Though the Pacific covers nearly half of the surface of the globe, it receives the drainage of a comparatively small portion of the land. In South America the watershed between the rivers which run into the Pacific and the Atlantic is nowhere more than 100 miles from the shores of the Pacific; while for the greater part it is only about 50 or 60 miles, and in some places much less. Thus the Pacific receives hardly more than one twenty-fifth part of the drainage of South America. In the Mexican isthmus, as far west as the isthmus of Tehuantepec, the watershed continues at a short distance from tho Pacific, never receding more than 90 miles, and frequently approaching it within less than 10 miles. West of the Isthmus of Tehuantepec it gradually recedes farther, and at 20° N. lat. it is more than 300 miles from the shores of the Pacific ; but farther north it again approaches to a distance varying between 120 and 200 miles. The countries of the Mexican isthmus, which send their drainage to the Pacific, pro bably constitute not more than one-eighth of that division of America. In North America (north of 32° N. lat.) the watershed lies much more towards the interior of the continent, being between 35° and 45° N. lat., about 750 miles from the Pacific; • but farther north it is only about 350 miles from it. The area of the countries of North America whose drainage runs to the Pacific probably does not exceed one-fifth of the whole surface. According to this rough estimate, the Pacific receives not quite one-tenth of the drainage of America.
In Asia the watershed between the rivers which fall into the Arctic Ocean on one side, and into the Pacific on the other, is likewise at no great distance from the latter, north of the mouth of the river Amur; but the basin of this river extends above 1200 miles into the interior of Asia, and it is followed on the south by the basins of the rivers Hoang-ho and Ynntse-kiang, which reach somewhat farther iuland. The basins of these three rivers, added to those of a few others of inconsiderable extent, may comprehend nearly one-seventh part of the area of Asia, and so much of the.drainage of that contiuent enters the Pacific. In Australia the line which divides the waters which run into the Pacific from those which flow off in another direction is hardly anywhere 100 miles from the great ocean, except in the north-eastern district, where it is about 900 miles : it may be estimated that about one-ninth of the drainage of that continent is poured into that sea.
The eastern, northern, and southern portions of the Pacific Ocean are remarkably free from islands. In an extent of sea far exceeding the Atlantic in area, and borderiug on the western shores of America, only a few isolated islands occur, and one cousiderablo group, the Galapagos Islands. Between the coast of South America and Aus tralia, south of 24° S. lat., only the largo islands called New Zealand are situated ; and in their vicinity there are a few small groups and islets : in the remainder of this vast extent of sea hardly four or five islands, or diminutive groups, are known to exist. But that portion
of tho sea lying between the two tropics, and extending from the western boundary of the Pacific ea0ward to 135° W. long., or over more than half the width of the ocean, is abundantly diversified with islands of various dimensions. To the north of the equator the islands and groups, though numerous, are much less so than to the south of the line. The islands of the Pacific are both low and elevated. The low islands are of very small extent, and are based on coral reefs, which encircle a small space of sea. This inclosed space resembles a lagoon, and these islands am often called Lagoon Islands. In close proximity to the corallino islands snundings have been made of great depth, clearly proving them to hp the crests of lofty submarine, mountains, with sides of remarkable steepness. Captain Fitzroy found no bottom, within a mile and a half of Keeling Island, with a line of 7200 feet The volcanic islands are of moderate extent, and generally rise to a great elevation in their centre. Besides the different groups which lie in a line along its western boundary, and which have been already mentioned, several groups of volcanic islands are dispersed in the ocean. The gronps of this description north of the equator are the Bonin Sims, Ladrone, and Sandwich Islands. The Galapagos are traversed by the equator. South of the equator are the volcanic gronpe of the Marquesas, Society, Navigator, and Friendly Islands, and the solitary Easter Island. Some of these volcanic islands are encircled by coral-reefs, as the Society, Navigator, and Friendly Islands; others have not such a circle of reefs, as the Sandwich, Ladrone, Bonin Sims., Galapagos, New Georgian Archipelago, and New Hebrides. The islands which do not belong to the volcanic or lagoon islands are few in number : tho largest of them is New Caledonia. A portion of the Pacific has a peculiar character. Flinders calls it the ' Corallian Sea,' and determines its extent by assigning Papua and Luisiade as its northern boundary; whilst the north-eastern coast of Australia up to Sandy Point (21° 40' S. lat.) incloses it on the west, and on the south a line drawn from Sandy Point to the Island of Pines near the southern coast of New Caledonia. On the east it seems to terminate at some distance from the New Hebrides. It extends more than 1000 miles in length, and about 600 miles in width. The whole space is covered with innumerable coral-reefs and banks, which have only a few feet of water on them, and are very dangerous to the navigator. This is probably both the largest and the most extra ordinary reef in any part of the world. It is divided from the continent of Australia by a space of sea free from islands, in general from 20 to 30 miles, and in some places from 50 to 70 miles wide. This arm of the sea has generally a depth of between I0 and 20 fathoms, but this depth increases towards the south to 40 and even 60 fathoms.