It was supposed that every whirlpool formed round a central rock; under it opened a great cavern or gulf, down which the waters rushed, and so the whirling was produced as in a basin emptying through a central hole. This notion was developed by Athanasius Kircher (1602-80). In his theory whirlpools marked the entrances to subterranean channels connecting different seas, and the phenomena of tides were produced by the alternate flow of water in opposite directions. Kircher gives a curious diagram of the Mälström or “umbilicus maris,” illustrating his supposition that the water, after pouring into the vast funnel, flowed along a channel under the Scandinavian peninsula and rose in the Gulf of Bothnia. When the level of this gulf had been raised to a sufficient height, he thought that the current was reversed, and, aided by a stream pouring through a subterranean tunnel from the White Sea, raised the tide on the coast of Norway. Carrying his theory a degree farther to account for the Gulf Stream and the Antarctic drift, Kircher, with great ingenuity, placed a grand vortex at the North pole, down which all the water of the ocean tumbled, and, passing through the earth’s axis, emerged at the South pole, thus keeping up “a circulation like that of the blood in the human body.”

The facts which gave rise to the wild theories of mediaeval geographers and the extravagant descriptions of early voyagers are impressive enough in themselves to rank amongst the grandest phenomena of nature. No one who has seen the tide-streams racing through the Pentland Firth at 12 miles an hour, now swirling along with a smooth dimpled surface, like molten glass, now meeting the countercurrent and leaping high into the air in columns of water and spray, or who has heard the roar of Corrievreckan as the Atlantic tide rushes between Scar ba and Jura against an easterly gale, will be disposed to deny the terrible danger to small open vessels or to wonder that horror strengthened imagination to the confusion and exaggeration of fact.

The formation of whirlpools is a natural result of water flowing rapidly in an irregular channel (see Hydromechanics, vol. xii. pp. 468, 510); it takes place in all rivers and in every tide-race of the sea, the depth, diameter, and velocity depending on accidental causes. The form of the surface of an ordinary whirlpool is given by Prof. J. Thomson as that generated by the revolution of a curve whose formula is y = c 3 ∕ x 2 , where y is the depth of any point on the curve below the general level remote from the whirl, X the distance of the point from the axis of revolution, and c a constant. Every point on the surface moves with the velocity a heavy body would attain in falling from the general level of the water surface to that point, and any point in the interior of the revolving mass has a velocity equal to that of the point on the surface immediately above itself. Prof. Thomson applied his researches on the motion of whirlpools to the construction of a particularly effective form of turbine.

All the famous whirlpools are situated in channels essentially similar in configuration and in tidal phenomena: their vortices are produced at certain phases of the tide or with certain directions of the wind; and they are all dangerous to navigation, but the danger is due to the cause which produces the whirlpools—the tidal race—not to the “roaring wells” themselves. Whirlpools in a tidal stream are not stationary, but travel along with the current, filling up and again forming in irregular succession. Small boats have repeatedly been drawn into these vortices in the northern fjords and capsized; and trading steamers in passing through a tideway are violently deflected from their course. It is on record that a seventy-four gun ship has been whirled right round in the vortices of the Straits of Messina. The fishermen of the Norwegian fjords and of the northern island groups,—Lofoten, Faroe, Shetland, and Orkney,—still believe that, if they can throw a heavy or bulky object into a whirlpool, it will close up without harming their boats. Lithgow in his Travels, speaking of the Pentland Firth, says, “I denote this credibly, in a part of the north-west end of this gulf there is a certain place of sea where these distracted tides make their rencountering rendezvous, that whirleth ever about, cutting in the middle circle a sloping hole, with which, if either ship or boat shall happen to encroach, they must quickly either throw over something into it, as a barrel, a piece of timber, and such like, or that fatal euripus shall then suddenly become their swallowing sepulchre.” This custom, it has been supposed, is sacrificial in its origin. Its continued practice, however, suggests what is probably the case, that the bulky object splashing into the whirl breaks the continuity of the surface and causes a collapse or filling-up. In windy weather, when there is a broken sea, vortices are not formed. The tidal stream passing through the irregular channels between islands gives rise to a complicated series of eddies and counter-currents, which, according to the British Admiralty’s Sailing Directions, make navigation dangerous, except when guided by local knowledge and aided by fair wind and a favourable tide. The general effect appears to be that, as the tide rises, the strength of the stream increases; and the counter-currents set up along the shore, as well as the overfalls produced by inequalities of the bottom, thoroughly mix the water to a depth of from 50 to 100 fathoms. This has been proved by observations of temperature amongst the tidal currents off the Mull of Cantyre, where at all times of the year there is no difference between the surface and bottom temperature. At high water a state of rest ensues; and ebb tide usually reverses the order of phenomena, the main stream, counter-currents, and eddies running in the opposite direction, but with nearly equal strength, until low tide brings another pause.

Charybdis, a whirlpool famous in classical literature, is situated in the Straits of Messina. The rise of tide at Messina does not exceed one foot, but the current may attain the velocity of nearly 6 miles an hour. Where the north-going flood tide meets the southrunning counter-current, and where the southerly ebb meets its induced northerly stream, great eddies or garofóla are formed, one of which is Charybdis. These depend very much on the wind for the intensity of their phenomena.

