The pillars of Hercules

The Mediterranean is like a bag with two necks filling at both ends. The current through the Dardanelles presents exciting varieties, but no perplexing mysteries. It is the discharge of the surplus of the Black Sea, and the current is subject to the influences of the northerly and southerly winds; being reversed when the latter long prevails. At Gibraltar all is disorder—the stream incessant—the level on both sides the same. The tide rises and falls, yet the current always runs out of the ocean and into the Mediterranean. So determined is this rush, that the gales of the Equinox neither quicken nor retard it, and the phases of the moon have no power over it. It bursts through all obstacles and transgresses all laws, and seems to move by a will of its own—too strong to be disturbed, too deep to be discovered. During my excursions I was engaged in examining these phenomena, and I will commence with stating the results of several months’ cogitation and inquiries.

I first applied myself to test the old explanation of an under-current, by endeavouring to float substances at various levels, and after great trouble in procuring lines, and having machines of various kinds made, I found that without a frigate’s tackle and crew no results could be obtained. I was thus reduced to mere scrutiny of the alleged facts, and of the alleged theory. The facts amount to this: a vessel, in 1754,[24] was fired into from the battery, it sank in face of the rock, and was afterwards cast up in the bay of Tangier.

A vessel, when it sinks, goes to the bottom, and if fragments of it are detached and are cast ashore, it is only because they float, that is, they rise to the surface. This story will not, therefore, serve the theory, even if authentic. There is nothing to prevent a ship or timber from floating out; for close in shore, on both sides, the tides of the ocean rise within the Straits to the height of four feet: of these, boats take advantage to get through against both wind and current. Sometimes, indeed, though it very rarely happens, the whole current is reversed; and vessels working during the night, and reckoning on being carried fifty miles to the eastward, have found themselves in the morning ninety miles to the westward of the point where they expected to be, that is to say, carried forty miles over the ground to the westward during the night.[25]

Having thus disposed of the only, but incessantly quoted fact, I proceed to the theory. Reasoning, however, there is none, for it amounts to nothing more than this: “What becomes of all this water? It cannot go to the Black Sea, from which the Mediterranean receives water; it cannot escape by a subterranean passage into the Red Sea, for the level of the Red Sea is higher by thirty feet. Then there is an under-current discharging the water back again into the ocean.”

Water moves by its weight. Unless there is difference of level, there is no motion. The resistance is from the bottom according to its roughness, and the vis inertiæ is felt at the top—thus the greatest speed is at about two-thirds of the depth; here there is no difference of level, nor is the water acted on superficially by any propelling power. There is no prevalence of winds to account for a current at the surface. So great is the momentum of the stream, that, unlike the currents of the Dardanelles, it is neither accelerated by favourable winds, nor even retarded by adverse storms. The idea of an over-current running against an under-current is so opposed to all experience, that to be admissible, proofs would be required, and it could never be received as an hypothesis to account for an unexplained phenomenon.

Thus, the theoretical explanations utterly fail; yet there is action without agent, momentum without motor, currents without winds or declivity, and a vessel constantly filling without escape or overflow. A mighty river rushes over its bed; but this river is not moved by its weight; it runs on a dead level[26] to the sea it reaches from the fountain whence it springs.—That fountain is the ocean itself! No wonder that this should be the first of ancient mysteries, and the last to be explained.

Before I had discarded the idea of an under-current, or had discovered the insufficiency of the evaporation to account for the indraught, I was sitting on Partridge Island, (a small rock within the Straits,) and gazing with astonishment at the enormous mass of water running by me, when the question occurred to me, what becomes of the salt? If the water evaporate, the salt remains; here then is the sluice of a mighty salt-pan—where is the produce? This has been going on for thousands of years; is there a deposit of salt at the bottom? If so, why have the abysses of the Mediterranean not been filled up? But salt is not deposited; how then is the Mediterranean not become brine? Then I saw that the evaporation would not account for the indraught, and before I descended from that rock, I had solved the problem. That solution is—an under-current produced by a difference of specific gravity between the water of the Mediterranean and the ocean.

If you take two vessels, and fill one with fresh water, and the other with salt, or the one with sea-water at its ordinary charge of 1030, and the other with sea-water of higher specific gravity, such as would result from evaporating a portion of it, say 1100, and colour differently the water in the two vessels, and then raise a sluice between them, you will instantly have two currents established in opposite directions. In fact, you produce currents of water, like currents of wind, by the converse of rarefaction.

“Recent discoveries,” says Humboldt, “have shown that the ocean has its currents exactly as the air. Living, as we do, upon the surface, they have been beyond our reach; but now, having obtained soundings to the depth of four miles, we have ascertained that there is a rush of icy water from the Pole to the Equator, just as there is a draft of air close to the earth into the centre of Africa. The Mediterranean offers an apparent anomaly of a higher temperature at great depths. This Arago explains by the fact, ‘That the surface of the water flows in as a Westerly current, whilst a counter current prevails beneath, and prevents the influx from the ocean of the cold current from the Pole.’[27] If there was nothing to determine the currents at the entrance of the Mediterranean, save the relative degrees of cold at great depths between it and the ocean, the cold water would run in at the lowest depths, and the warm water would run out on the surface, which is precisely the reverse of what it does.

