VII. Water may be the cause, but may operate in quite different ways from those supposed by Thales. Storms, etc., in subterranean seas may cause earthquakes 233 VIII. There must be such subterranean water. The Tigris and Arethusa prove it. Nero, the virtuous and the veracious, sent two officers to investigate the sources of the Nile; their account confirms the assumption 235 IX. Fire is another alleged cause. It either bursts out through opposing obstacles, as in the clouds (Anaxagoras), or burns away the foundation and causes a subsidence at the spot 236 X. Pieces of the earth falling in merely through the decay of age may produce the effect without fire or any external influence. This is Anaximenes’ opinion 237 XI. Fire is supposed by some to cause earthquakes by expanding the vapour which it first causes to be given off from the subterranean waters 238 XII. Archelaus sets down the cause as air pressing up the earth’s internal wind which is already condensed to bursting point 239 XIII. Aristotle and Theophrastus take evaporation to be the cause. Strato, much in the same way, thinks that differences of internal temperature are the cause 240 XIV. By some it is thought that air is the cause, but that its operation, along with water, is like that of blood and air in the vessels of the body. The earth, it is assumed in this case, admits air, which must find an exit. When it does so violently, the result is an earthquake 242 XV. The earth is porous, perforated at many points, and it is thus that the air enters 243 XVI. The earth is full of air, nourishing plants rooted in it, and exhaling enough to feed the sun and the other heavenly bodies. Air is the most movable of elements; therefore the earth, if it is full of air, must also have frequent movements 244 XVII. Obstruction of air, just as of water, causes greater impetuosity when it escapes. Wind is frequently associated with earthquakes, as at Chalcis 245 XVIII. Additional considerations to prove that the great cause of earthquakes is air, _i.e._ wind 247 XIX. Metrodorus of Chios compares the rumbling of an earthquake to the resonance of the voice in a tub; the underground caves impart the sound 248 XX. Various combinations of water and air supposed by Democritus and Epicurus to co-operate to the production of earthquakes 249