my fall; not that one ought to pray for a public disaster, but it is a great solace of death to see that the earth is likewise subject to death.
III It will be useful also to be assured that none of these things is the 1 doing of the gods, and that the moving of heaven or earth is no work of angry deities. Those phenomena have causes of their own. It is not by special command that they put forth their rage, but, just as in our own bodies, the disturbance arises from certain inherent imperfections; at the moment when they seem to inflict injury, they sustain it. Through our ignorance of the truth all these things are terrible, the more as their infrequency increases our alarm. Familiar occurrences seem less 2 serious; the unusual causes greater terror. But why is anything unusual in our estimation? The reason is that we grasp the meaning of nature only superficially, and not rationally; we dwell too exclusively on what she has done, and do not consider what she can do. Accordingly, we pay the penalty of this neglect in our terror of things that we suppose unprecedented, when they are not really unprecedented, but merely unusual. For instance, are not superstitious fears inspired both privately and even for the safety of the State, if either the sun has been seen in eclipse or if the moon, whose obscuration is more frequent, has partially or wholly been concealed? And is not this far 3 more so in the case of such sights as we have spoken of: torches driven athwart the heavens, the sky on fire over the greater part of its extent, comets, mock suns, stars appearing in the daytime, the sudden passage of stars that mark their trail with a bright light? Our wonder at these is in no case free from fear. As the cause of the fear is ignorance, is it not worth while to gain the knowledge that will dispel it? How much better it would be to inquire into the causes of the alarming sights, to bend, in fact, our whole mind to the task? Nothing, surely, could be found more deserving than that, of having the mind’s energies not only lent to it, but devoted to it.
IV Let us ask ourselves, therefore, what it is that stirs the earth to 1 its foundation, what moves a mass of such weight, what it is that is stronger than the earth, and that in its violence can shake such a load. Let us inquire why at one time the earth trembles, at another is loosened and sinks, and again is divided into parts and opens a chasm; or why on some occasions the intervals of destruction are prolonged, on others are suddenly cut short. What is the cause why it now consigns to its depths rivers of renowned greatness, and now causes fresh rivers to issue? why does it sometimes open up springs of hot water, sometimes freeze them with cold? and why at times are fires 2 caused to shoot out through some hitherto unknown opening in mountain or crag, while sometimes well-known fires, that have been famous for centuries, are suppressed? The earthquake produces a thousand strange sights, changing the aspect of the ground, levelling mountains, elevating plains, exalting valleys, raising new islands in the deep.
What are the causes that bring these things to pass? That is a subject well worthy our discussion. What, you say, will be the reward of our labour? That reward, I say, which surpasses all others, the knowledge of nature. Among the many serviceable lessons to be derived from 3 such researches, no feature is more commendable than this, that man is thereby made to dwell upon the sight of his own grandeur[79]; the study is pursued, not in hope of gain, but from the wonder it excites. Let us inquire, therefore, what it is that brings about all this. The inquiry is so fascinating to me that although long ago in my youth I published a volume on earthquakes, I am anxious to make another trial of my powers, and to see whether age has added anything to my knowledge, or, at any rate, to my industry.
[79] The meaning may rather be--the grandeur of the subject.
V The cause of earthquakes has been assigned variously by different 1 authorities to water, fire, air, and to the earth itself; some assign it to a combination of several of the causes, others, to a union of them all. Certain writers have stated that it was plain to them that some one of these causes produced the earthquake, but it was not plain which. Let us look at the various opinions in detail. First of all, I feel bound to say in general terms that the old views are crude and inexact. As yet men were groping their way round truth. Everything was new to those who made the first attempt to grasp it; only later were the subjects accurately investigated. But all subsequent discoveries must nonetheless be set down to the credit of those early thinkers.