Here is a body of water 740,000 square miles in extent, subject hourly to the increase of its specific gravity. Upon the surface, a crust of salt is left in the course of every year, sufficient to give a double charge to the depth of six fathoms. To adjust the difference thus created with the ocean, there is but a narrow inlet,—a mere crack upon the side of the vessel, an interval of six miles left in a circumference of four thousand. By this, in its deepest part, the heavier water will have to find its way out, and thus occasion an indraught of water above, besides the demand created by evaporation. It remains to be ascertained by experiment, that the specific gravity of the water in the Straits varies at different levels, and at what level it commences to move outwards. These experiments will present great practical difficulties from the tides at the sides, which will mingle the streams; and, from the shallowness towards the ocean, they must be made in the middle and at the Mediterranean side. The evaporation, and the differences of specific gravity, will give the means of calculating the amount of water passing through in both directions, and the depth and velocity of the two currents. But it may be inferred that the currents will have the greatest speed at the top and the bottom,—that their velocity will diminish towards the centre, and that a neutral space of dead water will remain, not only in consequence of the counter-impetus of the currents, but because of the nearer approach of specific gravity, and the mingling of the two waters, which would destroy the moving power.

With this solution we can at once understand the powerlessness of tides and storms, currents without difference of level, or prevalence of winds: the volume of the stream is accounted for, the mass of salt disposed of, and the apparent rebellion against the laws of Nature put down.

By tables kept for several years at Malta, it appears that the Mediterranean, at that point, varies in level between winter and summer no less than three feet. In winter, when there is no evaporation, and when the quantity of water falling in the immediate vicinity of the Mediterranean is greatest, the level is lowest. The cause, I should take to be the pressure of wintry wind. In like manner, those erratic movements in the Straits may result from difference of atmospheric pressure without and within.

These currents, by the testimony of the ancients, have not held from the beginning—they have been the results of successive modifications of the channel. This is singularly borne out by the traditions of the neighbouring people and the geological features of the coast.

Eldressi narrates, as an old and popular story of his day, that “the Sea of Cham (Mediterranean) was in ancient times a lake surrounded on all sides, like the Sea of Tabaristan (Caspian), the waters of which have no communication with any other seas. So that the inhabitants of the extreme west invaded the people of Andalusia, doing them much injury, which they, in like manner, did to the others, living always in war, until the time of Alexander,[28] who consulted his wise men and artificers about cutting that arid isthmus and opening a canal. Thereupon they measured the earth, and the depth of the two seas, and saw that the Sea of Cham was not much lower than the Sea Muhit, (Ocean); so they raised the towns that were on the coast of the Sea of Cham, changing them to the high ground. Then he ordered the earth to be dug out; and they dug it away to the bottom of the mountains on both sides; and he built there two terraces with stones and lime the whole length between the two seas, which was twelve miles; one on the side of Tankhe (Tangier), and one on the side of Andeluz. When this was done, he caused the mound to be broken, and the water rushed in from the great sea with violence, raising the waters of the Sea of Cham, so that many cities perished, and their inhabitants were drowned, and the waters rose above the dykes, and carried them away, and did not rest until they had reached the mountains on both sides.”

The Moors have also a Myth. “The sea,” they say, “was created fresh, but exalting itself against its Maker, gnats were sent to drink it up. It then humbled itself in the stomach of the gnats, and prayed to be relieved, so the gnats were ordered to vomit it forth again, but the salt remained from the stomach of the gnats an eternal sign of its disobedience.”

Before suggesting the interpretation of these Myths, I will point out the change which the coasts and channel have undergone.

The description of the Straits by Greek and Roman writers is so unlike their present appearance, that, but for the impossibility of doubting the identity of the objects, we must have supposed their words to apply to some undiscovered region. Who could recognize the deep sea and the iron-bound coasts of these narrows, in a plain of sand furrowed by rivers running in from the ocean, which it was difficult to reach, not from the strength of the stream, but the intricacies of the passage—who would imagine the necessity of constructing flat-bottomed boats to get across from Gibraltar to Ceuta, where now there is above one thousand fathoms, or of transferring the ferry to the Atlantic side of the Straits, where at present the depth is not one-sixth of what it is in the other, in order to get more water? Yet there is no doubt that these details apply to these spots, nor can we question the known accuracy of the writers, or escape from the concurrence of their testimony. We are reduced, then, to the necessity of admitting some great revolution in the features of the country, and a total change in the nature of the current.

The explanation is easy: the bank of sand, left by the retiring waters of the Deluge, which covers the western border of Africa, reached to the coast of Andalusia, and the remnants of it still lie on the eastern side of Gibraltar, and fill the caverns exposed to that side; on the depression of the Mediterranean by evaporation, the water of the ocean would filter in, the sand would be gradually removed. The amount of sand on the Mediterranean side of the “Rock,” shows that a plain nearly one thousand feet above the level of the sea, once stretched across to Africa, where now the channel is one thousand fathoms deep. This has been worked out by the overfall, while on the Atlantic side the water shoals to one hundred and eighty fathoms, presenting the character of an estuary with a bar from the rush outward of the under waters, since the Gut was sufficiently deepened to admit of the currents in opposite directions. The centre part of the channel is worn down to the rock or to gravel, every particle of sand has been removed from the bottom. Two inferences may be drawn: 1st. That the process of removal was likely to be accompanied by sudden inbursts, which would submerge the borders. 2nd. That the Mediterranean, in early ages, was fresh, and afterwards became, as it evaporated, very salt, until the channel was deepened to allow of its mixing with the ocean. What else is implied by the Myth of the midges and the fable of Alexander?