158 The Realm of Nature CHAP. 



about 1 7 per cent of the surface is dry land. The fine curve 

 in the figure shows the average distribution of 28 per cent 

 of land in all latitudes. All the great land masses of the 

 globe are widest in the north, and taper to a point toward the 

 south. Only a few small islands lie beyond 56 S. if the 

 unexplored Antarctic region is excepted. The inequality 

 of the distribution of land and water appears greatest in 

 the hemisphere having its centre near New Zealand, which 

 comprises two-thirds of the entire ocean surface and only 

 one-eighth of the land ; and in the opposite hemisphere 

 (with its centre in the English Channel) which contains only 

 one-third of the ocean and seven-eighths of the land of the 

 Earth. In the water hemisphere the proportion of land 

 is about y 1 ^ or 8 per cent ; in the land hemisphere it is 

 about J or 50 per cent, the areas of land and sea being 

 equal (see small maps on Plate XI.) 



215. Divisions of the Hydrosphere. The Caspian Sea 

 is the only large sheet of water which is cut off by land 

 from the rest of the hydrosphere, and its separation from 

 the ocean is comparatively recent ( 335). Otherwise the 

 hydrosphere is a connected whole, made up of four wide 

 open expanses called Oceans, from which smaller portions' 

 called Seas are more or less distinctly marked off by the land. 

 It is a matter of opinion where to draw the line between 

 oceans and seas ; the expanse of water within the Arctic 

 Circle, for example, is by some authorities considered the 

 smallest ocean, and by others with more show of reason it 

 is held to be the largest sea. Seas may be classed in three 

 groups (a) Inland Seas, entirely surrounded by land, of 

 which the Caspian is the only example ; (b) Enclosed 

 Seas, nearly surrounded by land but connected with the 

 ocean or with another sea by one channel, which is narrow 

 and shallow compared with the general breadth and depth ; 

 (c) Partially Enclosed Seas, which (a) have two or more 

 entrances, or (/3) are marked off from the ocean by a line 

 of islands, or (y) by an entirely submerged barrier. 



216. The Oceans. No natural boundaries mark off the 

 hydrosphere sharply into separate parts, but it is convenient 

 to distinguish four divisions called oceans, the positions 