It was a task demanding great courage to remove the veil that hid 2 nature, and, not satisfied with a superficial view, to look beneath the surface and dive into the secrets of the gods. A great contribution to discovery was made by the man who first conceived the hope of its possibility. We must, therefore, listen indulgently to the ancients.
No subject is perfected while it is but beginning. The truth holds not merely of the subject we are dealing with, the greatest and most complicated of all, in which, however much may be accomplished, every succeeding age will still find something fresh to accomplish. It holds alike in every other concern; the first principles have always been a long way off from the completed science.
VI Water is the first cause alleged: more authors than one adopt this 1 view, but it is not stated by all in the same terms. Thales of Miletus is convinced that the whole earth floats, and is upborne by moisture lying beneath it, which you may call either Ocean or the great sea, or still mere elemental water of a different character from the sea, the simple ingredient, moisture. In these waves, in his opinion, the globe is supported like some huge lumbering vessel in the water which bears it. It is unnecessary for me to reproduce his reasons for supposing 2 that the heaviest part of the world cannot be sustained in such a rare and nimble element as air: for the earth’s position is not the question here but its movement. By way of argument, to prove that water is the cause, he adduces the fact that in every considerable earthquake, as a rule, new springs burst out. So if a boat leans over to one side away from the straight, the result is that it ships water. And, generally speaking, in the case of all objects which water supports, if they are unduly sunk, the water either pours over them or at any rate rises to right and left above its ordinary height.
Now, no lengthened consideration is needed to prove the falsity of 3 this view. Why, if the earth were supported by water, and from time to time shaken by it, it would be in perpetual shock; the wonder would be not that it was tossed about sometimes, but that it was ever at rest.
Then, again, it would be shaken all over and not at a single point: we never find only half the ship tossed by the waves. But, according to present experience, a shock never occurs over the whole earth simultaneously, but is always felt at some particular spot. How, then, can it be that what is carried as a whole is not shaken as a whole, if the shock comes from the body by which it is carried?
But, it may be urged, why do waters burst out at the time of earth- 4 quakes? Well, in the first place, there has often been earthquake without any fresh supply of water appearing. Secondly, if the supposed cause of the water rushing forth were the true one, it would pour all round the sides of the earth, as we see happening under similar circumstances in sea and rivers: when boats sink, the increase of water shows itself chiefly over the sides. Finally, the outburst of waters which Thales describes would not be so small as he says, nor would it ooze in like bilge-water through a chink, but from the exhaustless reservoir that upbears all creation, a mighty deluge would ensue.
VII Some, who, like Thales, attribute earthquake to the effects of water, 1 give a different explanation of its operation. There are, they say, many kinds of waters running over the whole earth. In one place there are constant rivers whose size renders them fit for navigation, even without the aid of rains. There is the Nile, rolling down its huge volume all summer long: here are the Danube and the Rhine separating with their streams the peaceful from the hostile, the former checking attacks from the Sarmatians and forming the boundary between Europe and Asia, the latter keeping back the Germans, a nation ever keen for war. Then there are lakes of very wide extent, great pools surrounded 2 by tribes mutually ignorant of each other, marshes that no boat can struggle through, that cannot be passed even by the people that dwell on their borders. Add, then, the multitude of fountains, and of river sources that belch out of their recesses full-grown streams. Besides, there are many rushing torrents that gather only for a time, whose force is as shortlived as it is sudden. Now there are waters, in all this variety of form and character, within as well as above the 3 earth. Away there below some are borne along in vast bulk, and tumble their whole volume down the steep: others more sluggish are dammed back in shallows, and flow with gentle, quiet stream. And can any one deny that within those vast underground hollows waters are formed, and lie sluggish and inactive in many places? It needs no long proof to show that there must be many waters in the place where all waters are. The earth would not be able to produce so many rivers unless it poured them from a copious reserve.
This being so, sometimes below the earth a river must become swollen, 4 and leaving its banks assail with violence all obstacles that meet it. So there will be a movement of some point on which the river has made an onset, and which it will keep lashing until its waters fall.
