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The Indian Ocean is that of gulfs; for the two great Gulfs of Bengal and the Persian Sea impress upon it its character. It pushes, besides, into the interior two midland seas, the Persian Gulf and the Red Sea, which detach the peninsula of Arabia from the rest of the continent.
The Atlantic Ocean is that of inland seas. No one advances further into the lands, piercing into the very heart of the Old World and the New. There are, at least, four mediterraneans, without taking into the account the Polar Seas ; two on the European side, the Mediterranean, properly so called, divided into three great basins—the Eastern, the Western, the Black Sea, not to mention several others of small size—and the Baltic; two on the coasts of the New World, the Gulf of Mexico, and Hudson's Bay. Neither is the form of land-locked seas wanting here also; the Northern Ocean, on the coasts of the Old World ; the Caribbean Sea, in Central America, closed by the peninsula of Yucatan and the chain of the greater and lesser Antilles; the Gulf of St. Lawrence, locked by the peninsula of Nova Scotia and Newfoundland, are the proof. The great gulfs are represented by those of Guinea and of Biscay. The Atlantic Ocean is, then, the most articulated, the most indented of the oceans, and that which, by its blending with the lands, approaches the nearest to the character of the inland seas. It is, if we may venture to say so, the most maritime of the oceans, as the Pacific is the most truly oceanic.
The islands, finally, are one of the most interesting characteristics of the oceans. There are two species to be distinguished: the continental islands, which their proximity, their size, their geological character, their forming a line with the mountain chains of the firm land, prove to be a dependence of the continents; and the pelagic or oceanic islands, dispersed singly, or in groups, at a distance from the lands, over the vast surface of the ocean, of small dimensions, and always of a volcanic or coralline character.
The Pacific Ocean is far the richest in islands, whether continental or pelagic. The Indian archipelago and that of New Holland is the largest continental archipelago existing on the surface of the globe; and the thousands of pelagic islands with which the centre of this ocean is studded, have nowhere else their parallel.
The Atlantic Ocean possesses still, in the group of the Antilles, the British Isles, and those of the Mediterranean,
continental archipelagoes of great importance; but the pelagic islands are poorly represented there by the groups of the Azores, Madeira, the Canaries, Cape Verd, St. Helena, and some other small islands lost in the midst of the ocean.
The Indian Ocean is scanty in both. Madagascar and Ceylon represent the continental islands. Here and there a few volcanic islands, as Mauritius and Bourbon, represent the pelagic.
Each ocean, therefore, differs from the others in some peculiarities of character; and we readily conceive how these circumstances may modify their importance with regard to the facility or the difficulty they may bring into the relations of exchange which commerce establishes among
all the nations of the world.
The basin of the oceans is depressed below the face of their waters, as the continents are elevated in the atmosphere above the same surface level. It may then be said, that we know not one-half of the reliefs of the solid crust of our globe, for more than two-thirds are concealed from our observation by the seas that cover them.
In the neighbourhood of the continents the seas are often shallow, and their bottom seems to be only the continuation, by gentle slopes, of the relief of the continents which border them. Thus the Baltic Sea has a depth of only 120 feet between the coasts of Germany and those of Sweden, scarcely a twentieth part of that of Lago Maggiore, in the Italian Alps ; farther north, it becomes deeper. The Adriatic between Venice and Trieste has a depth of only 130 feet. In these two cases we see that the bed is only the continuation of the gentle inclination of the plains of Northern Germany and of Friuli. It is the same with the Northern Sea, and with those which wash the British Islands. Here is found a submarine plateau, which serves as a common basis for the coasts of France and the British Islands ; nowhere does it sink lower than 600 feet, and frequently it rises much higher. Between France and England the greatest depth does not exceed 300 feet; but at the edge of the plateau, south-west of Ireland, for example, the depth suddenly sinks to more than 2000 feet; we may say that here the basin of the Atlantic really begins.
The seas in the south of Europe are distinguished from the preceding by their much greater depths. The basin of
the Mediterranean may be called a basin broken through, and fallen in, resembling on a small scale what the Pacific Ocean is on a large one. All the short and abrupt slopes of the lands surrounding it fall rapidly towards the interior. The western basin, in particular, seems to be very deep; it is isolated from the Atlantic by a submarine ridge or neck, which in the narrowest part of the strait of Gibraltar, is not more than 1000 feet below the surface. But a little farther towards the east the depth falls suddenly to 3000 feet; and at the south of the coast of Spain, and of the Sierra Nevada, a depth of nearly 6000 has been ascertained by Captain Smith. Captain Berard indicates still greater depths on the coast of Algeria. If we may believe Marsigli, and if he has not made some mistake in the statement, there has been found in the prolongation of the Pyrenees the enormous depth of 9000 feet. Not far from Cape Asinara, on the north-west of Sardinia, the plummet has been sunk, without touching bottom, at a depth of nearly 5000 feet.
