The earth is believed to be a cooling and contracting body. With a reduction in size the outer crust becomes wrinkled. The crests of these wrinkles protrude above the water, which fills the troughs. Without altering the relative positions of the exposed land masses and the oceanic basins, a comparatively slight increase in the depth of the latter would cause more land to be uncovered. Or were the bed of the ocean to be raised slightly, the sea would flow over the coast regions and accessible low-lying valleys. Were the solid crust uniformly smooth, it would be completely drowned by a continu ous sea about two miles in depth.
By the present arrangement, the land area constitutes 28 per cent of the surface. The ocean covers about 72 per cent, but by evapora tion and condensation, some of the water is dis tributed over the continents to be retained in lakes or returned to the sea by rivers. Though special names are given to different portions of the ocean, it is a contindous body of water.
The surface temperature of the ocean varies with the latitude. Ice floes and icebergs form in the polar seas, while an average temperature of 80° is maintained in the tropical ocean. The daily or seasonal range is not great in any lati tude. Even in the tropics heat does not extend far below the surface and the ocean as a whole is a mass of cold water.
The amount of dissolved salts in sea water averages 3.5 per cent. The salinity is not uni form, as portions are regularly freshened. These are chiefly the belts of equatorial rains, the regions affected by the melting ice of the polar seas and those near coasts receiving the drainage of large land areas. As heat ex pands sea water, thus reducing its density, it generally happens that the surface water is saltier than the colder layers below. The dis tribution of heat and the modification of climate effected by ocean currents is of vast importance, Nearly half the sun's heat in the Torrid Zone is carried to higher latitudes. The Arctic regions receive more heat from the Gulf Stream than they do directly from the sun. The west ern coast of Europe from the North Cape to Gibraltar has a climate much warmer than that of the opposite coast of North America and this work of the Gulf Stream is in a measure duplicated on the western coast of North Amer ica by the Japan current. Cold currents from
the southern ocean soften the climate of the western tropical coasts of Africa and South America. If the water completely covered the earth, a double wave caused by the attraction of the moon would, on account of the earth's rotation, travel around the globe every day. This ideal arrangement of tides is not realized except in the southern ocean, and, with a general movement to the west, the tidal wave is deflected northward in the other oceans.
In determining the height of mountains or any part of the land surface, it is necessary to have some common level as a basis of com parison. For this purpose the surface of the sea has been chosen, but on account of the many movements to which the ocean Is subjected and because of the attraction exerted by elevated land masses, its surface is not level, and all comparative heights are of doubtful value. Ef forts have been made to determine the mean level of the whole land surface. While much has yet to be accomplished in securing the requisite data, it has been quite clearly shown that such a line lies about 10,000 feet below present sea-level. The abysmal area below this line is of equal extent with the area above it. As the exposed continental area occupies 28 per cent of the earth's surface, it follows that 22 per cent is covered by water less than 10,000 feet in depth. Quite a large part of this region slopes gently away from the coast line to a depth of 100 fathoms, forming the continental shelf. The area of this submerged shelf is very nearly the same as that of the low costal plains of which it is a continuation. Were the level of the sea' to be reduced 600 feet, 10,000,000 square miles would be added to the land area, and the present exposed surface would sustain a loss somewhat greater were the sea to rise an equal amount. The proportion of land is much greater in the northern than in the southern hemisphere and the large land masses have their greatest width in the north.