Or it may happen that the constant wear of a stream may eat away some quarter, dragging down thereby some mass above, by whose fall, in turn, the surface which rested on it is shaken. Now surely a man trusts too 5 much to the sight of the eyes and cannot launch out his imagination beyond, if he does not believe that the depths of earth contain a vast sea with winding shores. I see nothing to prevent or oppose the existence of a beach down there in the obscurity, or a sea finding its way through the hidden entrances to its appointed place. There, too, it occupies as much space as here, perhaps more, since the regions up on earth have had to be shared with so many living creatures; but the hidden regions being desert without inhabitant give freer scope to the waves of the nether ocean. And who is there to hinder the sea from 6 swelling there and being tossed by all the winds that every interstice of the earth, and every species of atmosphere can create? So, then, when a storm greater than ordinary has arisen, it may beat upon some one side of the earth with too great vehemence and move it. For on the surface likewise, many places which had been far from the sea have felt the violence of its sudden approach: villas almost out of sight of it have been invaded by the waves which used only to be heard in the distance. The nether sea, too, can approach and retire; neither of which movements can take place without shock to the earth that stands above it.
VIII I do not, indeed, suppose that you will long hesitate to believe that 1 there are underground rivers and a hidden sea. From what other cause could the rivers burst out and come to the surface unless the source of the moisture were shut up within the earth? For instance, when one sees the Tigris interrupted and dried up in the middle of its course, not diverted as a whole, but gradually with imperceptible losses first lessen and then waste away, where do you suppose it goes to if not to the depths of the earth, especially as you see it emerge again not less in volume than its former stream? And what are you to say when you 2 see the Alpheus, so celebrated by the poets, sink in Achaia and, having crossed beneath the sea, pour forth in Sicily the pleasant fountain Arethuse? And don’t you know that among the explanations given of the occurrence of the inundation of the Nile in summer, one is that it bursts forth from the ground, and is swollen not by rain from above but by water given out from within the earth?
I have myself heard from their own lips the story told by the two 3 non-commissioned officers sent to investigate the sources of the Nile by our good Emperor Nero, a monarch devoted to virtue in every form, but especially solicitous for the interests of truth. The King of Ethiopia had supplied them with assistance and furnished letters of introduction to the neighbouring kings, and so they had penetrated into the heart of Africa and accomplished a long journey. “We came 4 indeed,” I give their own words, “to huge marshes, the limit of which even the natives did not know, and no one else could hope to know; so completely was the river entangled with vegetable growth,[80] so impassable the waters by foot, or even by boat, since the muddy overgrown marsh would bear only a small boat containing one person.
There,” my informants went on, “we saw with our eyes two rocks from which an immense quantity of water issued.” Now whether that is the real source or only an addition to the river; whether it rises there 5 or merely returns to the surface after its previous course underground; don’t you think that, whatever it is, that water comes up from a great lake in the earth? The earth must contain moisture scattered in numerous places and collected at depth in order to be able to belch it out with such violence.
IX Fire is the cause assigned by some for earthquakes, but they are not 1 agreed as to its method of action. First among them is Anaxagoras, who is of opinion that pretty much the same cause produces concussion in the earth as in the atmosphere. In the nether parts of earth, air (gas) causes explosions of thick atmosphere massed in clouds with the same violence as on earth clouds are wont to be burst. Fire is struck out by this collision of clouds and by the rush of the atmosphere that is forced out. This fire in seeking an exit meets obstructions and 2 bursts through all obstacles, until it has either found a way of escape to the light through the narrow passages, or has made one for itself by violence and destruction. Other writers who still believe the cause to lie in fire do not suppose that this is its method of action: they think the fire presents itself in more than one place and burns away everything in the vicinity. Then if the parts eaten away fall in at any time, a shock follows in the portions which are deprived of their supports; they first totter and then collapse; nothing encounters them to support their weight. Then chasms and vast gulfs are opened up, or 3 it may be, after hanging a long time in the balance, the ground settles down over what is still left standing. We see the same thing happen ordinarily as often as a part of the city suffers from a fire. The joists are burnt through, or what gave support to the upper part of the buildings is undermined. Then the roofs after tossing about for a long time fall in; their swaying and oscillating continue until they find a resting-place on solid ground.
