Sir Robert Sibbald describes a very remarkable marine whirlpool among the Orkney islands, which would prove dangerous to strangers, though it is of no consequence to the people who are used to it. It is not fixed to any particular place, but arises in various parts of the limits of the sea among these islands. Wherever it appears, it is very furious, and boats would inevitably be drawn in and perish with it, but the people who navigate them are prepared for it and always carry a bundle of straw or some such matter in the boat with them. This they fling into the vortex which immediately swallows it up, and, seemingly pleased with this propitiatory offering, subsides into smoothness, but soon after re-appears in another place.
A remarkable and sudden rising of the spring-tide takes place at the mouth of several rivers, for instance, the Indus (where the surprising phenomenon nearly caused the destruction of the fleet of Alexander the Great), the Hooghly, the Dordogne, &c. In the Seine it is observed on a scale of great magnitude. While the tide gradually rises near Havre and Harfleur, a giant wave is suddenly seen to surge near Quillebœuf, spanning the whole width of the river (from 30,000 to 36,000 feet). After this mighty billow has struck against the quay of Quillebœuf, it enters a more narrow bed and flows stream-upwards with the rapidity of a race horse, overflowing the banks on both sides, and not seldom causing considerable loss of property by its unexpected appearance. The astonishment it causes is increased when it takes place during serene weather, and without any signs of wind or storm. A deafening noise announces and accompanies this sudden swelling of the waters, which owes its first origin to the silent action of gravitation, and is the result of the diminishing velocity of the tide-wave over a shallow bottom.
While the tide-wave advances over the deep and open seas with an astonishing rapidity, its progress up the channel of a river is comparatively very slow, partly on account of the reason just mentioned, and partly from its meeting a current flowing in an opposite direction.
Thus, the tide takes no less than twelve hours for its progress from the mouth of the Thames to London, about the time it requires to travel all the way from Van Diemen's Land to the Cape of Good Hope. Consequently, when it is high-water at the mouth of the Thames at three o'clock in the afternoon, for instance, we have not high-water at London Bridge before three o'clock in the following morning, when it is again high water at the Nore. But, in the mean time, there has been low water at the Nore and high water about half-way to London, and while the high water is proceeding to London, it is ebbing at the intermediate places, and is low water there when it is high water at London and at the Nore. If the tide extended as far beyond London as London is from the Nore, we should have three high waters with two low waters interposed. The most remarkable instance of this kind is afforded by the gigantic river of the Amazons, as it appears by the observations of Condamine and others, that, between Para, at the mouth of the colossal stream, and the conflux of the Madera and Marañon, there are no less than seven simultaneous high waters with six low waters between them. Thus, four days after the tide-wave was first raised in the Southern Ocean, its last undulations expire deep in the bosom of the South American wilds.
The Mediterranean is generally supposed to be tideless, but this opinion is erroneous; and in the Adriatic, the flux of the sea is far from being inconsiderable, for, at Venice, the difference between high and low water is sometimes no less than six or even nine feet. Mr. W. Trevelyan, during a summer residence in the old port of Antium, on the Roman coast, found from a series of accurate observations, that the tides regularly succeed each other and attain a height of fourteen inches. In the eastern Mediterranean new measurements have proved that they are still more considerable, while in the western part of that inclosed sea they are almost imperceptible.
The differences of level caused by the Mediterranean tides, are indeed too inconsiderable to attract the general notice of the inhabitants on the coast, but in the famed Euripus, the narrow channel which separates the island of Eubœa or Negropont from continental Greece, the tide produces the striking phenomenon of very irregular fluctuations of the waters, from one end of the channel to the other.
This phenomenon was of course completely inexplicable to the ancient philosophers, and Aristotle is even said to have drowned himself in the Euripus in a fit of despair, since, with all his prodigious sagacity, he could not possibly solve the mystery. For us, who know that peculiar formations of the sea-bed and coasts are capable of considerably augmenting the force of the floods, and that tidal waves rushing into a narrow channel in opposite directions, and at different times, must necessarily produce irregular fluctuations of the waters, the phenomenon of the Euripus has ceased to be a mystery.