Kriimmel's calculation gives the proportional areas at a high temperature as follows:— The vertical distribution of temperature in the open ocean is better known than that of salinity. Such information has already been given above (p. 689) in the table of salinity. The regional differences of temperature at like depths become less as the depth increases. Thus at 30o fathoms or roughly Soo metres greater differences than F hardly ever occur between 60° N. and 60° S., in 800 fathoms or 1,5oo metres the differences are less than 15°, and in 1,5oo fathoms or 3,00o metres less than 7°. Even in the tropics the high temperature of the surface is con fined to a very shallow layer; thus in the Central Pacific where the surface temperature is 82° F the temperature at 200 metres is only 52° F. The whole ocean must thus form but a cold dwelling-place for the organisms of the deep sea. Sir John Mur ray calculates that at least 8o% of the water in the ocean has a temperature always less than 40° F, and a recent calculation by Kriimmel gave in fact a mean temperature of 39° F for the whole ocean.
The vertical distribution of temperature in mid-ocean in the greater depths, i.e., depths of more than 200 metres or zoo fathoms, shows first a peculiar situation under the equator and secondly a great difference between the same latitudes north and south. If we use the above table of five points in the Atlantic we find that the water, from a depth of ioo metres to one of at least 600 metres, is colder under the equator than at lat. 30° S. and lat. 30° N., e.g., the water, 200 metres down at the equator, is 9° F colder than at the Bermudas and respec tively). One gathers from this that even at these moderate depths much water in the equatorial zone is rising from the underlying depths, while in mid-latitudes it is sinking. Particularly is this the case in the Sargasso sea between the Azores and the Bermudas. This region shows relatively the highest temperature even down to depths of 2,000 or 3,00o metres; it is considered as the start ing point of the North Hemisphere Depth Current or Tiefen strom. (See fig. 2.) The North Atlantic is on the whole much warmer than the South Atlantic ; this is seen by a comparison of the temperatures at equal depths at 30° lat. N. or S., and still more at 6o° lat. In the region 6o° N. and 30° W., south of Iceland, one does not find polar or arctic temperatures in the depths, but at 6o° S. and 30° W., near South Georgia, they are well marked. Similarly in the Indian and Pacific oceans, the water at a depth of from ioo to 600 metres, near the equator is colder than that north and south of it; and in both these oceans the water at greater depths in middle latitudes is colder in the Southern than in the Northern hemispheres, depth for depth.
Everywhere, as in the case of salt content, one finds the north ward-reaching influence of the huge, cold, antarctic body of water. The Indian ocean has no connection with the north polar region, the Pacific has ice in quantity only in the Sea of Okhotsk and parts of the Bering sea, and it is separated from the Arctic basin by the narrow Bering strait. But in the depths the Atlantic is also well sheltered from the north polar water by great sub marine ridges. Thus it happens that the world ocean is much colder in southern than in northern latitudes.
Generally, the temperature, like the salt content, decreases with the depth. In polar waters this is not the case; here the temperature increases below the surface layers, which had been cooled by ice and cold air to 29.5° F, until at i,000 metres it is somewhere around 32° or 33° F, and diminishes slowly thence to the bottom. Therefore the arrangement of the temperature is usually anatherm Ova above warm) but in the polar seas it is katatlierm (ear6. under) or dichotherm (3ixa apart). There are also deep regions described as homotherm, where from the level of the barrier to the bottoin, the temperature remains uniform. Thus south-west of Cape Province (S. Africa) in the Cape Trough, the temperature decreases continuously until it reaches 33.3° F in the greatest depths, while north of the Wal fisch ridge and east of the south Atlantic rise in the so-called Congo Deep the temperature remains uniform at 36.4° F from 3,500 metres down to the bottom. Thus in the Central American sea below 1,700 metres, the depth on the bar, no water is found at a temperature lower than that prevailing in the open ocean at that depth, viz., 39.6° F, not even at the bottom of the great Bartlett Deep in 6,290 metres. Such homothermic masses of water are characteristic of all deep enclosed seas. Thus in the Malay sea the various basins are homothermic below the depth of the rim, at the temperature prevailing at that depth in the open ocean. In other enclosed seas shut off from the ocean by a very shallow sill the rule holds good that the homothermic water below the level of the sill is at the lowest temperature reached by the surface water in the coldest season of the year, provided always that the stratification of salinity is such as to permit of convection being set up. To this group belongs the Arctic sea. The Mediterranean sea also belongs to this group; its various deep basins are nearly homothermic (at the winter surface tem perature) below the level of their respective sills.