X Anaximenes affirms that the earth is itself the cause of the earth- 1 quake, and that nothing encounters it from without to give it a shock. Within it, he thinks, certain parts of its substance fall of themselves, either loosened by moisture, or eaten away by fire, or shaken off by the violence of air. But even in absence of such active cause there is not wanting sufficient to account for the loss or removal of some portion of the earth. In the first place, all things fall through age, for nothing is safe from the ravages of time, which waste even the solidest and strongest edifice. In old buildings parts fall without being knocked off, merely because they have more weight than strength. So in the earth’s body as a whole it comes to pass 2 that portions are loosened by age, and being loosened, fall, causing shock to the things above them. This they do primarily while they are leaving their place; for nothing, especially if it is large, can be wrenched off without movement of that to which it adhered. But further, when the objects have fallen, they meet the solid earth and rebound like a ball. When a ball falls, it jumps up and bounces repeatedly, just as often, in fact, as it recoils from the ground for a new flight.
If the loosened objects within the earth are carried down into stagnant waters, this accident of itself causes a shock to the vicinity through the wave cast up by the weight of the objects shot suddenly down from a great height.
XI Some attribute these earthquakes to fire, but give different explanations of its action. When fire causes intense heat at various points beneath the earth, it must roll up a great cloud of vapour, which can find no exit, and which dilates the air by its high temperature. If the pressure of the vapour is excessive, it scatters all obstructions; but if it is comparatively moderate, it merely causes movement of the earth. We observe water smoke when fire is applied.
What the fire does to this water in a narrow pot, one may suppose is done on a much greater scale when a violent and wide-spreading fire causes immense extents of water to boil. It then by evaporation from the overflowing waters shakes violently whatever it strikes.
XII Many of the greatest authorities are persuaded that earthquakes are 1 to be attributed to air. Archelaus, who is well versed in the records of antiquity, speaks thus: Winds are carried down into the earth’s hollows and recesses. When they are all full, and the atmosphere is condensed to the utmost extent, the air, which continues to come in, forces and thrusts the former air, and with frequent blows first compresses and then dislodges it. The air in its endeavour to find 2 room forces all the narrow passages and tries to burst its barriers.
Through the struggle of the air as it seeks for an escape it comes to pass that the earth is moved. This explains why the approach of an earthquake is preceded by still and quiet of the atmosphere; the force of the air which is wont to rouse the winds is held in check in its nether abode. Even on the present occasion of the earthquake in 3 Campania, although the season was winter, the atmosphere was perfectly still and calm for several days before it.[81] Well, then, did an earthquake never take place when there was a wind blowing? On very rare occasions have there been two winds blowing simultaneously. Still, such a thing is possible, and is wont to occur. But if we admit it as an established fact that two winds can be in activity at one and the same time, why shouldn’t it happen that [at times] one of them agitates the upper air, the other the nether?[82] [81] The text is uncertain, and the argument down to the end of the chapter rather obscure.
[82] The argument seems to be: Two winds can blow simultaneously. One may be beneath the earth (causing or during earthquake), one above.
Therefore, stillness of the upper atmosphere is not a necessary concomitant of earthquake. The fact has at times been otherwise.
XIII In this category you may rank Aristotle and his disciple Theo- 1 phrastus, a man of pleasant though not of superhuman eloquence, as the Greeks considered him, and of easy, polished style. Let me unfold in more detail what they hold in common: There is always evaporation of some kind going on from the earth, which is at one time dry, at another has an admixture of moisture. When this, rising from the lowest parts of earth, has been raised to the utmost extent, and has no place beyond into which to issue, it is borne back and returns upon itself.
