Pachydermata

Pachydermata (Gr., 'thick-skins'), a term applied by Cuvier to hoofed mammals (Ungulates) which are not ruminants—e.g. elephants, hyrax, hog, hippopotamus, tapir, rhinoceros, horse, &c.—and which have thick skins. For many good reasons the term is no longer much used. See MAMMALS.

Pacific Ocean.—Position and Extent.—The Pacific Ocean is the largest of the great divisions of the ocean, occupying as it does about one-half of the water-surface of the globe and more than one-third of the whole area of the world. It is almost landlocked towards the north, communicating with the Arctic Ocean by the narrow and shallow Behring Strait, only about 40 miles in width, whereas towards the south it opens widely into the great deep Southern and Antarctic Oceans. Looking upon its southern boundary as the Antarctic Circle, its length from north to south is about 9000 miles, while its greatest breadth at the equator is over 10,000 miles. Its area is approximately nearly 70,000,000 sq. m.

History.—The Pacific was first seen by Europeans in 1513, when a Spaniard, Balboa, with a few followers, viewed its waters from the summit of a mountain in Panama; Columbus was aware of its existence, but did not live to see it. The first European to sail upon it was Magellan, who in 1520 entered it after threading his way through the strait bearing his name, and he gave it the designation 'Pacific,' by which it is known to the present day. From about this time trade was established between Europe and the Pacific coasts through the Strait of Magellan and round Cape Horn. Sir Francis Drake was the first Englishman to sail upon it, entering it in 1577, and afterwards sailing across it as far as the Moluccas. The explorers of the 17th century discovered Australia, New Zealand, and other islands, and during the 18th century the work of exploration was carried on by numerous voyagers, whose names are famous in the annals of geographical discovery. Many of them attempted to find passages between the Atlantic and Pacific through the Arctic Ocean; but the problem remained unsolved until Maclure in 1850 discovered the North-west Passage, and Nordenskiöld in 1874 the North-east Passage. The routes are, however, of no practical utility. During the 19th century many exploring and surveying expeditions have completely investigated the region of the Pacific, and among the more recent scientific expeditions special mention may be made of that of H.M.S. Challenger.

River-systems.—Compared with the enormous expanse of the Pacific the area of land draining into it is comparatively insignificant—7,500,000 sq. m., being less than half of that draining into the Atlantic. By far the greater proportion of the land of North and South America drains into the Atlantic, the Andes and Rocky Mountains, which form the watershed, running north and south in more or less close proximity to the Pacific coast. The largest American river is the Yukon in the extreme north, which is over 2000 miles in length, and flows into Behring Sea. Proceeding south, we find the Fraser (600 miles long), the Columbia or Oregon (750), the Sacramento (420), and the Colorado (1100). The South American rivers draining into the Pacific are little more than mountain-streams. The Asiatic rivers flowing into the Pacific include some of the largest and most important rivers of the world. There is the Amur, 3060 miles in length, flowing into the sea of Okhotsk, and with its tributaries draining an area of nearly 900,000 sq. m.; the Hoang-ho, over 3000 miles long, and the Yang-tse-kiang, 3200 miles in length, falling into the Yellow Sea, the combined drainage area of which two rivers is estimated to exceed 1,250,000 sq. m.; whilst flowing into the China Sea there are the Choo-kiang, the Mekhong, and the Menam. The rivers of Australia draining into the Pacific are of slight importance and small size. The total annual rainfall on the catchment basin of the Pacific is estimated at about 5000 cubic miles; the annual river discharge at a little over a fifth of that amount.

