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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 whol