Between Sicily and the coast of Africa, at Cape Bon, a second neck, from 50 to 500 feet in depth, separates the western from the eastern basin of the Mediterranean. The latter seems to be less deep than the former; nevertheless, depths of from 2000 to 3000 feet have been determined in the neighbourhood of the Ionian Islands and the southern coast of Asia Minor.
The line of the islands and the peninsulas forming, along the eastern coast of Asia, the numerous locked seas which we have already named, seems to indicate the ancient border of a continent. Within this line these seas have only an inconsiderable depth. The seas which bathe the archipelago of the Sunda Islands, and of Southern China, scarcely anywhere reach the depth of 300 feet. Farther north, we find scarcely four or five hundred feet, even at a distance of more than 100 miles from the coasts. The deeps of the ocean begin only outside of the line of the islands.
Since Dampier, it has often been said that the sea is always deep at the foot of high and steep shores, and shallow at the edge of low coasts. The facts which have just been cited prove that this observation, correct in many cases, has only a relative value, and does not hold good universally. Those shallow seas of Eastern Asia are edged in great part by very high lands. The massive point of the south of Africa ends with abrupt coasts, and yet it is necessary to go out more than 100 miles before finding 600 feet of water. According to this rule, we should expect to find no greater depth of sea than at the Western foot of the lofty Andes, the declivities of which sweep down so suddenly into the Pacific Ocean; and nevertheless, under the parallel of Lima, this ocean has only 600 feet, more than forty miles from the coast. On the other hand, the low plains of the Landes of Bordeaux, on the coast of France, which lie along the Gulf of Biscay, look out upon a sea, the bottom of which, at a short distance, sinks already lower than a thousand feet.
In Central America, according to Humboldt, the Gulf of Mexico, 100 miles north of Yucatan, has a depth of only 600 feet; it is the submarine continuation of the plains of the Mississippi. Beyond the line of the Antilles, on the contrary, in the volcanic basin of the Caribbean Sea, Captain Sabine indicates a temperature taken at 6000 feet below the surface.
With regard to the depths of the open sea, they are still but little known; we have, however, a few in the Atlantic Ocean. Captain C. H. Davis, U.S.N., sunk the lead 7800 feet in the neighbourhood of the coasts of the United States, about 250 miles south of Nantucket. It is the greatest that has been reached in the middle latitudes of the Atlantic. Captains Scoresby) and Parry have found the bed of the Polar seas very deep, but variable. Scoresby did not touch the bottom at 76° north latitude with a sounding line of 7200 feet in length. Captain Ross has exceeded 6000 feet in Baffin's Bay. But the most astonishing depths that have been ascertained are found in the Southern Atlantic. Captain James Ross has found 16,000 feet west of Cape of Good Hope, and he has sounded with the plummet 27,600 feet, without touching bottom, west of St. Helena. The first of these measures is equal to the height of Mont Blanc; the second, to that of Dhawalaghiri. Thus the greatest known sea depth, added to the elevation of the highest mountain of the globe, gives us about 55,600 feet for the thickness of the layer of our globe, upon which our investigations have given some information.-Guyot.
1 Now Rev. Dr. Scoresby.
If we knew only the winds that blow in our temperate regions, we should almost despair of arriving at the knowledge of any law regulating their course. What is more fickle, more capricious, than the winds which suddenly change their direction, their force and temperature, without apparent cause, and inaccessible to our means of observation? They are the symbols of changeableness itself. But it is not so when we enter upon the equatorial seas, where, from one end of the year to the other, a gentle and regular wind blows from the east to the west with great constancy, and carries slowly and without violence the ships from the coasts of the Old World to those of the New; these are the trade winds. We know the astonishment and alarms of the companions of Columbus on noticing these winds, the constant direction of which towards the west seemed to render their return impossible. In the East India seas the winds blow six months from the north-east and six months from the south-west. These are the monsoons. This regularity of the tropical winds indicates the existence of permanent causes, of which it is, perhaps, possible to give some account.
At any rate, the phenomenon takes a certain course, which is annually repeated, and of which we ought to take cognizance; for, in case of need, the knowledge of the run of the atmospheric currents, independently of their causes, may be sufficient for our purpose.
The winds are the consequence of a disturbance of equilibrium in the layers of the atmosphere; and the tendency of their motion is to restore the equilibrium which has been destroyed; as soon as that is accomplished the movement ceases, and everything settles into a calm.
The more we study the causes of these disturbances of the atmospherical equilibrium, and of the winds, the more we see that they are reduced, essentially, almost entirely, to differences of temperature between neighbouring places. Here, again, the law of differences is the principle of movement, the condition of life.
One of the chief conditions of the equilibrium of the atmosphere is, that any level layer of the atmosphere should have the same density at all points. If this condition is not fulfilled, the denser portions flow under the less dense, while