does not easily
accumulate in great quantity in the position required； since the
surface of the earth secretes far more of the evaporation than its
depths。 Wherever an earthquake of this kind does occur a quantity of
stones comes to the surface of the earth （as when you throw up
things in a winnowing fan）； as we see from Sipylus and the
Phlegraean plain and the district in Liguria； which were devastated by
this kind of earthquake。
Islands in the middle of the sea are less exposed to earthquakes
than those near land。 First； the volume of the sea cools the
evaporations and overpowers them by its weight and so crushes them。
Then； currents and not shocks are produced in the sea by the action of
the winds。 Again； it is so extensive that evaporations do not
collect in it but issue from it； and these draw the evaporations
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
to the irregularity of the clouds and the hollows that intervene where
their density is interrupted。 This then； is thunder； and this its
cause。
It usually happens that the exhalation that is ejected is inflamed
and burns with a thin and faint fire： this is what we call
lightning； where we see as it were the exhalation coloured in the
act of its ejection。 It comes into existence after the collision and
the thunder； though we