Many features and phenomena of the ocean have been incidentally noted in the foregoing pages; but there are points, hitherto untouched, which deserve our attention.
Its saltness is due, not merely to the presence of chloride of sodium, or what we call common salt, but to a large number of other minerals, including the chlorides of magnesium and potassium, the sulphates of magnesia and lime, carbonate of lime, sulphuretted hydrogen, bromide of magnesia, hydrochlorate of ammonia, iodine, iron, copper, and even silver, varying in proportion according to locality. The copper plates of a ship examined at Valparaiso showed unmistakable traces of silver deposits. Calculations have been made showing that the ocean contains 2,000,000 tons of silver. In 1,000 grains of sea-water there are thirty-eight grains of these ingredients and some little organic matter. The saltness of the sea is generally greater towards the poles, but to this statement there are exceptions. In parts of the Irish Channel the water contains salts equal to the fortieth of its weight, the saline matter rising to one-sixteenth of its weight off the coast of Spain. In many places the ocean is less salt at the surface than at the bottom. Its saltness increases its density and its buoyancy.
Maury, a recognised authority, finds in the saline properties of the sea one of the principal forces from which the currents in the ocean proceed. “The brine of the ocean,” says he, “is the ley of the earth; from it the sea derives dynamical powers, and the currents their main strength.” Let us suppose a long tank or, say, swimming-bath, divided in the middle by a water-tight wall, on one side of which should be fresh and on the other salt water, at equal levels. It is obvious that were the division removed the waters would not stand side by side as before, for the denser water would have a tendency not merely to mingle with the lighter, but to form a current under it. So salt waters of different densities.
CHART OF THE ATLANTIC OCEAN.
“The ocean,” says Figuier, “is a scene of unceasing agitation; ‘its vast surface rises and falls,’ to use the image suggested by Schleiden, ‘as if it were gifted with a gentle power of respiration; its movements, gentle or powerful, slow or rapid, are all determined by differences of temperature.’ ” Heat increases its volume, and therefore [pg 91]lightens it; cold increases its density, and it will naturally descend. These are, then, among the obvious reasons of its currents. The duration and force of winds and the tides are both disturbing influences. Such an oceanic marvel as the great Gulf Stream could only be explained after a careful study of all the operating causes of its existence. Dr. Maury has well described it. He says:—“There is a river in the bosom of the ocean: in the severest droughts it never fails, and in the mightiest floods it never overflows; its banks and its bottom are of cold water, while its current is of warm; it takes its rise in the Gulf of Mexico, and empties itself into the Arctic seas; this mighty river is the Gulf Stream. In no other part of the world is there such a majestic flow of water; its current is more rapid than the Amazon, more impetuous than the Mississippi, and its volume is more than a thousand times greater.” This great current of water particularly influences the climates of Northern Europe, and especially those of Britain and Ireland.
The Gulf Stream, as it issues from the Florida Channel, has a breadth of thirty-four miles, a depth of 2,200 feet, and moves at the rate of four and a half miles an hour. “Midway in the Atlantic, in the triangular space between the Azores, Canaries, and Cape de Verd Islands, is the great Sargassum Sea, covering an area equal to the Mississippi Valley; it is so thickly matted over with the Gulf weed (Sargassum bacciferum) that the speed of vessels passing through it is actually retarded, and to the companions of Columbus it seemed to mark the limits of navigation: they became alarmed. To the eye, at a little distance, it seemed sufficiently substantial to walk upon.” The difference of temperature between the Gulf Stream and the waters it traverses constantly gives birth to tempests and cyclones. In 1780 a terrible storm ravaged the Antilles, in which 20,000 persons perished. The ocean quitted its bed, and inundated whole cities; the trunks of great trees and large parts of buildings were tossed wildly in the air. Numerous catastrophes of this kind have earned the Gulf Stream the title of the “King of the Tempests.” So well had Maury studied the Gulf Stream and its storms, that he was enabled to point out the exact position of a vessel overtaken by a terrible gale. “In the month of December, 1859,” says Figuier, “the American packet San Francisco was employed as a transport to convey a regiment to California. It was overtaken by one of these sudden storms, which placed the ship and its freight in a most dangerous position—a single wave, which swept the deck, tore out the masts, stopped the engines, and washed overboard 129 persons, officers, and soldiers. From that moment the unfortunate steamer floated upon the waters, a waif abandoned to the fury of the wind. The day after the disaster the San Francisco was seen in this desperate situation by a ship, which reached New York, although unable to assist her. Another ship met her some days after, but, like the other, could render no assistance. When the report reached New York two steamers were despatched to her assistance; but in what direction were they to go? what part of the ocean were they to explore? The authorities at the Washington Observatory were appealed to. Having consulted his charts as to the direction and limits of the Gulf Stream at that period of the year, Dr. Maury traced on a chart the spot to which the disabled steamer was likely to be driven by the current, and the course to be taken by the vessels sent to her assistance.” The steamers went straight to the exact spot, and found the wreck; and although by that time the crew and passengers had been taken off by three passing vessels, it was certainly a triumph of science.
WAVES OFF THE CAPE OF GOOD HOPE.
The tides are produced by two pairs of great waves which travel round the earth each day—a greater pair caused by the attraction of the moon, a lesser pair caused by the sun. The moon, by reason of its nearness to the earth, produces by far the greater influence, but the tides are also subject to all kinds of local influences. The eastern coast of Asia and western side of Europe are exposed to extremely high tides; while in the South Sea Islands they scarcely reach the height of twenty inches. There is hardly any tide in the Mediterranean, separated as it is from the ocean by a narrow strait. “The highest tide which is known occurs in the Bay of Fundy, which opens up to the south of the isthmus uniting Nova Scotia and New Brunswick. There the tide reaches forty, fifty, and even sixty feet, while it only attains the height of seven or eight in the bay to the north of the same isthmus. It is related that a ship was cast ashore upon a rock during the night so high, that at daybreak the crew found themselves and their ship suspended in mid-air, far above the water.” The winds have an immense influence on the height of tides, and also on the waves. The highest known waves are found off the Cape of Good Hope ([p. 89]) at the period of high tide, under the influence of a strong north-west wind which has traversed the Atlantic, pressing its waters round the Cape. “The billows there,” says Maury, “lift themselves up in long ridges, with deep hollows between them. They run high and fast, tossing their white caps aloft in the air, looking like the green hills of a rolling prairie capped with snow, and chasing each other in sport. Still, their march is stately and their roll majestic. Many an Australian-bound trader, after doubling the Cape, finds herself followed for weeks at a time by these magnificent rolling swells, furiously driven and lashed by the ‘brave west winds.’ These billows are said to attain the height of thirty, and even forty feet; but no very exact measurement of the height of waves is recorded.” Those off Cape Horn are rather less in height. Spray is dashed over the Eddystone Light, 130 feet high. After a great storm in Barbadoes in 1780, some old and heavy cannons were found on the shore, which had been thrown up from the bottom of the sea. If waves in their reflux meet with obstacles, whirlpools result, such as those in the Straits of Messina, between the rocks of Charybdis and Scylla made famous by Homer, Ovid, and Virgil, and once much dreaded, but now little feared.