Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/134

PACIFIC OCEAN The vast extent of the Pacific Ocean gives full scope for the current-producing action of tides and winds, while the smooth continental boundary on its eastern side, the numerous groups of islands which break its surface, and its indented western coast, combine to modify the direction of the main streams and to produce innumerable minor currents, some permanent, and others varying from time to time in velocity and direction. The chief cause of these currents is believed to be traceable to the direct or indirect action of wind ; but here it is proposed to refer merely to their general geography and physical effects, without dis cussing the theory of their formation. A general surface drift of the cold waters of the Antarctic Ocean, having a temperature lower than 40 Fahr. at all seasons, bears north-east towards Cape Horn, where it divides into two branches ; one, the Cape Horn current, passes on into the Atlantic, and the other sweeps northward along the west coast of South America until it strikes the Peruvian shore, which deflects it westward. The cooling effect of this current on the water all along the coast is illustrated very clearly by the abrupt north ward turn of the isothermals (see METEOROLOGY, figs. 8 and 9), which is more conspicuous in the chart for the southern winter than in that for the summer. In summer, however, there is a more striking evidence of this current s cooling power to be seen in the arrangement of the isothermals. The northern line of 70 Fahr. reaches as far south as 18 N. lat., and that of 80 makes a short loop from 1 8 N. to the equator ; but the southern isothermal of 80 does not touch the American coast at all, and that of 70 lies farther from the equator than 30 S. lat., so that the increase of temperature from the south is very gradual ; so much so that at the Galapagos Islands, under the equator, the temperature of the surface water is only 70, while a few hundred miles to the west it is over 80. Penguins essentially Antarctic birds are found living on the shores of these islands. In consequence of this current, the highest surface temperature at all seasons of the year is found distinctly ,to the north of the equator in the eastern Pacific. The Peruvian current forms the southern fork of the great equatorial current, which runs due west. This current is very broad, and divided by a narrow counter- current flowing in an opposite direction through its centre. The two branches of the equatorial current occupy very approximately the two areas of falling barometer between the north and south belts of high pressure and the central trough of minimum barometric readings. This difference of atmospheric pressure on each side produces the north east and south-east trade winds, and to these the current probably owes its regularity and constant direction. The counter-current lies in the narrow belt of low barometric pressure to which the trades blow, and probably originates from the banking up of the waters to the westward. Its rate and position consequently vary greatly at different times of the year. The " Challenger," on her cruise between the Sandwich and Society Islands, found these currents to run with considerable force. In the " Narra tive" of the cruise (chap.xviii.) the fact is alluded to thus: " From Hawaii Island to the 10th parallel the direction of the current was westerly, and its average velocity 18 miles per day, ranging from 10 to 23 miles. From the 10th to the 6th parallel the direction was easterly, and its average velocity 31 miles per day, ranging from 7 to 54 miles per day. From the 6th parallel of north latitude to the 10th parallel of south latitude the direction was again westerly, and the average velocity 35 miles per day, ranging from 17 to 70 miles per day. From thence to Tahiti the general tendency of the current was westerly, but its velocity was variable. The axis of greatest velocity of the counter-equatorial current was between the 7th and 8th parallels of north latitude. The axis of greatest velocity of the equatorial current was on the parallel of 2 north, where its speed amounted to 3 miles per hour." The equatorial current strikes on the East Indian Archipelago, where it is split up by the narrow channels and shallow waters, and diverted into numberless minor currents. The two main divisions, which have acquired a high temperature from prolonged exposure to the tropical sun, ultimately leave the archipelago ; the southern arm curves southwards, carrying its warm water to the east coast of Australia and to New Zealand, whence it is diverted towards the east, and becomes merged again in the general north-easterly antarctic drift. The north equatorial current, which varies in volume and velocity with the monsoons, strikes the coast of Asia between the Philippines and Japan, and is deflected in a north-easterly direction as the Kuro-Siwo or Japan current wholly a warm oceanic river during the S.E. monsoon similar to the Gulf Stream of the Atlantic. The Japan current sends many branches into the inland seas and channels of the north-eastern coast of Asia, but the main body of water flows northward until it bifurcates in 40 N. lat., send ing one fork among the Kurile Islands and along the Kamchatka peninsula into Behring Sea, whence it escapes- by Behring Strait into the Arctic Ocean. A small counter-current of arctic water flows southward through Behring Sea, but it is not of sufficient volume to make its influence felt very decidedly on the general temperature of the surface water in the vicinity. The second and larger branch of the Japan current crosses the North Pacific, and, curving southward by Alaska and British Columbia, part of it returns as the north equatorial current, while the rest forms the variable Mexican current that runs along the coasts of California and Mexico. The general direction of surface circulation in the Pacific may be remembered by supposing the ocean divided into a northern and southern half by the equatorial counter- current. In the northern half the water circulates in the direction of the hands of a watch, i.e., it passes up the west coast and down the east, while in the southern half the rotation is in the opposite direction down the west coast and up the east; but the latter half does not exhibit the complete cycle so distinctly as the former. The centre of each area of circulation is occupied by a small Sargasso Sea, the northern being the more clearly defined, but neither approaches the well-known Sargasso Sea of the North Atlantic either in definiteness, extent, or amount of weed. Temperature of Surface Wafer. The distribution of Surface temperature in the surface water of the Pacific varies con- tcrn P eri siderably during the year. The equatorial region is of course comparatively little affected by the change of season, but there is a general rise of temperature in the northern parts of the ocean, and a fall in the southern, during the northern summer, and a similar rise in the south and fall in the north during winter. The charts exhibit a general northward move in the isothermals during the former season, and a southward tendency in the latter. The change in the position of the lines is greatest in the temperate zones. The charts of ocean surface tempera ture (see METEOROLOGY, figs. 8, 9) for February and August show the direction of the isothermals at two opposite seasons ; and reference to them will make it plain that in temperate regions the lines of equal temperature follow the parallels of latitude much more closely in the Pacific than in the Atlantic, while their displacement with the change of season takes place in a direction nearly north and south. There are notable instances of divergence from these rules, such as the peculiarity of the isothermal of 80 already alluded to. Another circumstance is the fact that the temperature of the surface water on the western side of a great continent is much lower than that on the eastern side in the same latitude ; it seems as if the west side of