Coasts and Seas.—Generally speaking, the American and Australian coasts bordering the Pacific are mountainous and free from indentations, while the Asiatic coasts are low and fertile, with many gulfs and bays, and fringed with island groups enclosing numerous seas. The Alaskan shores of North America are low and swampy, while the coast farther south is rocky and rugged, with numerous inlets and off-lying islands. The most considerable indentation of the whole American Pacific coast is the Gulf of California, the Gulfs of Panama and Guayaquil being the only others of importance. The southern extremity of South America presents a complete contrast to the rest of the coast-line, being broken up into numerous bays with scattered islands, the winding Strait of Magellan separating Tierra del Fuego from the mainland. The contour of the Asiatic coast-line is much more diversified than that of America, being especially characterised by the off-lying seas more or less completely enclosed and cut off from communication with the open ocean. Behring Sea is separated from the Pacific basin by the peninsula of Alaska and the Aleutian Islands, communicating with the Arctic Ocean through Behring Strait. The Sea of Okhotsk is divided from Behring Sea by the peninsula of Kamchatka, and from the basin of the Pacific by the Kurile Islands. The Sea of Japan is cut off from the ocean by the Japanese islands, from the Sea of Okhotsk by the island of Saghalien, and from the Yellow Sea by the peninsula of Corea. The Yellow Sea is an extensive indentation of the Chinese coast, and is so named from the large amount of ochreous material brought down by the great rivers Hoang-ho and Yang-tse-kiang, which flow into it. The China Sea is separated from the Pacific by the island of Formosa, the Philippine Islands, the island of Palawan, and Borneo, and from the Indian Ocean by the Malay peninsula; it includes the two extensive Gulfs of Tonquin and Siam. The islands of the East Indian Archipelago cut up this part of the Pacific into several more or less distinct seas, known as the Sulu, Celebes, Java, Banda, and Arafura Seas, the last named lying between the north coast of Australia and New Guinea, and including the Gulf of Carpentaria. The Coral Sea is enclosed by the north-east coast of Australia, New Guinea, New Britain, the Solomon Islands, the New Hebrides, and New Caledonia, and communicates with the Arafura Sea by Torres Strait. The Pacific coast of Australia is mountainous and free from any considerable inlets, the most important harbours being Moreton Bay and Port Jackson, the latter one of the finest in the world. Bass Strait separates the island of Tasmania from Australia. The main islands of New Zealand are separated by Cook Strait, and the principal bays are the Gulf of Hauraki, Bay of Plenty, Hawke Bay, and Pegasus Bay.

Islands.—The Pacific Ocean is remarkable for the innumerable small islands and island groups which stud its surface, but the area occupied by the truly oceanic islands is very small; they are principally congregated towards the central and western portions of its basin, the eastern portion, for some considerable distance off the American coasts, being comparatively free from islands. The principal continental islands may be briefly enumerated: commencing at the southern point of

South America, and proceeding northwards along the American coast, then southwards along the Asiatic coast, we have Tierra del Fuego and the islands off the coast of Chili; Vancouver, Queen Charlotte, Prince of Wales, and other islands off the coast of British North America; Kodiak Island, off the Alaskan coast; the Aleutian chain of islands, stretching from the Alaskan peninsula towards the Asiatic coast and enclosing Behring Sea; the Kurile Islands, stretching from the peninsula of Kamchatka to the Japanese Islands; Saghalien; the islands of Japan; Formosa and Hainan, off the Chinese coast; the Philippine Islands; Borneo, Celebes, Sumatra, Java, and other islands of the East Indian Archipelago; New Guinea; New Caledonia; Australia and Tasmania; and New Zealand. The oceanic islands of the Pacific are all either of volcanic or coral origin, the volcanic islands lying within the zone of coral-reef builders being fringed with coral-reefs, while there are large numbers of islands entirely of coral formation—coral atolls. The principal groups are the Sandwich Islands, in the centre of the North Pacific basin, $18^\{\backslash circ\}$ to $22^\{\backslash circ\}$ N. lat., consisting of eight larger and four smaller islands, containing many active and extinct volcanoes, including the well-known Kilauea in Hawaii, said to be the largest active crater in the world; the Bonin Islands, south-east of Japan; the Ladrone or Mariana Islands, between $13^\{\backslash circ\}$ and $20^\{\backslash circ\}$ N. lat., containing several active volcanoes; the Caroline Islands, south of the Ladrones, mostly of coral formation; the Marshall Islands, east of the Carolines, entirely of coral formation; the Gilbert Islands, on the equator, of coral formation and densely populated; in the South Pacific there are the Solomon Islands, the New Hebrides, the Fiji Islands, the Friendly Islands, the Samoa or Navigator Islands, the Society Islands, all fringed by coral-reefs, and the Paumotu or Low Archipelago, an extensive group of coral islands lying between $10^\{\backslash circ\}$ and $25^\{\backslash circ\}$ S. lat., besides the volcanic Galapagos group on the equator about 600 miles off the coast of Ecuador, and others of less extent and importance.

Depth.—The Pacific was formerly looked upon as rather a shallow ocean, but we now know that some of the greatest depths in the world occur in it, and that on the whole it is deeper than the Atlantic, its mean depth being about 2500 fathoms. The eastern basin is comparatively uniform in depth, between 2000 and 3000 fathoms, except a large area under 2000 fathoms extending from off the coast of Chili in a westerly direction for over $40^\{\backslash circ\}$ of longitude, while off the north-west coast of North America the 2000 fathom line lies a considerable distance off-shore. The western basin is much more diversified, numerous groups of islands, shallow water, and immense depths occurring irregularly; the greatest depths yet sounded are found in this region of the Pacific. The Challenger's deepest sounding, 4575 fathoms (nearly $5\backslash frac\{1\}\{2\}$ miles), was in the sea between the Caroline and Ladrone Islands, while the American ship Tuscarora found a depth of 4655 fathoms to the north-east of Japan, where a large area of very deep water extends off the Kurile Islands and Japanese coast; more recently depths of over 4000 fathoms have been discovered off the coast of Chili, and a British surveying ship has sounded in 4530 fathoms east of the Fiji Islands, which is the deepest sounding recorded south of the equator. There are many detached patches throughout the Pacific with depths of over 3000 fathoms. The seas bordering on the western basin of the Pacific vary considerably in depth: the depth in the Sea of Okhotsk, the Yellow Sea, and the Java Sea does not apparently exceed 700 fathoms; and the Behring Sea, the Sea of Japan, and the Arafura

