In this category you may rank Aristotle and his disciple Theo- 1 phrastus, a man of pleasant though not of superhuman eloquence, as the Greeks considered him, and of easy, polished style. Let me unfold in more detail what they hold in common: There is always evaporation of some kind going on from the earth, which is at one time dry, at another has an admixture of moisture. When this, rising from the lowest parts of earth, has been raised to the utmost extent, and has no place beyond into which to issue, it is borne back and returns upon itself. The struggle of the air in its ebb and flow tosses to and fro all obstructions it meets, and, whether its egress is stopped or whether it escapes through the narrow openings, it causes movement of the earth and uproar. To the same school of opinion belongs Strato, who 2 made a special study of this department of science, and was a diligent student of natural philosophy. His verdict on the matter is this: Cold and heat always move away from one another in opposite directions, and cannot remain in the same place. Cold flows into the spot whence the influence of heat has departed; and, conversely, there is heat in the place whence cold has been banished. The statement is beyond doubt, but the contrariety of the two may become plain to you from the following: In the winter season, when there is cold on the earth’s surface, the 3 wells are warm, and caves and all underground retreats equally so. The heat, yielding possession of the upper regions to the cold, retreats down there. When it reaches the lower regions, and is accumulated there to the utmost, the denser it is, the more powerful is it. To this a further supply is added, to which what has already gathered, and is compressed into a narrow space, of necessity gives way. The same thing happens from the opposite cause when a greater quantity of cold is borne down to these recesses. All the heat that lurks there gives way 4 to the cold, and retires to the narrow passages, and is driven onward with great impetuosity. The nature of the two, as I have said, does not allow agreement, or abode in the same place. In its flight, then, and eager haste to escape at all hazards the air pushes back and tosses about all that lies near it. This is why, previous to an earthquake, a roaring is usually heard, through the tumult of the winds in the earth’s bowels. For not otherwise, as our poet Virgil says, could 5 The earth bellow beneath our feet and the lofty peaks be moved, were not this the work of the winds. In this contest again there are ups and downs. There are cessations in the massing of the heat and, in turn, in its emission. Then the cold, too, is restrained and gives way, but some day soon it will be more powerful again. While, therefore, the alternating forces rush to and fro, and the air moves hither and thither, the earth is shaken. XIV