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8
We have already shown that wet and dry must both give rise to an
evaporation:
earthquakes are a necessary consequence of this fact. The
earth is essentially dry, but
rain fills it with moisture. Then the
sun and its own
fire warm it and give rise to a quantity of
wind
both outside and inside it. This wind sometimes flows outwards in a
single body, sometimes inwards, and sometimes it is divided. All these
are necessary laws. Next we must find out what body has the greatest
motive force. This will certainly be the body that naturally moves
farthest and is most violent. Now that which has the most rapid
motion
is necessarily the most violent; for its swiftness gives its impact
the greatest force. Again, the rarest body, that which can most
readily pass through every other body, is that which naturally moves
farthest. Wind satisfies these conditions in the highest degree
(fire only becomes flame and moves rapidly when wind accompanies
it): so that not water nor earth is the cause of earthquakes but
wind-that is, the inrush of the external evaporation into the earth.
Hence, since the evaporation generally follows in a continuous
body in the direction in which it first started, and either all of
it flows inwards or all outwards, most earthquakes and the greatest
are accompanied by calm. It is true that some take place when a wind
is blowing, but this presents no difficulty. We sometimes find several
winds blowing simultaneously. If one of these enters the earth we
get an earthquake attended by wind. Only these earthquakes are less
severe because their source and cause is divided.
Again, most earthquakes and the severest occur at night or, if by
day, about noon, that being generally the calmest part of the day. For
when the sun exerts its full power (as it does about noon) it shuts
the evaporation into the earth. Night, too, is calmer than day. The
absence of the sun makes the evaporation return into the earth like
a sort of ebb tide, corresponding to the outward flow; especially
towards dawn, for the winds, as a rule, begin to blow then, and if
their source changes about like the Euripus and flows inwards the
quantity of wind in the earth is greater and a more violent earthquake
results.
The severest earthquakes take place where the
sea is full of
currents or the earth spongy and cavernous: so they occur near the
Hellespont and in
Achaea and Sicily, and those parts of Euboea which
correspond to our description-where the sea is supposed to flow in
channels below the earth. The hot springs, too, near Aedepsus are
due to a cause of this kind. It is the confined character of these
places that makes them so liable to earthquakes. A great and therefore
violent wind is developed, which would naturally blow away from the
earth: but the onrush of the sea in a great mass thrusts it back
into the earth. The countries that are spongy below the surface are
exposed to earthquakes because they have room for so much wind.
For the same reason earthquakes usually take place in spring and
autumn and in times of wet and of
drought-because these are the
windiest seasons. Summer with its heat and winter with its frost cause
calm: winter is too cold,
summer too dry for winds to form. In time of
drought the
air is full of wind; drought is just the predominance of
the dry over the moist evaporation. Again, excessive rain causes
more of the evaporation to form in the earth. Then this secretion is
shut up in a narrow compass and forced into a smaller space by the
water that fills the cavities. Thus a great wind is compressed into
a smaller space and so gets the upper hand, and then breaks out and
beats against the earth and shakes it violently.
We must suppose the action of the wind in the earth to be
analogous to the tremors and throbbings caused in us by the force of
the wind contained in our bodies. Thus some earthquakes are a sort
of tremor, others a sort of throbbing. Again, we must think of an
earthquake as something like the tremor that often runs through the
body after passing water as the wind returns inwards from without in
one volume.
The force wind can have may be gathered not only from what happens
in the air (where one might suppose that it owed its power to
produce such effects to its volume), but also from what is observed in
animal bodies. Tetanus and spasms are motions of wind, and their force
is such that the united efforts of many men do not succeed in
overcoming the movements of the patients. We must suppose, then (to
compare great things with small), that what happens in the earth is
just like that. Our theory has been verified by actual observation
in many places. It has been known to happen that an earthquake has
continued until the wind that caused it burst through the earth into
the air and appeared visibly like a hurricane. This happened lately
near Heracleia in
Pontus and some time past at the island Hiera, one
of the group called the Aeolian islands. Here a portion of the earth
swelled up and a lump like a mound rose with a noise: finally it
burst, and a great wind came out of it and threw up live cinders and
ashes which buried the neighbouring town of Lipara and reached some of
the towns in Italy. The spot where this eruption occurred is still
to be seen.
Indeed, this must be recognized as the cause of the fire that is
generated in the earth: the air is first broken up in small
particles and then the wind is beaten about and so catches fire.