Sea are all under 1500 fathoms; while the China Sea, Celebes Sea, Sulu Sea, and the Banda Sea are in some places over 2000 fathoms in depth. The bulk of water filling the Pacific is estimated at 170,000,000 cubic miles.

Winds and Currents.—The surface-currents of the Pacific Ocean depend to a great extent upon the direction of the prevailing winds, the principal of which are the two trade-winds, blowing more or less constantly, the one from the north-east, the other from the south-east. Between these two regions is what is called the equatorial belt of calms, which is found all the year round north of the equator in the eastern Pacific, but in the west Pacific it is south of the equator during the summer of the southern hemisphere, and during the southern winter it is replaced by a regular southerly breeze; north and south of the trade-winds, also, there are two other belts of calms. In addition to the trade-winds, there are the monsoons, which blow with great regularity, but the direction of which changes according to the season. Monsoons are especially prevalent in the west Pacific, their general direction being south-east, north-east, or north-west, and they cause surface-currents, the direction of which likewise changes with the season. The differences between the temperature and atmospheric pressure over the land and over the water cause monsoonal winds. In mid-ocean the winds are found to have a greater velocity than in the vicinity of the land.

The Pacific is practically cut off, as far as the circulation of the deep water is concerned, from communication with the Arctic Ocean in the north, but towards the south it has uninterrupted communication with the Antarctic. A cold surface-current flows constantly northwards from the Antarctic, dividing into two at Cape Horn, one entering the Atlantic, the other flowing along the coasts of Chili and Peru, thence turning to the westwards; but the cold water frequently met with along the eastern coasts of America is evidently brought from oceanic depths by the action of off-shore winds. The great equatorial current flows to the westward, divided by a counter-current running in an opposite direction into two branches, the northern one on approaching the Asiatic coast being deflected northwards and finally north-eastwards as the Japan current, which is comparable to the Gulf Stream in the Atlantic; the southern branch is diverted to the southward, flowing along the shores of Australia and New Zealand, thence curving eastwards, and ultimately merging into the Antarctic surface-current. There are many minor currents, and branches of these more important ones, diverted by the numerous groups of islands. The broad currents, circling in the one direction in the North Pacific and in the opposite direction in the South Pacific, enclose in their centres two miniature Sargasso Seas somewhat similar to that of the North Atlantic, though not so well marked.

Temperature of the Water.—The temperature of the surface-waters of the Pacific varies with the season, but in the tropical regions the variation is very small. Between the latitudes of $45^\{\backslash circ\}$ N. and $45^\{\backslash circ\}$ S. the temperature of the surface is always above $50^\{\backslash circ\}$ F., while north and south of these latitudes it is nearly always below $50^\{\backslash circ\}$ F. The highest temperature occurs among the islands of the Malay Archipelago and off the Mexican coast, where the mean temperature rises to $85^\{\backslash circ\}$ F., and in the sea between Japan and New Guinea the temperature in August reaches $84^\{\backslash circ\}$ F. In the South Pacific the temperature of the surface-water is apparently higher than that of the air, while in the North Pacific the reverse is the case in some places. The temperature of the water below the surface as a general rule decreases as the depth increases, the lowest temperature occurring at the bottom in great depths, where the bottom temperature appears to be nearly constant all the year round, usually about 35° F. This refers only to the open ocean, for in the enclosed seas of the western basin of the Pacific the minimum temperature is usually found some distance above the bottom, depending upon the depth of water over the barrier cutting off the sea from the general oceanic circulation. (See SEA, Vol. IX., p. 272). The temperature of the intermediate water in the open ocean decreases rapidly at first from the surface downwards, and then slowly down to the bottom, irrespective of the surface temperature, which may vary from below 60° to over 80° F.

Salinity.—The salinity of the surface-waters of the ocean changes with the season; increase of evaporation raises, while precipitation in the form of rain lowers, the salinity. In the South Pacific there is a region of high salinity in the neighbourhood of the Society Islands, the maximum salinity being 1.02750 (taking pure water at 4° C. as unity); in the North Pacific the salinity is never so high, the maximum being 1.02650, while in some regions the salinity falls to 1.02485.

For the deposits see the article SEA; see also works cited at CHALLENGER, CORAL, POLYNESIA.