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Let us now go on to consider the authors who have alleged as causes 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 to water. When it is accumulated at one place, which becomes too small to contain it, it inclines in some particular direction, and opens up a passage for itself, at first by its mere weight, afterwards by the gathering force of its current. Being long shut up it cannot escape except down an incline, and it cannot drop straight down with any gentleness, or without violent shaking of the parts through which and on which it falls. Now, if after it has begun its rapid downward movement it is checked at any point, and the force of the current is thrown back upon itself, it is driven back on the earth which encounters it, and attacks the earth at the point where it is most insecure. Moreover, the ground is sometimes so saturated with the moisture it has received into its heart that it subsides to a lower level and its very foundation is destroyed. The pressure is then exerted on the part toward which the weight of the descending waters most inclines. Air, too, sometimes urges the water. If it presses with some degree of violence, it naturally moves the part of the earth toward which it has urged the gathering of the waters. Sometimes, again, the air is driven into passages through the earth, and in its search for a way of escape causes a general movement. The earth, as we know, is pervious to wind; air is too subtle to be excluded, too violent to be resisted when excited to rapid movement.
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Turning from Democritus to Epicurus, we find the latter to assert that all the foregoing may be causes of earthquake, but he tries to introduce some additional ones. He criticises other authors for affirming too positively that some particular one of the causes is responsible, as it is difficult to pronounce anything as certain in matters in which conjecture must be resorted to. As he says, then, water is capable of producing earthquake by washing and rubbing off certain portions, the weakening of which removes the support of what was upborne by them when unimpaired. The force of air is also capable of moving the earth. Perhaps the air within the earth is set in violent agitation by other air entering from without. Or, perchance, it may be that the earth receives an internal blow from the sudden fall of some portion of it, and derives thence the shock. Or, perchance, some portion of the earth is upheld, as it were, by certain pillars and stakes, the injury or withdrawal of which causes a tremor to run through the mass they support. Or, perchance, a quantity of hot air turning to fire and assuming the character of lightning courses along to the widespread destruction of all obstacles it encounters. Or, perchance, some wind stirs the sluggish marshy waters, whose stroke in consequence shakes the earth; or the tossing of the air, increasing to violence through the mere movement, is carried from the lowest depths right up to the surface of the earth. Still, Epicurus is satisfied that there is no more potent cause of earthquake than air.