The struggle of the air in its ebb and flow tosses to and fro all obstructions it meets, and, whether its egress is stopped or whether it escapes through the narrow openings, it causes movement of the earth and uproar. To the same school of opinion belongs Strato, who 2 made a special study of this department of science, and was a diligent student of natural philosophy. His verdict on the matter is this: Cold and heat always move away from one another in opposite directions, and cannot remain in the same place. Cold flows into the spot whence the influence of heat has departed; and, conversely, there is heat in the place whence cold has been banished. The statement is beyond doubt, but the contrariety of the two may become plain to you from the following: In the winter season, when there is cold on the earth’s surface, the 3 wells are warm, and caves and all underground retreats equally so. The heat, yielding possession of the upper regions to the cold, retreats down there. When it reaches the lower regions, and is accumulated there to the utmost, the denser it is, the more powerful is it. To this a further supply is added, to which what has already gathered, and is compressed into a narrow space, of necessity gives way. The same thing happens from the opposite cause when a greater quantity of cold is borne down to these recesses. All the heat that lurks there gives way 4 to the cold, and retires to the narrow passages, and is driven onward with great impetuosity. The nature of the two, as I have said, does not allow agreement, or abode in the same place. In its flight, then, and eager haste to escape at all hazards the air pushes back and tosses about all that lies near it. This is why, previous to an earthquake, a roaring is usually heard, through the tumult of the winds in the earth’s bowels. For not otherwise, as our poet Virgil says, could 5 The earth bellow beneath our feet and the lofty peaks be moved, were not this the work of the winds. In this contest again there are ups and downs. There are cessations in the massing of the heat and, in turn, in its emission. Then the cold, too, is restrained and gives way, but some day soon it will be more powerful again. While, therefore, the alternating forces rush to and fro, and the air moves hither and thither, the earth is shaken.
XIV There are some who think that, while air and no other cause produces 1 earthquake, it operates in a different way from that which Aristotle supposed. Listen to what they say: Our body is irrigated with blood, and with air which courses everywhere along its own routes. We have some comparatively narrow vessels through which they cannot do more than pass; some wider, in which they accumulate, and from which they are distributed to the members. So this whole body of the earth at 2 large has passages alike for water, which performs the function of blood, and for wind, which might be called simply the breath of its life. These two encounter each other at some points, at some points they are stationary. While in our bodies good health is enjoyed, the movement of the veins preserves its rate undisturbed; but when there is malady the pulse beats more rapidly, the deep breathing and panting betoken laboured, wearied effort. In like manner the earth remains unshaken while it maintains its natural position. But if any 3 flaw occur in it, there is a shaking, just as of a body suffering from disease; for the air which flowed through it with regularity is violently smitten, and causes its veins to quiver; but not, let me add, in the way, described a little above,[83] imagined by those who will have it that the earth is a living creature. In that case the earth, just as an animal does, would feel the agitation equally all over. When a fever seizes any of us, it does not delay for a time its attack upon some parts, but with uniform regularity spreads over them all.
[83] There seems a slight lapse of memory here. Cf. pp. 126, 196.
Perhaps you had better assume, therefore, that air from the surround- 4 ing atmosphere enters the earth. As long as it has free egress, it glides through it without doing harm; but if it meet some obstacle to block its way, then it is, to begin with, weighted with the atmosphere that pours in on the rear; by and by it escapes with difficulty through some chink, and makes its way with the greater violence the narrower the opening is. That cannot take place without a struggle, and a struggle involves shaking of the earth. But if the confined air cannot find even a chink by which to issue, it is massed and 5 becomes furious, and is driven round in this direction and in that, overthrowing or bursting one thing after another. It is excessively subtle, and at the same time exceedingly powerful; it can worm its way into obstructions however great, splitting and scattering whatever it enters. When this occurs, then there is a regular tossing of the earth.
