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These movements being dissimilar, their causes are likewise different. Let us deal first with the shaking movement. If great loads are being conveyed by a row of many waggons, and the wheels, under the unusual strain, fall into the ruts of the road, one feels the earth shaken. Asclepiodotus has put it on record that on one occasion the fall of a rock that was torn off from the mountain-side caused by the tremor the collapse of some houses in its vicinity. Just the same thing may occur beneath the earth; parts of the overhanging crags may be loosened and fall with great weight and noise upon the floor of the cavern beneath, and with a violence proportionate to the weight of the mass and the height of the fall. The whole roof of the subterranean valley is disturbed by an occurrence of this kind. It is conceivable, too, that rocks are not always wrenched off by their own weight; when rivers roll over them, the constant moisture weakens the joints of the stone, and day by day bears away part of its fastening, causing abrasion, so to speak, of the skin in which the stone is enclosed. The long waste of ages, through constant daily rubbing, by and by so weakens the fastenings that they cease to be able to sustain their burden. Then blocks of vast size fall down, then the crag hurled headlong will not suffer anything to stand that it strikes in the rebound from its fall, but
Comes away with a roar; and all things seem suddenly to rush headlong, as our countryman Virgil says. Such must be the cause of the earthquake that shakes the ground beneath. Now I must pass on to the second kind.