We find Aristotle, in his treatise on natural events, rejecting the explanations of three other philosophers as untenable, and propounding a theory of his own. Anaximenes, of Miletus, suggested the drying and moistening of the earth occasioned irregular contraction and expansion, and from the cracking and readjusting shocks resulted. Democritus, of Abdera, shook his earth by means of vast subterranean bodies of water which some force compelled to move from one cavity to another. Doubtless the peculiar wave-like motion of the earth in many earthquakes suggested his theory. Anaxagoras, of Clazomenae, believed that ether--by which the old Greeks seem to mean air--was confined in underground cavities, and in its efforts to escape upward produced the vibration of the earth. Aristotle substitutes for the disturbing agent wind, which has flowed into fissures and caverns and is endeavoring to flow out again. Virgil and Pliny stand by the old Greek; and it is quite probable that Shakespeare acquired his idea from one of the three. And these, with Anaxagoras, are but little out of the way; for as seen in the discussion of volcanic action, the explosive or disturbing agent is generally steam, though other gases are present in large quantities. We have already noticed that earthquakes and volcanoes are produced by the same causes; but as the myths of many nations do not connect the two, it is evident that such people did not recognize their essential identity. But after knowing they are but variations in results, we cannot so readily explain the reason of the variations. Certain facts are well established; and from these common premises widely different conclusions have been deduced. We know to-day that in active volcanic regions, an earthquake almost invariably precedes an eruption; and a violent one has never, within the historic period, followed an eruption. So the most reasonable inference is, that the earthquake merely betokens the presence of a vast quantity of imprisoned vapor which has not found an outlet; and that so soon as a volcanic vent is found, the pressure is relieved, and the earthquake subsides. But this leaves us just where the theorists of volcanic agency have stopped. The question of the sudden formation of volumes of gases in sufficient quantities to produce such terrible effects is to be solved.