12. The nature of the ebb and flow has been sufficiently treated of by Posidonius and Athenodorus. Concerning the flux and reflux of the currents, which also may be explained by physics, it will suffice our present purpose to observe, that in the various straits these do not resemble each other, but each strait has its own peculiar current. Were they to resemble each other, the current at the Strait of Sicily[350] would not change merely twice during the day, (as Eratosthenes himself tells us it does,) and at Chalcis seven times;[351] nor again that of Constantinople, which does not change at all, but runs always in one direction from the Euxine to the Propontis, and, as Hipparchus tells us, sometimes ceases altogether. However, if they did all depend on one cause, it would not be that which Eratosthenes has assigned, namely, that the various seas have different levels. The kind of inequality he supposes would not even be found in rivers only for the cataracts; and where these cataracts occur, they occasion no ebbing, but have one continued downward flow, which is caused by the inclination both of the flow and the surface; and therefore though they have no flux or reflux they do not remain still, on account of a principle of flowing which is inherent in them; at the same time they cannot be on the same level, but one must be higher and one lower than another. But who ever imagined the surface of the ocean to be on a slope, especially those who follow a system which supposes the four bodies we call elementary, to be spherical.[352] For water is not like the earth, which being of a solid nature is capable of permanent depressions and risings, but by its force of gravity spreads equally over the earth, and assumes that kind of level which Archimedes has assigned it. 13. To what we cited before concerning the temple of Ammon and Egypt, Eratosthenes adds, that to judge from appearances, Mount Casius[353] was formerly covered by sea, and the whole district now known as Gerra lay under shoal water touching the bay of the Erythræan Sea,[354] but was left dry on the union[355] of the [Mediterranean] Sea [with the ocean]. A certain amphibology lurks here under this description of the district lying under shoal water and touching the bay of the Erythræan Sea; for to touch[356] both means to be close to, and also to be in actual contact with, so that when applied to water it would signify that one flows into the other. I understand him to mean, that so long as the strait by the Pillars of Hercules remained closed, these marshes covered with shoal-water extended as far as the Arabian Gulf, but on that passage being forced open, the Mediterranean, discharging itself by the strait, became lower, and the land was left dry.