For the earth either opens to give room to the wind, or, after giving room, is deprived of its foundation and subsides into the very cavern from which it allowed the wind to issue.
XV Some entertain the following opinion: The earth is porous at many points, possessing not merely those first shafts which it received as ventilators at its creation, but many subsequently opened up by various changes. In some places water has washed away the soil that was on the surface; part has been eaten away by torrents, while parts have been exposed by the disruptive action of great tides. Through the interstices thus produced air enters. If it so happen now that the sea has shut it in and driven it deeper, and the waves prevent its escape by the same road, egress and regress being alike closed, the air rolls about within the earth. Its natural tendency is to hurry straight forward, but as that path is closed, it presses upward and lashes the earth, whose weight lies heavy upon it.
XVI I must further mention a view held by the majority of writers, which 1 probably I shall myself support. The earth does not lack air within; that everybody knows. I do not mean merely the air which holds it together and unites its parts, which exists even in stones and dead bodies; but I mean that fresh vital air which supports all life. Unless the earth possessed this store of air, how could she infuse it into so many trees and crops, which derive their life from this and no other source? How could she nourish all the different roots that sink into 2 the soil in one place and another, some merely attached to the surface, others sunk deeper, had she not an abundant supply of the breath of life, which produces so many varied growths and rears them with its nourishing draught? These are the slighter arguments that I hitherto urge. Why, all the heaven we see, which is shut in by fiery ether, the highest portion of the universe, all these stars, whose number cannot be conceived, all this concourse of heavenly bodies, and, to mention only one more, this sun, that urges his course so close to us, many times larger than the whole circuit of the earth--all these draw their nourishment from materials of earth which they share among them, and are sustained, of course, by nothing else than the breath of the 3 earth. This is their nourishment, this their pasturage. Now the earth would be unable to nourish so many bodies of such size, larger even than itself, unless it were full of breath, which it exhales from every part of it day and night. For there must be a large reserve of that from which so much is sought and taken; in fact, the supply to be drawn from it is created for the occasion. The earth would not possess a 4 perennial supply of air sufficient for the wants of so many heavenly bodies, unless the elements issued and returned alternately and were transmutable into one another. But apart from this, it is necessary that the earth be abundantly filled with it, and be able to draw it forth from her hidden store. There is no doubt then that a great quantity of air lurks in the interstices of the earth, and a widely diffused atmosphere occupies the hidden spaces underground. If that is true, of necessity the earth must often be moved, since it is full of a most movable substance. No one, I suppose, can doubt that there is nothing so restless, so capricious, so fond of disturbance as air.
XVII It follows, therefore, that air should obey the law of its being; 1 what is wont to be moved will sometimes move other things. And when?
Whenever its free course is checked. As long as it is not hindered it flows quietly along. When it is opposed and held back it becomes furious, bursting all obstacles just like that Araxes that ever spurned a bridge.
As long as the river has a free easy channel it rolls down its waters 2 in due and regular succession. But if through chance or by human agency rocks are placed in its way to check its course, then it gathers fresh strength from the barrier, and the more numerous the obstacles opposed to it, the greater the force that it musters to overcome them. For all the water that accumulates behind, constantly increases, and being at last unable to bear its own weight manifests its violence through the havoc it works in its descent, and escapes headlong down its channel, bearing the very obstacles that blocked its path. The same thing occurs with air, only that, in proportion to its greater strength 3 and mobility, it is the more rapidly carried onward, and bursts the more violently all that encloses it. From this, of course, there is a disturbance in the part of the ground under which the struggle has occurred. The truth of this assertion may be proved from the consideration that often when an earthquake has taken place, involving a breach of only some part of the earth, wind has issued from it for several days. This is recorded to have taken place in the earthquake 4 in which Chalcis suffered, as you will find in Asclepiodotus, Posidonius’ pupil, in his discussion of my own topic of Physical Inquiries. In other authors, too, you will find it stated that after a chasm had opened up at one spot, in no long time wind issued from it, having no doubt made for itself the way along which it travelled.
