Turn the broad helm, the fluttering canvas urge From Maelstrom's fierce innavigable surge.

CANTO III. l. 93.

On the coast of Norway there is an extensive vortex, or eddy, which lies between the islands of Moskoe and Moskenas, and is called Moskoestrom, or Maelstrom; it occupies some leagues in circumference, and is said to be very dangerous and often destructive to vessels navigating these seas. It is not easy to understand the existence of a constant descending stream without supposing it must pass through a subterranean cavity to some other part of the earth or ocean which may lie beneath its level; as the Mediterranean seems to lie beneath the level of the Atlantic ocean, which therefore constantly flows into it through the Straits; and the waters of the Gulph of Mexico lie much above the level of the sea about the Floridas and further northward, which gives rise to the Gulph-stream, as described in note on Cassia in Vol. II.

The Maelstrom is said to be still twice in about twenty-four hours when the tide is up, and most violent at the opposite times of the day. This is not difficult to account for, since when so much water is brought over the subterraneous passage, if such exists, as compleatly to fill it and stand many feet above it, less disturbance must appear on the surface. The Maelstrom is described in the Memoires of the Swedish Academy of Sciences, and Pontoppiden's Hist. of Norway, and in Universal Museum for 1763, p. 131.

The reason why eddies of water become hollow in the middle is because the water immediately over the centre of the well, or cavity, falls faster, having less friction to oppose its descent, than the water over the circumference or edges of the well. The circular motion or gyration of eddies depends on the obliquity of the course of the stream, or to the friction or opposition to it being greater on one side of the well than the other; I have observed in water passing through a hole in the bottom of a trough, which was always kept full, the gyration of the stream might be turned either way by increasing the opposition of one side of the eddy with ones finger, or by turning the spout, through which the water was introduced, a little more obliquely to the hole on one side or on the other. Lighter bodies are liable to be retained long in eddies of water, while those rather heavier than water are soon thrown out beyond the circumference by their acquired momentum becoming greater than that of the water. Thus if equal portions of oil and water be put into a phial, and by means of a string be whirled in a circle round the hand, the water will always keep at the greater distance from the centre, whence in the eddies formed in rivers during a flood a person who endeavours to keep above water or to swim is liable to be detained in them, but on suffering himself to sink or dive he is said readily to escape. This circulation of water in descending through a hole in a vessel Dr. Franklin has ingeniously applied to the explanation of hurricanes or eddies of air.