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ocean. The Ptolemaic school, on the other hand, 

 looked on the Atlantic MM Indian Ocean* aa 

 enclosed *eas like the Mediterranean, and held 

 that the east and west points of the known world 

 approached so close to each other that a ship sail- 

 ing from Spain might easily reach the eastern 

 coast* of Asia. This mistaken notion led, fourteen 

 centuries after Ptolemy, to the discovery of America 

 by Columbus. The discm -ei ic of Coliiirhus, Vasco 

 da Gama, and Magellan, during tin- thirty years 

 from 149*2 to I.VJ-.', added a hemisphere to tile chart 

 of the world. These voyages doubled at a single 

 bound all that was previously known of the surface 

 of the glolte, and in a special manner enlarged our 

 knowledge of the sea. Down to the time of the 

 V/KI//I ii'H'r and similar recent scientific expeditions 

 all our knowledge of the sea might, literally speak- 

 ing, be called superficial ; it was limited to the 

 upper layers of oceanic waters and to the shallower 

 depths surrounding continents and islands. Owing 

 to the recent introduction of improved apparatus 

 and methods, the most profound depths of the 

 ocean have been examined with precision and 

 success, so that we now possess a large amount of 

 deiiuit" information concerning the physical and 

 biological conditions of all regions of tne'ocean. 



Area, The waters of the sea cover about 

 143,259,300 s<j. m., or about five-sevenths of the 

 surface of the earth. The areas of the main divi- 

 sions of the ocean are estimated as follows : 



Sq. mill*. Fir cent 



Pacific (from Arctic Circle to 40" S.) 58,100,000 40-&4 



AUMllL-( ) 30,150,003 21-61 



In.lian (lonthern boundary 40 S.% 18,100,000 12-63 



Southern ( from 40 & to Antarctic Circle )..27,300,(XK) 19-05 



Arctic ( within the Arctic Circle) 5,000.000 3-49 



Antarctic (wltliin the Antarctic Circle).... 4,650,000 3-25 



143,300,000 100-00 



Depth. The solid glolie or lithosphere, viewed 

 as to its superficial aspect, may be regarded as 

 divided into two great planes : one of these corre- 

 sponds to the dry land or up|>er surface of the 

 continental masses, and occupies about two-sevenths 

 of the earth's surface; the other, cnrres]tonding to 

 the abysmal regions of the ocean, is depressed over 

 2A miles below the general level of the continental 

 plane, and occupies about four-sevenths of the 

 earth's surface. The transitional area, uniting 

 these two planes, forms the sides or walls of the 

 ocean basins, ami occupies about one-seventh of 

 the earth's surface. The depressed regions of the 

 globe, represented by the ocean basins, are filled 

 with sea-water up to within about 376 fathoms 

 (2250 feet) of the general level of the continents, 

 the average depth of the water in the ocean basins 

 lieing on the other hand about 2080 fathoms 

 (12.480 feet). Were the solid crust of the earth 

 to be reduced to one uniform level by removing 

 the elevated continental nns-cs into the depressed 

 abymiml aieas, the surface of the earth would then 

 be covered by a universal ocean or hydrosphere 

 with a depth of al>oiit '2 miles. This depth of 

 2 miles below the present sea-level has been called 

 by Dr Mill the mean sphere level. The average 

 depths of the main divisions of the ocean are : 



Pacific Ocean. . 2SOOfa(homi 



Atlantic 2900 ,. 



Indian BOO 



The greatest depth recordi-d is 5155 fathoms, 

 in the South Pacilic (IS06), near the Kermadec 

 Islands; the ClutUenijer sounded in 4475 fathoms, 

 in the Noith 1'm-ilic. north-west of the Caroline 

 Islands. In the South Pacific a depth of 4428 

 fathoms was found just south of the Friendly 

 Islands, and 417" fa thorns otT the west coast of 

 Chili. In the Atlantic the greatest depth is 4561 

 fathoms, off Porto Kico, West Indies. Kons records 

 a sounding in the Antarctic Ocean where he found 



Southern Ocean. .2300 tathonw. 

