In general, the salt content of the depths of the great oceans is less than that of the surface; it is only the manner and rate of the decrease that is different in northern and in southern latitudes. In addition the polar seas have another factor of vertical distri bution of salt content. In the icy seas, down to a depth of ioo or 200 metres, as a result of freezing, the water is very cold but poor in salt ; under this, from 200 to i,000 metres, is a layer of warmer, saltier water; from there to the bottom the temperature di minishes, but only a very little, and the salt content remains almost unaltered (fig. 2). The following table combines the latest figures for salt-content and temperature at five latitudes along meridian 3o° W.
In a few enclosed seas—differing from the ocean—the salt content increases with the depth, e.g., in the Baltic and Black seas, because salty water flows into the seas along the floor from the North and Mediterranean seas respectively; the result is that in the middle of the Baltic, near Gothland, the salt content at the surface is only 6 or while from a depth of 10o metres it rises to I 1 or 13 Temperature of the Oceans.—The temperature of the sur face of the sea is obtained from samples dipped by a bucket. The measurement of temperature in the depths involves stopping the ship and employing thermometers of special construction. The best instrument and the one in general use to-day is the outflow or reversing thermometer, first introduced by Aime in the Medi terranean in 1841-45, but greatly improved and simplified by Negretti and Zambra, London, and Richter Wiese, Berlin. The inciple is to have a constriction in the tube above the bulb so proportioned that when the instrument is upright it acts in every way as an ordinary mercurial thermometer but when it is inverted the thread of mercury breaks at the constriction. Mag naghi introduced a convenient method of inverting the thermom eter by means of a propeller actuated on beginning to heave in the line; greater precision and certainty are obtained by using a lever actuated by a weight slipped down the line to cause the re versal. All thermometers sunk into deep water must be protected against the enormous pressure to which they are exposed.
The warming of the ocean is due practically to solar radiation alone ; such heat as may be received from the interior of the earth can only produce a small effect and is fairly uniformly dis tributed. On account of the high specific heat and of the tur bulence of sea-water the diurnal range of temperature at the surface is very small. According to A. Buchan's discussion of the two-hourly observations on the "Challenger" the total range be tween the daily maximum and minimum in the warmer seas is between 0.7° and o.8° F, and for the colder seas still less (0.2° F), compared with 3.2° F in the overlying air. The maxi mum usually occurs between 1 and 2 .30 P.M., the minimum shortly before sunrise. The temperature of the surface water is generally a little higher than that of the overlying air, the daily average difference being about o.6° F, varying from 0.9° lower at i P.M
to 1.6° higher at 1 A.M. There are few observations available for ascertaining the depth to which warmth from the sun penetrates in the ocean. The investigations of Aime in 1845 and Hensen in 1889 indicate that the amount of cloud has a great effect. Aime showed that on a calm, bright day in the Mediterranean the temperature rose o.i° C between the early morning and noon at a depth of about 20 metres. The penetration of warmth from the surface is effected by direct radiation, by convection by particles rendered dense by evaporation increasing salinity, and through the turbulence of movement in the upper layers. After a storm the whole of the water in the North sea assumes a homothermic condition, i.e., the temperature is the same from surface to bottom, and this occurs not only south of the Dogger bank, where the condition is normal, but also, though less frequently, in the deeper water farther north. Similar effects are produced in nar row waters by the action of tidal currents, and the influence of a steady wind blowing on or off-shore has a powerful effect in mixing the water.
The warmest parts of the Indian ocean and Western Pacific have a mean annual temperature of 82° to 84° F, but such high temperatures are seldom found in the tropical Atlantic. In the Indian ocean between 15° N and 5° S. the surface temperature in May averages 84° to 86° F, and in the Bay of Bengal the tem perature is 86°, and no part of the Atlantic has so high a monthly mean temperature at any season. G. Schott's investigations show that the annual range of surface temperature in the open ocean is greatest in N., with 18.4° F, and in 30° S., with 9.2° F; on the contrary, near the equator it is less, only 4° F in io° N., and in high latitudes it is also small, 5.2° F in 50° S. The figures quoted above are differences between the average surface tem peratures of the warmest and of the coldest month. As to the absolute extremes of surface temperature, Sir John Murray points out that 90° F frequently occurs in the western part of the tropical Pacific, while among seas the Persian gulf in summer months reaches 96° F, only under blood-heat, and the Red sea follows closely with a maximum of 94°. The greatest change of temperature at any place has been recorded to the north-east of Japan with a minimum of 27° F and a maximum of 83°. In those localities, however, it is not the same water which varies in temperature with the season, but the water of different warm and cold currents which periodically occupy the same locality as they advance and retreat. The zones of surface tem perature are arranged roughly parallel to the equator, especially in the southern hemisphere. Between 40° N. and 40° S. the currents produce a considerable rearrangement of this order. The arrangement of the isotherms thus affords a basis for valu able deductions as to the direction of ocean currents.