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 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 3 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