 Antarctic ., . . 630 n 

 Arctic n .. 030 



no Inittom nt 4000 fathoms. By far the larger 

 n ol the sea Moor lies between the depths of 

 1000 and .'(OKI fat hums, equal to nearly 7H per cent. , 

 while about 17J per cent, is found in depths less 

 than 1000 fathoms, and about -U percent, in depths 

 greater than 3000 fathoms. The bulk of water in 

 the whole ocean is estimated at 323,800,000 cubic 

 miles. 



Temperature. The temperature of the surface- 

 waters of the ocean varies from 28 K. in the polar 

 regions to 85 or 86 in equatorial regions. In 

 many places the surface-layers are subject to great 

 annual changes due to the seasons and the direc- 

 tion of the wind. The temperature of the water at 

 the liottom of the ocean over the abysmal areas 

 ranges from ,'<27 F. to 36'8 F. In some large 

 basins separated from each other by low ridges the 

 temperature may differ to the extent of one or tw o 

 degrees, but the temperature is apparently con- 

 stant at any one spot throughout the vear. The 

 great mass of the ocean consists of cold water 

 i.e. of water below 40 or 45 F. ; at a depth of 

 little over half a mile the water in the tropics has 

 generally a temperature below 40 F. In the open 

 ocean the temperature usually decreases as the 

 depth increases, the coldest water being found at 

 the bottom. In enclosed or partially enclosed seas, 

 cut off by barriers from the great ocean basins, the 

 temperature remains uniform from the height of 

 the barrier down to the bottom ; for instance, in 

 the Mediterranean the temperature is about 56 

 from 200 fathoms down to 2000 fathoms ; in the 

 Siilu Sea, 50'5 from 400 fathoms to 2500 fathoms ; 

 in the Celebes Sea, 38 "6 from 800 fathoms to the 

 bottom in 2600 fathoms. In regions where there 

 are heavy rains, or where rivers pour fresh water 

 into the sea, alternating layers of colder and 

 warmer water have been observed within a hun- 

 dred fathoms from the surface. 



Circiiltttion. The circulation of oceanic waters 

 is maintained by the action of the prevailing winds 

 on the surface-layers. In the oceanic areas the 

 prevailing winds are governed by the large anti- 

 cyclonic areas situated towards the centres of the 

 North and South Atlantic and North and South 

 Pacific. The winds blow out from and around 

 these antieyclonic areas. ' For instance, in the 

 southern hemisphere the warm salt water of the 

 tropical regions is driven to the south along the 

 eastern coasts of South America, Africa, and 

 Australia, till on reaching a latitude of between 

 50 and 55 S. it sinks on l>eing cooled, and spreads 

 slowly over the floor of the ocean to the north and 

 south. A similar circulation takes place in the 

 northern hemisphere, although much modified by 

 the peculiar configuration of the land-masses; for 

 instance, the cold salt water at 30" F. which occupies 

 the deeper parts of the Arctic basin is largely made 

 up of the dense Gulf Stream water, which sinks to 

 the liottom on l>eing cooled in the Norwegian Sea. 

 The water evaporated from the sea-surface is borne 

 to the land-mosses and condensed on the mountain- 

 slopes. It is estimated that over 6500 cubic miles of 

 t his water is returned to the sea l>y rivers annually, 

 bearing along with it a burden of soluble salts and 

 earthy matters in suspension ; in this way the ocean 

 has in all probability liccome salt in the course of 

 ages. The saltest waters are found in the regions 

 of greatest evaporation ; for instance, in the Red Sea, 

 Mediterranean, ami in the trade- wind regions of 

 the great ocean basins. It has been shown that by 

 the action of ofT-shore winds cold water is brought 

 up from the deep sea to supply the place of the 

 surface-water carried seawards oil' the west coast of 

 Africa and America, and off Cape Guardafui on the 

 east coast of Africa ; and the alisence of coral-reefs 

 off these coasts is believed to l>e due to the great 

 range and variation in the temperature of the water 



