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