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We must now hear what Metrodorus of Chios desires to urge by way of 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 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.