by Aristotle
Democritus says that the earth is full of water and that when a quantity of [365b1] 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 [5] 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 heavy rain, since, as we have explained, the earth grows dry in time of drought and breaks up, whereas the [10] rain makes it sodden and destroys its cohesion.
But if this were the case the earth ought to be found to be sinking in many places. Again, why do earthquakes frequently occur in places which are not [15] excessively subject to drought or rain, as they ought to be on the theory? Besides, on this view, earthquakes ought always to be getting fewer, and should come to an end entirely some day: the notion of contraction by packing together implies this. So if [20] this is impossible the theory must be impossible too.
8 · We have already shown that wet and dry must both give rise to an exhalation: earthquakes are a necessary consequence of this fact. The earth is [25] 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. If [30] this cannot but be so, we must next 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 [35] swiftness gives it impact the greatest force. Again, the rarest body, that which can most readily pass through every other body, is that which naturally moves farthest. [366a1] If, then, the nature of wind is of this kind, wind must be the body with the most motive force; for 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 exhalation.
Hence, since the exhalation generally follows in a continuous body in the direction in which it first started, and either all of it flows inwards or all outwards, [5] 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 [10] 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 [15] 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 exhalation into the earth. Night, too, is calmer than day because of the absence of the sun. So the flood turns inward [20] again, like a sort of ebb tide, in the opposite direction 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 is greater and a more violent earthquake results.
[25] 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—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. [30] It is the confined character of these places that makes them so liable to earthquakes. A violent wind which would naturally blow away from the earth, is thrust back into the earth by the onrush of the sea in a great mass. The countries that are spongy [366b1] 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 [5] 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 exhalation. Again, excessive rain causes more of the exhalation to form in the earth. Then this secretion is shut up in a [10] 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 [15] bodies. Thus some earthquakes are a sort of tremor, others a sort of throbbing. Again, we must think that the earth is affected as we often are after urinating—for a sort of tremor runs through the body as the wind returns inwards from without in [20] 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 [25] 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 [30] 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, [367a1] one of the group called the Aeolian islands. Here a portion of the earth swelled up and sort of crested lump rose with a noise: finally it burst, and a great wind came out [5] of it and threw up 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 [10] 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 [15] 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 overflows into the void beyond.
Again, our theory is supported by the facts that the sun appears hazy and is [20] 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 wind which dissolves and rarefies the air begins to withdraw into the earth; and [25] there must be calm and cold towards sunrise and dawn. The calm we have already explained. There must as a rule be calm because the wind flows back into the earth, and it must be most marked before the more violent earthquakes; for when the wind [30] is not part outside the earth, part inside, but moves in a single body, its strength must be greater. The cold comes because the exhalation 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 [367b1] the breath we blow from our mouth: close by it is warm, as it is when we huff, but there is so little of it that it is scarcely noticed, whereas at a distance it is cold for the [5] same reason as wind.) Well, when this disappears into the earth the vaporous exhalation concentrates because of the moisture 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 [10] 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 breakers are very thick and crooked, but when the sea [15] is calm they are slight and straight (because the secretion is small). As the sea is to the earth so the wind is to the cloudy air; so, when the wind drops, this very straight and thin cloud is left, a sort of breaker in the air.
[20] 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 [25] 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 path approaches the point at which the eclipse is going to [30] 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 [368a1] 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 [5] violent, and there it must remain in a cramped space like water in a vessel 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 source which created the exhalation and the impulse of the wind clearly does not at once exhaust the whole of [10] 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 exhalation left to have any perceptible effect on the earth at all.
Subterranean noises, too, are due to the wind; sometimes they precede [15] 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 it is too weak by reason of thinness to [20] 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 marvel-mongers say. [25]
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 cause whether it exerts its force 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 [30] 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 source 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 [368b1] 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 opposite wind, will burst out and cause a flood. This is what happened in Achaea. There a south wind [5] was blowing, but outside33 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 [10] 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 exhalations at a given place are [15] 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 the former have their source inside the earth, so that the exhalations all move in one direction; the sun has less power over them than over [20] those in the air so that, when once they have been given a start by its motion, which is determined by its various positions, they flow in one direction.
When the wind is present in sufficient quantity it causes an earthquake which is horizontal like a tremor; except occasionally, in a few places, it runs vertically, upwards from below, like a throbbing. It is the vertical direction which makes this [25] kind of earthquake so rare. The source does not easily accumulate in great quantity in the position required, since the surface of the earth secretes far more than its depths. Wherever an earthquake of this kind does occur a quantity of stones comes [30] 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. For the volume of the sea cools the exhalations and overpowers them by its weight and so crushes them. Again, the sea flows rather than shakes when mastered [369a1] by the winds. Again, it is so extensive that exhalations do not collect in it but issue from it, and these draw the exhalations from the earth after them. Islands near the [5] continent 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.
[10] 9 · Let us go on to explain lightning and thunder, and further whirlwind, fire-wind, and thunderbolts; for the cause of them all must be supposed the same.
As we have said, there are two kinds of exhalation, moist and dry, and their [15] combination 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 [20] 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 [25] densest part of the cloud.) Now the heat that escapes disperses to the upper 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 is forcibly carried on and collides with the neighbouring clouds, and the sound of this collision is what we call thunder. This [30] 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 [369b1] 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.
[5] It usually happens that the wind 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 [10] hearing. The rowing of triremes illustrates this: the oars are going back again b
efore 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 [15] 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 unreasonable on [20] either theory, but especially when 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—for when the sky is clear there is no lightning. The theory seems altogether too hasty.
The view that the heat of the sun’s rays intercepted in the clouds is the cause of [25] these phenomena is equally implausible: 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 [30] like supposing that water, snow, and hail existed all along and emerged when the time came and were 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 account must fit both. Again, how can any distinction be [370a1] made about the intercepting between this case and that of interception in denser substances? 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 [5] on their theory to take place on a proportionate scale.34 Boiling is due to the wind 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 [10] question.