8. These river deposits are prevented from advancing further into the sea by the regularity of the ebb and flow, which continually drive them back. For after the manner of living creatures, which go on inhaling and exhaling their breath continually, so the sea in a like way keeps up a constant motion in and out of itself. Any one may observe who stands on the sea-shore when the waves are in motion, the regularity with which they cover, then leave bare, and then again cover up his feet. This agitation of the sea produces a continual movement on its surface, which even when it is most tranquil has considerable force, and so throws all extraneous matters on to the land, and
“Flings forth the salt weed on the shore.”338
This effect is certainly most considerable when the wind is on the water, but it continues when all is hushed, and even when it blows from land the swell is still carried to the shore against the wind, as if by a peculiar motion of the sea itself. To this the verses refer—
“O’er the rocks that breast the flood
Borne turgid, scatter far the showery spray,”339
and,
“Loud sounds the roar of waves ejected wide.”340
9. The wave, as it advances, possesses a kind of power, which some call the purging of the sea, to eject all foreign substances. It is by this force that dead bodies and wrecks are cast on shore. But on retiring it does not possess sufficient power to carry back into the sea either dead bodies, wood, or even the lightest substances, such as cork, which may have been cast out by the waves. And by this means when places next the sea fall down, being undermined by the wave, the earth and the water charged with it are cast back again; and the weight [of the mud] working at the same time in conjunction with the force of the advancing tide, it is the sooner brought to settle at the bottom, instead of being carried out far into the sea. The force of the river current ceases at a very little distance beyond its mouth. Otherwise, supposing the rivers had an uninterrupted flow, by degrees the whole ocean would be filled in, from the beach onwards, by the alluvial deposits. And this would be inevitable even were the Euxine deeper than the sea of Sardinia, than which a deeper sea has never been sounded, measuring, as it does, according to Posidonius, about 1000 fathoms.341
10. Some, however, may be disinclined to admit this explanation, and would rather have proof from things more manifest to the senses, and which seem to meet us at every turn. Now deluges, earthquakes, eruptions of wind, and risings in the bed of the sea, these things cause the rising of the ocean, as sinking of the bottom causes it to become lower. It is not the case that small volcanic or other islands can be raised up from the sea, and not large ones, nor that all islands can, but not continents, since extensive sinkings of the land no less than small ones have been known; witness the yawning of those chasms which have ingulfed whole districts no less than their cities, as is said to have happened to Bura,342 Bizone,343 and many other towns at the time of earthquakes: and there is no more reason why one should rather think Sicily to have been disjoined from the mainland of Italy than cast up from the bottom of the sea by the fires of Ætna, as the Lipari and Pithecussan344 Isles have been.
11. However, so nice a fellow is Eratosthenes, that though he professes himself a mathematician,345 he rejects entirely the dictum of Archimedes, who, in his work “On Bodies in Suspension,” says that all liquids when left at rest assume a spherical form, having a centre of gravity similar to that of the earth. A dictum which is acknowledged by all who have the slightest pretensions to mathematical sagacity. He says that the Mediterranean, which, according to his own description, is one entire sea has not the same level even at points quite close to each other; and offers us the authority of engineers for this piece of folly, notwithstanding the affirmation of mathematicians that engineering is itself only one division of the mathematics. He tells us that Demetrius346 intended to cut through the Isthmus of Corinth, to open a passage for his fleet, but was prevented by his engineers, who, having taken measurements, reported that the level of the sea at the Gulf of Corinth was higher than at Cenchrea,347 so that if he cut through the isthmus, not only the coasts near Ægina, but even Ægina itself, with the neighbouring islands, would be laid completely under water, while the passage would prove of little value. According to Eratosthenes, it is this which occasions the current in straits, especially the current in the Strait of Sicily,348 where effects similar to the flow and ebb of the tide are remarked. The current there changes twice in the course of a day and night, like as in that period the tides of the sea flow and ebb twice. In the Tyrrhenian sea349 the current which is called descendent, and which runs towards the sea of Sicily, as if it followed an inclined plane, corresponds to the flow of the tide in the ocean. We may remark, that this current corresponds to the flow both in the time of its commencement and cessation. For it commences at the rising and setting of the moon, and recedes when that satellite attains its meridian, whether above [in the zenith] or below the earth [in the nadir]. In the same way occurs the opposite or ascending current, as it is called. It corresponds to the ebb of the ocean, and commences as soon as the moon has reached either zenith or nadir, and ceases the moment she reaches the point of her rising or setting. [So far Eratosthenes.]
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 Sicily350 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 Casius353 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