In ancient maps of the united Aral-Caspian Sea two whirlpools are represented. Near the position laid down there are in the river Amu-Daria two whirlpools at the junction of several channels. These have been recently examined and found to arise from the river flowing over two conical hollows in its bed, respectively 120 and 60 feet deep; these do not appear to have been formed by running water, but closely resemble craters of mud volcanoes.

The most violent tidal current is said to be that of Salten Fjord to the south of Bodö on the north-west coast of Norway, known by the name of the Saltenström, and dreaded on account of its turbulence and its numerous vortices. Opposite Salten Fjord, on the western side of Vest Fjord, the wild jagged range of the Lofoten Islands runs like a row of shark’s teeth from south to north. Between two of the southernmost of the group, Moskenaes and Mosken, runs the Moskenström, a tidal current which after low water commences to flow towards the north-east, then gradually changes its direction to east, to south, and at high water to southwest; after half an hour’s cessation the ebb begins to flow towards the south-west, at half ebb due west, and then gradually turns through north to north-east at low water. The current thus rotates round Mosken once in 12 hours. It runs with a velocity of 7 miles an hour when a strong wind blows in the same direction, and, as the sea-bed is very irregular and rises abruptly from 200 fathoms seaward of Mosken to 20 fathoms in the channel and Vest Fjord, the flow is very turbulent, with occasional whirlpools and opposing currents set up along the shore, giving it a character very similar to that of the Strait of Messina. This is the place of the Malstrom.

In the Faroe Islands several dangerous tide-races exist in which are dreaded whirlpools, the two worst being’ the Quaerne off Sand Island and one round the rock of Sumboe-musk off Süder Island.

The Shetland and Orkney Islands are traversed by a system of formidable tideways called roosts, dangerous to fishing boats and very frequently forming whirlpools. It is sufficient to refer to the swiths or wells of Swona in Orkney, to the whirlpool of the Swelchie off Stroma, and the Merry Men of Mey, also in the Pentland Firth. The channel of Jura Sound in the Hebrides, which contains Corrievreckan whirlpool, resembles that of Mosken and Messina in being narrow and of very irregular configuration. The sea to the west is about 70 fathoms deep; a trough over 50 fathoms in depth and quarter of a mile wide runs through towards the east, deepening about the centre to 105 and 120 fathoms in consecutive holes. The channel is less than a mile in width, and the water is shallow (15 to 20 fathoms) from the shore out to the central trough, where the deepening is abrupt. To this fact, and not to the supposed existence of “a submerged rock of pyramidal form shooting up from a depth of 100 fathoms,” are due, in all probability, the tidal stream, running sometimes at 9 miles an hour, and the great vortices which are occasionally formed, as well as the dangerous counter-currents and overfalls.

Folklore.— Many marvellous stories are told of the dwellers in whirlpools. Fish are supposed to be more abundant there than anywhere else. Whales have from earliest times been associated, if not confounded, with these gulfs; and all manner of sea-beings, —krakens, trows (trolls), and mermaids,—claim sanctuary beneath the turmoil. Ramus, two centuries ago, tried by dint of misapplied philological ingenuity to identify Charybdis with the Malstrom, alleging that the fact of Ulysses’s voyage to northern seas was plainly recorded in the names of islands and headlands. For the myth of Charybdis, see Scylla and Charybdis. Whirlpools have been brought forward to explain the origin of the tides; but a well-known Norse folk-tale in one of its forms gives to a particular vortex—the Swelchie—an additional and more important office, that of maintaining the salinity of the ocean. “Malstrom” (millstream) probably refers to the rapid current resembling a mill-race. The Quaerne in the Faroe Islands suggests the same idea from its name. A mist of poetical romance has always played over the roaring surges of Corrievreckan, and this finds expression in many poems and tales. Horrible sea-monsters made the gulf their home in the earliest times; but they gave place to the seductive mermaid who captivated a Macphail of Cólonsay, and entertained him in her coral caves and fairy palaces beneath the sea for many years. Scott in his Border Minstrelsy gives another version of this story, in which Macphail outwits the mermaid and remains faithful to the maid of Colonsay. The name of the whirlpool is sometimes derived from coire bhreaccn, “the speckled cauldron,” referring to the foam that usually variegates its surface; but the legend makes it the corry or gulf of Vrekan, a prince of Norway, who, having come there to woo a Hebridean chieftain’s daughter, was swept into the whirlpool, and drawn down from sight for ever. George Macdonald has embodied this legend in a highly dramatic poem.

Literature.— References to whirlpools occur incidentally in many places. See A. Kircher’s Mundus Subterraneus, vol. i., Amsterdam, 1664; Pontoppidan’s Natural History of Norway, 1755 (this work cites the opinions of earlier writers, Ramus, Arraboe, &c.). For a good description of Charybdis, see Nicholson's Journal, vol. i. (1798) p. 12; of Corrievreckan, Athenaeum, 3d September 1864. Full and trustworthy details of the actual state and dangers of special whirlpools will be found in the Sailing Directions or Coast Pilots oi the British Admiralty for the various seas. (H. R. Μ.)