XVIII The chief cause of earthquake, therefore, is air, an element 1 naturally swift and shifting from place to place. As long as it is not stirred, but lurks in a vacant space, it reposes innocently, giving no trouble to objects round it. But when any cause coming upon it from without rouses it, or compresses it, and drives it into a narrow space, in the first instance, to be sure, it merely retires and roams about its enclosure. But when opportunity of escape is cut off, and resistance meets it on all hands, then...With deep murmur of the mountain It roars around the barriers;...which, after long battering, it dislodges and tosses on high, growing the more fierce, the stronger the obstacle with which it has 2 contended. By and by, when it has traversed the whole space in which it was enclosed, and has failed to find a way of escape, it recoils from the side on which its impact was greatest. It is then either distributed through the secret openings which the earthquake of itself causes here and there, or escapes through a new rent. So uncontrollable is this mighty power. No bolt can imprison wind; it loosens every bond, bears with it every weight, and insinuating itself into the smallest crannies wins its release; for by the invincible power of nature it is free, especially when roused, and asserts its right for itself. Air is 3 a thing no man can tame; nothing will be found which, When the winds struggle and the tempests roar, Can restrain them by its sway and rein them by bonds and prison.
Doubtless the poets wished the place in which the winds lay pent up underground to be considered a prison. But they did not perceive either that what was shut up is no longer wind, or that what is wind can no longer be shut up. What is shut up is at rest, and the atmosphere is at a standstill; whereas all wind is in flight. Besides these arguments, there is a consideration by which it becomes manifest that motion 4 is brought about by air, namely, that our bodies never tremble except when some cause produces disturbance of the internal air,[84] which is contracted by fear, grows sluggish in old age, languishes when the veins are numbed, is checked with cold, or after some attack of fever is quite driven from its wonted course. As long as it flows unimpeded, and moves in its wonted fashion, there is no quivering of the body.
When anything intervenes to prevent its functioning, then being no longer able to maintain what it upheld by its vigour, it fails, causing a collapse of everything that it had sustained when unimpaired.
[84] Or spirit: there is almost a play upon the ambiguous meaning of the term.
XIX We must now hear what Metrodorus of Chios desires to urge by way of 1 opinion. I do not allow myself the liberty of passing over unnoticed even opinions that I disapprove; it is better to have the largest possible variety of views, and to condemn rather than omit what we do not approve. Well, then, what has Metrodorus to say? He compares the 2 subterranean disturbances to the voice of a person who puts his head into a barrel and begins to sing out. In that case there is a kind of quavering as the voice extends and resounds through the whole hollow space; slight as the movement is, it passes all round the vessel in which it is enclosed, grazing its sides and causing disturbance all through. In the same way the vast empty caverns that stretch down beneath the earth have atmosphere of their own, on which other air coming from above falls with violence. The agitation produced differs in no wise from that of the empty vessels which I have just mentioned, when they resound through shouting into them.
XX Let us now go on to consider the authors who have alleged as causes 1 all the different factors mentioned, or, at any rate, several of them.
Democritus is one of those who think that several are concerned. He asserts that the earthquake is produced sometimes by air, sometimes by water, sometimes by both. He pursues the argument in the following way: Some portion of the earth is hollow, in which a large quantity of water has gathered. Part of this water is thinner and less dense than the rest. When it is driven back by a heavy mass descending upon it from above, it comes violently against the earth, causing a commotion of it. The fluctuating movement of the water cannot take place without corresponding movement of the body on which it impinges. Besides, what we said a little above regarding air must be repeated in regard 2 water. When it is accumulated at one place, which becomes too small to contain it, it inclines in some particular direction, and opens up