CHAPTER II.
THE CURRENTS OF THE STRAITS.
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.