A phenomenon in these islands affords further evidence of the fact
that winds move below the surface of the earth. When a south wind is
going to blow there is a premonitory indication: a sound is heard in
the places from which the eruptions issue. This is because the sea
is being pushed on from a distance and its advance thrusts back into
the earth the wind that was issuing from it. The reason why there is a
noise and no earthquake is that the underground spaces are so
extensive in proportion to the quantity of the air that is being
driven on that the wind slips away into the void beyond.
Again, our theory is supported by the facts that the sun appears
hazy and is darkened in the absence of
clouds, and that there is
sometimes calm and sharp frost before earthquakes at sunrise. The
sun is necessarily obscured and darkened when the evaporation which
dissolves and rarefies the air begins to withdraw into the earth.
The calm, too, and the cold towards sunrise and dawn follow from the
theory. The calm we have already explained. There must as a rule be
calm because the wind flows back into the earth: again, it must be
most marked before the more violent earthquakes, for when the wind
is not part outside earth, part inside, but moves in a single body,
its
strength must be greater. The cold comes because the evaporation
which is naturally and essentially hot enters the earth. (Wind is
not recognized to be hot, because it sets the air in motion, and
that is full of a quantity of cold
vapour. It is the same with the
breath we blow from our mouth: close by it is warm, as it is when we
breathe out through the mouth, but there is so little of it that it is
scarcely noticed, whereas at a distance it is cold for the same reason
as wind.) Well, when this evaporation disappears into the earth the
vaporous exhalation concentrates and causes cold in any place in which
this disappearance occurs.
A sign which sometimes precedes earthquakes can be explained in
the same way. Either by day or a little after sunset, in fine weather,
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
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 see it earlier because
sight is quicker than
hearing. The rowing of triremes illustrates this: the oars are going
back again before the sound of their striking the water reaches us.
However, there are some who maintain that there is actually fire
in the clouds. Empedocles says that it consists of some of the sun's
rays which are intercepted:
Anaxagoras that it is part of the upper
ether (which he calls fire) which has descended from above. Lightning,
then, is the gleam of this fire, and thunder the hissing noise of
its extinction in the cloud.
But this involves the view that lightning actually is prior to
thunder and does not merely appear to be so. Again, this
intercepting of the fire is impossible on either theory, but
especially it is said to be drawn down from the upper ether. Some
reason ought to be given why that which naturally ascends should
descend, and why it should not always do so, but only when it is
cloudy. When the sky is clear there is no lightning: to say that there
is, is altogether wanton.
The view that the heat of the sun's rays intercepted in the clouds
is the cause of these phenomena is equally unattractive: this, too, is
a most careless explanation. Thunder, lightning, and the rest must
have a separate and determinate cause assigned to them on which they
ensue. But this theory does nothing of the sort. It is like
supposing that water,
snow, and
hail existed all along and were
produced when the time came and not generated at all, as if the
atmosphere brought each to hand out of its stock from time to time.
They are concretions in the same way as thunder and lightning are
discretions, so that if it is true of either that they are not
generated but pre-exist, the same must be true of the other. Again,
how can any distinction be made about the intercepting between this
case and that of interception in denser substances such as water?
Water, too, is heated by the sun and by fire: yet when it contracts
again and grows cold and freezes no such ejection as they describe
occurs, though it ought on their the. to take place on a proportionate
scale. Boiling is due to the exhalation generated by fire: but it is
impossible for it to exist in the water beforehand; and besides they
call the noise 'hissing', not 'boiling'. But hissing is really boiling
on a small scale: for when that which is brought into contact with
moisture and is in process of being extinguished gets the better of
it, then it boils and makes the noise in question. Some-Cleidemus is
one of them-say that lightning is nothing objective but merely an
appearance. They compare it to what happens when you strike the sea
with a rod by night and the water is seen to shine. They say that
the moisture in the cloud is beaten about in the same way, and that
lightning is the appearance of brightness that ensues.
This theory is due to ignorance of the theory of reflection, which
is the real cause of that phenomenon. The water appears to shine
when struck because our sight is reflected from it to some bright
object: hence the phenomenon occurs mainly by night: the appearance is
not seen by day because the daylight is too in, tense and obscures it.
These are the theories of others about thunder and lightning: some
maintaining that lightning is a reflection, the others that
lightning is fire shining through the cloud and thunder its
extinction, the fire not being generated in each case but existing
beforehand. We say that the same stuff is wind on the earth, and
earthquake under it, and in the clouds thunder. The essential
constituent of all these phenomena is the same: namely, the dry
exhalation. If it flows in one direction it is wind, in another it
causes earthquakes; in the clouds, when they are in a process of
change and contract and condense into water, it is ejected and
causes thunder and lightning and the other phenomena of the same
nature.
So much for thunder and lightning.
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