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bulk remains at a temperature below 40° F. More than go per cent of the sea-floor is thus covered, including all those vast regions, in depths greater than 2000 fathoms, where the temperature is, uniformly found to be within the range of a few degrees above the freezing point of fresh water. Salt water freezes at a lower temperature than fresh water: sea water with a salinity of 35 per thousand freezes at 28° F. and it is not uncommon to find tempera
tures but little above this in the waters of the polar seas. About 16 per cent of the entire ocean surface has a temperature below 40° F. In the surface waters of lower latitudes, there is a progressive rise in temperature, reaching the average of 81° near the equator ; but the position of the isothermal lines is much modified by prevailing winds, by the situation of land-masses, and by distribution of areas of barometric maxima and minima. The warmer surface waters occupy only a comparatively small depth even in the tropics.
The following table gives the mean temperatures for the whole ocean, as calculated from all observations in all latitudes at the specified depths:
Depth in Mean Fahrenheit
300 44 7
- 1200 35-8
While the mean daily variation of temperature in the surface waters of the open ocean is probably less than i° F., the range from season to season may amount to 50° F. in those regions, like the oceanic areas around Japan, Newfoundland, and the Cape of Good Hope, where the surface is occupied successively by waters of polar and of equatorial origin; but, on the other hand, there are very extensive regions both in the tropical and polar waters where the range does not exceed a few degrees during the course of the year. With descent into the depths, the seasonal changes in temperature tend rapidly to diminish.
The variation of the density of sea water according to salinity and temperature is a prime factor in oceanic circulation. The average geographical distribution of density of the surface waters is shown in the accompanying chart, while the mean density at successive depths is given in the following table:
Deptn in fathoms Density
100 •. 1.0261