Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/133

 PACIFIC OCEAN 117 Behring Sea opposite the island of St Lawrence. Its tributaries have not been fully explored, so the area which they intersect is unknown, but probably it is very large. The Asiatic division of the Pacific river-system is very much more extensive than the American, and includes many streams of great size and of considerable commercial importance. In the north the Amur is more than 2000 miles long, and it receives many tributaries, which rise on the north in the Stanovoi mountains, and on the west and south on the borders of the great table-land of the Gobi, the central Asiatic desert ; altogether its basin measures nearly 900,000 square miles. The Hoang-ho (Hwang-ho or Whang-ho) and the Yangtze-keang both rise near the Kuen-lun mountains of Tibet amongst the extensive terraces which form the eastern slope of the great table-land of Central Asia. The Hoangho has a length of 2(500 miles, and in its course it sweeps in a northerly curve close to the In-Shan mountains ; then, after being crossed repeatedly by the Great Wall of China, it turns sharply to the south, and finally runs due east into the Yellow Sea. The Yangtze-keang follows a southward direction from its source, but ultimately turns to the north-east and enters the Yellow Sea not far from the mouth of the Hoang-ho. It is one of the longest rivers in the world, for, including its windings, it measures 3200 miles from its source to the sea. These two rivers drain more than a million and a quarter square miles ; and it is principally owing to the large amount of suspended matter which they carry down that the sea into which they fall is called the Yellow Sea. The other rivers of importance are the Choo-keang, the Mekong, and the Menam. The last two run into the Gulf of Siam, after watering the peninsula of Siam and Cochin China, Few rivers enter the Pacific on the east coast of Australia, and in conse quence of the proximity of the mountains to the shore they are short and unimportant. Atmospheric Pressure and Prevailing Winds. When the mean atmospheric pressure for the year over the entire . sur f ace O f the world is considered, it is found that there are two broad belts of high pressure which encircle the globe, one on each side of the equator. There is a wide area of slowly diminishing pressure between them, includ ing a narrow central band along which the barometric readings attain a minimum. Two other regions of low pressure surround the poles, and extend to a considerable distance. That around the North Pole is connected with an area of still lower pressure over the North Pacific, and there is another permanent depression, which is even deeper, in the vicinity of Iceland. Atmospheric pressure is the fundamental meteorological phenomenon, and the mean pressure for the year affords a clue to the cause of all such regular and continuous phenomena as trade winds and ocean currents, and to the distribution of temperature. Similarly a study of the isobars at different seasons throws light upon all periodical occurrences in the way of winds and currents. A low barometer is always accompanied by a high per centage of atmospheric aqueous vapour; consequently the equatorial belt of continuous low pressure is a region of almost continuous rain, excessive cloud, and constant calm or light variable winds. The effect of a difference in atmospheric pressure being established between two places is to produce a flow of air from the region of high towards that of low pressure, and the winds in their turn largely determine the surface movements or drift currents of the ocean. The region of calms between the north and south trades in the Pacific is both narrower, more irregular, and less clearly marked than the corresponding belt in the Atlantic. In the East Pacific it lies, at all seasons, con siderably north of the equator ; but during the southern summer it is found south of the line in the western parts of the ocean, and disappears entirely in the northern summer, as the calms of the Indian Ocean do also. The reason of the southern position of the west end of the calm belt seems to be the simultaneous occurrence of low atmo spheric pressure in the interior of Australia and an ex ceptionally high barometer in Asia. In the southern winter the depression over Asia and the increase of pressure over Australia form an unbroken barometric gradient, and the result io that the calms are replaced by a southerly breeze of great regularity. The region of calms included between the zones of the two trade winds, and towards which they blow, is not the only one with which they are associated ; for the opposite meteorological conditions that characterize the northern border of the north-east trades and the southern margin of the south east winds produce two fringing bands of calms. These regions are characterized by a high barometer, a sunny sky, and occasionally sudden squalls, contrasting with the depressed barometer and dull, wet weather of the equatorial region. In January the low atmospheric pres sure over the North Pacific produces winds which affect the climatological conditions of the shores in very different ways. At Vancouver Island the prevailing wind is south west, and consequently the winter on the shores of British Columbia is mild and moist. The opposite coast of Asia is visited during the same season by northerly winds, north-east in Alaska, north-north-east in Kamchatka, and north-west in Japan ; and, as a result, the weather in these regions in winter is dry and bitterly cold. The West Pacific and the Indian Ocean are the regions of monsoons, winds that blow as steadily as the trades, but which change their direction with the season. During the periods of transition the steady breeze gives place to variable winds, occasional calms, and sometimes terrific hurricanes. The general direction of the monsoons in the Pacific between April and October is southerly and south-easterly, and from November to April they blow from the north-east, and on nearing the continent of Asia from the north-west. Monsoonal winds are found connected with all continents ; they are produced by the great differences in the tempera ture and pressure which prevail over the land at different seasons as compared with the adjacent ocean. The mon soons give rise to oceanic currents which flow in the same direction as the wind, and like it run opposite ways during alternate half years. Although the velocity of the wind over the open sea is always greater than that near shore or on land, it was shown by the observations of the &quot; Chal lenger,&quot; in the Pacific and other oceans, that there is no distinct diurnal variation in the wind s force at sea, though very decided periods of maxima and minima were noticed in the vicinity of land (see METEOROLOGY, vol. xvi. p. 125). Currents. The system of surface circulation in the Currents. Pacific is much more complicated and less clearly defined than that in the Atlantic, as might be expected from the less constant character of the winds. The latter ocean has two wide channels of communication with the Arctic Sea, while, so far as currents are concerned, the Pacific is land locked to the north Behring Strait being narrow and shallow ; consequently water enters the Pacific almost entirely from the south, where there is uninterrupted communication with the Antarctic Ocean. There is no direct information as to the movements of ocean water at depths greater than 200 or 300 fathoms ; it is known, however, from indirect evidence, that movements do occur. Although the subject of under-currents at depths less than those just mentioned has been extensively studied, it is only with respect to surface currents that anything very definite is as yet known.