Page:EB1911 - Volume 20.djvu/470

 Atlantic. The result is that north-westerly winds, which in winter are exceedingly dry and cold, blow over the western or Asiatic area; westerly winds prevail in the centre, and south-westerly and southerly winds off the American coast. In the southern hemisphere there is a transition to the low-pressure belt encircling the Southern Ocean, in which westerly and north-westerly winds continue all the year round.

The distribution of temperature in the waters of the Pacific Ocean has been fully investigated, so far as is possible with the existing observations, by G. Schott. At the surface an extensive area of maximum temperature (over 20° C.) occurs over 10° on each side of the equator to the west of the ocean. On the eastern side temperature falls to 22° on the equator and is slightly higher to N. and S. In the North Pacific, beyond lat. 40°, the surface is generally warmer on the E. than on the W., but this condition is, on the whole, reversed in corresponding southern latitudes. In the intermediate levels, down to depths not exceeding 1000 metres, a remarkable distribution appears. A narrow strip of cold water runs along the equator, widest to the east and narrowing westward, and separates two areas of maximum which have their greatest intensity in the western part of the ocean, and have their central portions in higher latitudes as depth increases, apparently tending constantly to a position in about latitude 30° to 35° N. and S. A comparison of this distribution with that of atmospheric pressure is of great interest. High temperature in the depth may be taken to mean descending water, just as high atmospheric pressure means descending air, and hence it would seem that the slow vertical movement of water in the Pacific reproduces to some extent the phenomena of the “doldrums” and “horse latitudes,” with this difference, that the centres of maximum intensity lie off the east of the land instead of the west as in the case of the continents. The isothermal lines, in fact, suggest that in the vast area of the Pacific something corresponding to the “planetary circulation” is established, further investigation of which may be of extreme value in relation to current inquiries concerning the upper air. In the greater depths temperature is extraordinarily uniform, 80% of the existing observations falling within the limits of 1·6° C. and 1·9° C. In the enclosed seas of the western Pacific, temperature usually falls till a depth corresponding to that of the summit of the barriers which isolate them from the open ocean is reached, and below that point temperature is uniform to the bottom. In the Sulu Sea, for example, a temperature of 10·3° C. is reached at 400 fathoms, and this remains constant to the bottom in 2500 fathoms.

The surface waters of the North Pacific are relatively fresh, the salinity being on the whole much lower than in the other great oceans. The saltest [sic] waters are found along a belt extending westwards from the American coast on the Tropic of Cancer to 160° E., then turning southwards to the equator. North of this salinity diminishes steadily, especially to the north-west, the Sea of Okhotsk showing the lowest salinity observed in any part of the globe. South and east of the axis mentioned salinity becomes less to just north of the equator, where it increases again, and the saltest waters of the whole Pacific are found, as we should expect, in the south-east trade-wind region, the maximum occurring in about 18° S. and 120° W. South of the Tropic of Capricorn the isohalines run nearly east and west, salinity diminishing quickly to the Southern Ocean. The bottom waters have almost uniformly a salinity of 34-8 per mille, corresponding closely with the bottom waters of the South Atlantic, but fresher than those of the North Atlantic.

The surface currents of the Pacific have not been studied in the same detail as those of the Atlantic, and their seasonal variations are little known except in the monsoon regions. Speaking generally, however, it may be said that they are for the most part under the direct control of the prevailing winds. The North Equatorial Current is due to the action of the

north-east trades. It splits into two parts east of the Philippines, one division flowing northwards as the Kuro Siwo or Black Stream, the analogue of the Gulf Stream, to feed a drift circulation which follows the winds of the North Pacific, and finally forms the Californian Current flowing southwards along the American coast. Part of this rejoins the North Equatorial Current, and part probably forms the variable Mexican Current, which follows the coasts of Mexico and California close to the land. The Equatorial Counter-Current flowing eastwards is largely assisted during the latter half of the year by the south-west monsoon, and from July to October the south-west winds prevailing east of 150° E. further strengthen the current, but later in the year the easterly winds weaken or even destroy it. The South Equatorial Current is produced by the south-east trades, and is more vigorous than its northern counterpart. On reaching the western Pacific part of this current passes southwards, east of New Zealand, and again east of Australia, as the East Australian Current, part northwards to join the Equatorial Counter-Current, and during the north-east monsoon part makes its way through the China Sea towards the Indian Ocean. During the south-west monsoon this last branch is reversed, and the surface waters of the China Sea probably unite with the Kuro Siwo. Between the Kuro Siwo and the Asiatic coast a band of cold water, with a slight movement to the southward, known as the Oya Siwo, forms the analogue of the “Cold Wall” of the Atlantic. In the higher

