DISTRIBUTION OF SALINITY In each of the three oceans there are two maxima of salinity of the surface water—one in the north, the other in the south tropical belt, separated by a zone of minimum salinity in the equatorial region, and giving place poleward to regions of still lower salinity. The three oceans differ somewhat between them selves. The north Atlantic maximum is the highest with water of 37.9 per mille salinity; the maximum in the south Atlantic is 37.4; in the north Indian ocean, 36-7; the south Indian ocean, 36.4; the south Pacific, 36.9; and the north Pacific has the lowest maximum of all, only 35.7. The comparatively fresh equatorial belt of water has a salinity of 35.0 to 34-o in the Atlantic and in the Indian ocean, 34.5 in the western and 33.5 in the eastern Pacific. Taking each of the oceans as a whole, the Atlantic has the highest general surface salinity with 35.37.
The salinity of enclosed seas naturally varies much more than that of the open ocean. The saltiest include the eastern Mediter ranean with Per mille, the Red sea with 41 to 42 per mille in the Gulf of Suez, and the Persian gulf with 4o near Bushire. The fresher enclosed seas include the Malay and the East Asiatic fringing seas with 3o to 34.5 per mille, the Gulf of St. Lawrence with 3o to 31, the North sea with 35 north of the Dogger bank, diminishing to 32 further south, and the Baltic, which freshens rapidly from between 25 to 31 in the Skagerak to 7 or 8 east ward of Bornholm and to practically fresh water at the heads of the Gulfs of Bothnia and Finland. The Arctic sea presents a great contrast between the salinity of the surface of the ice-free Nor wegian sea with 35 to 35-4, and that of the central Polar basin, which is dominated by river water and melted ice, and has a salinity less than 25 per mille in most parts.
The causes of difference of surface salinity are mainly meteoro logical. The belt of equatorial minimum salinity corresponds with the excessively rainy belt of calms and of the equatorial counter current, the salinity diminishing towards the east. The tropical maxima of salinity on the poleward side of the trade-winds coin cide with the regions of minimum rainfall, high temperature, strong trade-winds and consequently of maximum evaporation. Evapora tion is naturally greatest in the enclosed seas of the nearly rain less subtropical zone such as the Mediterranean and Red sea. Where the evaporation is at a minimum, the inflow of rivers from a large continental area and the precipitation from the atmosphere at a maximum, there is necessarily the greatest dilution of the sea water, the Baltic and the Arctic seas being conspicuous examples.
The salt content of the sea water in the depths of the oceans has been investigated with certainty only during the last io or 20 years. While the variation at the surface is somewhere between 3o and 4o%,, the variation at depths of more than 200 metres lies only between and 34.5%o. These variations are there occa sioned not by meteorological phenomena, but by the currents of the deep sea. If we study the Atlantic, which is best known (fig. 2), we see that there is a great difference between the part in the northern hemisphere and that in the southern. In north latitudes the salt content diminishes steadily with the depth from 37 to but in south latitudes the diminution proceeds only to a depth of Boo or i,000 metres, where a first minimum of is reached. From Boo metres or l,000 metres to 2,000 or 3,00o metres the salt content increases to 34.9%, in order to decrease from this layer again down to the sea floor, where a second minimum of is observed. The particulars of the course of the isohal sines or lines of equal salinity are explained best through looking at the deep currents along the arrow lines in the figure. In high south latitudes the heavy water, poor in salt, sinks from the sur face into the depths and moves northward right over the Equator into the northern hemisphere at a moderate depth of from 600 1,000 metres as the so-called Intermediate Current or Zwisclienstrom. In the northern mid-latitudes the salty, heavy water sinks from the surface into the depths and moves, at a depth between I,000 and 3,00o metres, towards the southern hemisphere as the Depth current or Tiefenstrom. Finally at a still deeper level, on the ocean floor, the water moves from the southern to the northern hemisphere (the Floor current or Bodenstrom). Thus one finds currents with very different origins and varying salt content mov ing in various directions resulting from a combination of both vertical and horizontal movements. This scheme of the divisions of salt content and movement of water applies also to the Indian sea depths, and, fundamentally at least, to the Pacific. Most im portant for the origin of these movements is the fact that round the earth in the neighbourhood of Antarctica are present immense masses of water of high specific gravity which flow away north ward at the surface of the sea and which then, somewhere around Lat. 50° S., sink towards the depths as the Intermediate current or Zwischenstrom. Without this immense south polar sea the properties of the deep sea water would be very different; the north polar seas, shut off from the south by ridges, can send forth no such great streams.