When air has completely filled a large vacant space within the earth, 1 and has begun to struggle and meditate escape, it lashes again and again the sides of the enclosure within which it lurks, and right over which, as it happens, cities are sometimes situated. The shaking is at times so violent that buildings standing above the area of disturbance are thrown down. Sometimes it goes to such lengths that the walls by which the whole roof of the cavern is supported fall right down into that vacant underground space, and cities sink entire into the unfathomed depths. Long ago, if one may believe the story, Ossa and 2 Olympus were united; subsequently they were separated by an earthquake, and the one great mountain was split into two. Then the Peneus made its escape, draining the marshes with which Thessaly was overspread, and drawing off the waters, which from want of exit had hitherto formed a lake. It was an earthquake that let loose Ladon, the river which flows between Elis and Magalenopolis. What, it is asked, do these facts go to prove? Simply that air gathers in the spacious caves--for what 3 other name can I apply to the empty places under the earth? Were this not so,[86] great spaces of the earth would be convulsed, and many of them would totter to ruin at one and the same time. As it is, only small portions suffer, nor does a shock ever extend as much as two hundred miles. Look at the recent one, the marvellous tales of which have filled the whole world; it did not pass beyond Campania. Need I say that when Chalcis felt the earthquake shock Thebes did not fall? 4 when Aegium suffered, Patras, which is quite close by, only learned by report about the earthquake? That mighty shock, which swallowed up the two cities Helice and Buris, stopped short of Aegium. Plainly, then, the movement extends only such distance as the empty space underground stretches. [86] _I.e._ were the air distributed all through the earth. XXVI