Gulf Stream in the region of the Azores becomes deflected southward until it falls in again with the equatorial current and turns westward. In the centre of the whirl thus produced there is a tract of still water covered with floating seaweed (Sargassum bacciferum), and called the Sargasso (grassy) Sea.

189. Currents of the Pacific.-Part of the equatorial current, on reaching the Indian Archipelago, is deflected northward along the coast of China, and then, under the name of the Japan Current, curves eastward towards the coast of North America, where it turns southwards to feed the equatorial current, and thus renew its circuit. Another part of the western drift, impinging on Australia, turns southwards as the New South Wales Current, and passing New Zealand, merges in the anti-trades' drift of the southern seas.

190. Currents of the Indian Ocean.-The southern part of the equatorial current, when near the African coast, turns southward, and rushing through between Madagascar and the continent, is known as the Mozambique Current. North of the equator, the marine currents follow the guidance of the monsoons (see par. 252), and therefore change their directions with the seasons.

191. Inland Seas.--Inland seas have of course their local drift-currents depending upon the prevailing winds. But besides this, a circulation is kept up between their waters and those of the oceans, depending upon the relation that exists in each between the evaporation and the supply from rivers and rainfall. In the Mediterranean, the evaporation is in excess of the influx of water from rivers. There is therefore an eastward flow from the Atlantic through the Straits of Gibraltar to restore the level. At the same time there is an under-current, though to a smaller amount,

running the other way, owing to the greater saltness and heaviness of the Mediterranean water. In the Black Sea, the river water and rainfall are in excess of evaporation, and the superfluous waters form a surface-current through the Bosphorus, while there is a counter deep current of salter water from the Ægean. The Red Sea has practically neither rainfall nor rivers, and the evaporation would lower the level 20 feet in a year. If there were only an influx from the Indian Ocean through the Straits of Babelmandeb to keep the sea full, the water would in time become brine; there is therefore a counter-current which prevents the salinity from becoming excessive. During part of the year, the surface-current is from south to north; for the rest of the year, it is from north to south, according to the prevailing wind. The many rivers falling into the Baltic keep it overflowing into the North Sea, and it is prevented from becoming a fresh-water lake by a counter under-current from the same sea.

TEMPERATURE OF THE SEA.

192. Band of Uniform Temperature. The shallow stratum of water forming the ocean,' says Sir Wyville Thomson, 'is very definitely split up into two layers, which, so far as all questions concerning ocean movements and the distribution of temperature is concerned, are under very different conditions. At a depth varying in different parts of the world, but averaging perhaps 500 fathoms, we arrive at a layer of water of a temperature of 40° F.; and this may be regarded as a kind of neutral band separating the two layers. Above this band the temperature varies greatly over different areas, the isothermobathic lines being sometimes tolerably equally distributed, and at other times crowding

together towards the surface; while beneath it the temperature almost universally sinks very slowly, and with increasing slowness, to a minimum at the bottom.' This is true, however, only of the ocean where it is

Fig. 30.--Section of the Atlantic, from the Azores to Madeira. Showing soundings (in fathoms) and isothermal lines obtained in H.M.S. Challenger (Captain G. S. Nares), 1873.

open to the influx of the cold currents from the poles.

In the Mediterranean, for example, the surface temperature ranges from 80° in summer to 54° or 55° in winter. But this excess of summer heating does not extend beyond a depth of 100 fathoms: below this, the temperature of 54° or 55° is maintained constantly at all depths. In the Atlantic, in the same latitude, the temperature of the upper stratum of 100 fathoms does not essentially differ from that in the Mediterranean; but at greater depths, the temperature sinks to 49°, 40°, 38°, 36°, and in some places to the freezing-point of fresh water, and even below it (29.6°).

In many parts of the open ocean there are basins inclosed all round by submarine ridges rising considerably above the level of the bottom. However deep the interior of the basin may be, the temperature never sinks below that of the stratum at the lowest level of the brim. The rocky barrier shuts out the colder strata below as they well northwards along the bottom.

193. The Surface Temperature of the Sea, where it is not affected by currents from a colder or a warmer region, approximates to that of the air above it. But this rule is subject to great modifications. Over the deeper parts of the equatorial Atlantic, the surface mean temperature varies from 75° to 80°. In the Red Sea, the mean temperature is from 77.0° to 81.5°; but a temperature of 94.0° was once recorded near Aden. On the coast of Siam, a temperature of 91.0° has been observed. The cause of the comparatively low temperature over the deeper parts of the Atlantic is to be looked for in the glacial waters at the bottom, which must be continually rising up to supply the waste caused by evaporation and by the equatorial drift. It is remarkable that below the upper 60 or 80 fathoms directly affected by the solar heat, all the water in the

North Atlantic as far as lat. 40° is warmer than at the same depth under the equator.

194. Density of the Ocean.-The average specific gravity of sea-water is 1.02655; but exceptional circumstances cause considerable variations. Ships and other floating objects are thus slightly more buoyant (i.e., sink less deep) in the sea than in fresh water.

195. Influence of the Sea on Climate.-The sea, in virtue of its circulation, mitigates the extremes of heat and cold. The currents above described carry the heat of the tropics into temperate and polar regions, and the cold of the poles into the tropics. This is best exemplified in the North Atlantic. The shores of the British Islands, of Norway, and even Spitzbergen, are bathed with waters that have travelled from the West Indies, as is proved by branches of trees and other products that they bring along with them; and the genial temperature of these waters makes itself felt over the adjoining lands through its influence on the atmosphere. On the opposite side of the Atlantic, a cold current from Davis Strait flows along the coasts as far as Florida, and produces the contrary effect. The mean annual temperature of the north of Ireland in N. lat. 55°, is 50° F.; to find the same temperature in America, we must go as far south as near New York in N. lat. 41°. The isothermal of 40° F. passes Quebec in N. lat. 46-48°, through Newfoundland, and runs north-east to the coast of Norway in N. lat. 66°. The Japan Current affects the climate of Alaska and British Columbia in much the same way as the Gulf Stream affects that of Northern Europe. The warm waters deflected southwards from the equatorial current (see pars. 184 and 189) into the Antarctic Ocean, are believed, along with the prevailing west and north-west winds, to check the indefinite extension