latitudes of the South Pacific the surface movement forms part of the west wind-drift of the Roaring Forties. On the west coast of South America the cold waters of the Humboldt or Peruvian Current corresponding to the Benguela Current of the South Atlantic, make their way northwards, ultimately joining the South Equatorial Current. The surface circulation of the Pacific is, on the whole, less active than that of the Atlantic. The centres of the rotational movement are marked by “Sargasso Seas” in the north and south basins, but they are of small extent compared with the Sargasso Sea of the North Atlantic. From the known peculiarities of the distribution of temperature, it is probable that definite circulation of water is in the Pacific confined to levels very near the surface, except in the region of the Kuro Siwo, and possibly also in parts of the Peruvian Current. The only movement in the depths is the slow creep of ice-cold water northwards along the bottom from the Southern Ocean; but this is more marked, and apparently penetrates farther north, than in the Atlantic.

See Reports of expeditions of the U.S.S. “Albatross” and “Thetis.” 1888–1892; A. Agassiz, Expedition to the Tropical Pacific, 1899–1900, 1904–1905; H.M.S. “Challenger,” 1873–1876; “Egeria.” 1888–1889 and 1899; “Elisabeth,” 1877; “Gazelle,” 1875–1876; “Planet,” 1906; “Penguin,” 1891–1903; “Tuscarora,” 1873–1874; “Vettor Pisani,” 1884; “Vitraz,” 1887–1888; also observations of surveying and cable ships, and special papers in the Annalen der Hydrographie (for distribution of temperature see G. Schott, p. 2, 1910).

Up to a certain point, the islands of the Pacific fall into an obvious classification, partly physical, partly political. In the west there is the great looped chain which fringes the east coast of Asia, and with it encloses the series of seas which form parts of the ocean. The north of the chain, from the Kuriles to Formosa, belongs to the empire of Japan; southward it is continued by the Philippines (belonging to the United States of America) which link it with the vast archipelago between the Pacific and Indian oceans, to which the name Malay Archipelago is commonly applied. As the loop of the Kuriles depends from the southern extremity of Kamchatka, so from the east of the same peninsula another loop extends across the northern part of the ocean to Alaska, and helps to demarcate the Bering Sea; this chain is distinctly broken to the east of the Commander Islands, but is practically continuous thereafter under the name of the Aleutian Islands. Islands form a much less important feature of the American Pacific coast than of the Asiatic; between 48° N. and 38° S. there are practically none, and to the north and south of these parallels respectively the islands, though large and numerous, are purely continental, lying close under the mainland, enclosing no seas, and forming no separate political units. South-eastward of the Malay Archipelago lies “the largest island and the smallest continent,” Australia; eastward of the archipelago, New Guinea, the largest island if Australia be regarded as a continent only. With Australia may be associated the islands lying close under its coasts, including Tasmania. Next follow the two great islands and attendant islets of New Zealand.

There now remains a vast number of small islands which lie chiefly (but not entirely) within an area which may be defined as extending from the Philippines, New Guinea and Australia to 130° W., and from tropic to tropic. These islands fall principally into a number of groups clearly enough defined to be well seen on a map of small scale; they are moreover divided, as will be shown, into three main divisions; but whereas they have enough characteristics in common to render a general view of them desirable, there is no well-recognized name to cover them all. The name Polynesia was formerly taken to do so, but belongs properly to one of the three main divisions, to which the name Eastern Polynesia was otherwise given; Oceania and Oceanica are variants of another term which has been used for the same purpose, though by no means generally. Moreover usage varies slightly as regards the limits of the three main divisions, but the accompanying table shows the most usual classification, naming the principal groups within each, and distributing them according to the powers to which they are subject.

The following islands may be classified as oceanic, but not with any of the three main divisions: the Bonin Islands, north of the Marianas, belonging to Japan; Lord Howe and Norfolk Islands (to