Another peculiarity requires explanation: some wells are full for 1 six hours and dry for six alternately: why is this so? It is hardly necessary to name the rivers individually which are at certain months broad, at certain narrow, and to give separate causes of this, seeing I can give a common explanation that applies to all. An ague returns at the same hour, gout always keeps its appointment, the custom of women, unless interrupted, observes its stated period, birth is ready at the proper month. In like manner waters have their intervals of recurrence, at which to withdraw and at which to return. Now, some intervals are shorter, and the more striking on that account; some are longer, 2 but no less certain. And what is strange in that, when you see that the succession of events, and all nature, by decree preserve their appointed order? Winter has never mistaken its time. Summer has always blazed forth in its season. The changes of spring and autumn have occurred according to their wont. Solstice and equinox alike have kept their appointed days. Beneath the earth likewise there are laws of nature, less familiar to us, but no less fixed. Be assured that there exists below everything that you see above. There, too, there are antres vast, immense recesses, and vacant spaces, with mountains overhanging on either hand. There are yawning gulfs stretching down into the abyss, which 3 have often swallowed up cities that have fallen into them, and have buried in their depths their mighty ruins. These retreats are filled with air, for nowhere is there a vacuum in nature; through their ample spaces stretch marshes over which darkness ever broods. Animals also are produced in them, but they are slow-paced and shapeless; the air that conceived them is dark and clammy, the waters are torpid through inaction. Most of these creatures are blind, such as moles and underground rats, which have no sense of sight, since it is unnecessary for them. From these depths fish are, according to Theophrastus, dug up in certain localities. XVII