Since over the whole of the eastern half of the A., from about n. lat. 45 northwards, the prevailing winds are south-westerly, there is over the same region a general flow of the water of the ocean towards the n.e., passing the British isles, and thence along the coast of Norway, to some distance e. of the North cape. It is to this circumstance that the mild temperatures of north-western Europe must be referred. The amelioration of the winter climates from this cause' is very great, amounting to about 30° in the Hebrides, and to fully 40° in the Lofoden islands. This effect is directly brought about, not by the winds alone, but by the influence of the winds and sea combined. The influ ence of currents on the temperature of the ocean is so great, that even in Aug., the isothermal of 50° touches the n. of Norway in lat. 72° n., whereas to s.e. of Newfound land the same isothermal descends to about lat. 42° n. Again, on the meridian of 74° w., the change of temperature from lat. 40° to 35° n., or in 300 m., is 18°.0; whereas on the meridian of 20° w. from lat. 40* to 10°, a distance of 1800 m., the change of the temperature of the sea is only 15°.0.
The temperature of the A. about the equator is, if we except the part between 20° and 35° w. long., above 80°: that of the gulf of Guinea reaches the maximum of 85° in April ; from Oct. to May it is above 80°; in June and Sept. about 80°; and in July and Aug. it falls below 80°: that of the Caribbean sea is above 80° from July to Oct., during the rest of the year below 80°, except in July. ' Between 10° and 30° lat. n., the temperature of the eastern part of the A. is always from 3° to 7° colder than the western, and the maximum and minimum temperatures take place later in the year in the Caribbean sea than off the African coast.
Much has been done recently, particularly by II.M.'s ships Porcupine and Challenger, in throwing light on the physical geography of the A. The most important of the observations are those of deep and bottom temperatures, from their connection with oceanic circulation, and the distribution of life in the depths of the sea, and the bearings of the questions thereby raised on geological speculation (see art. Snn). Animal life abounds at much greater depths than was formerly supposed; although beyond 6000 ft. it gradually diminishes. A great part of the bottom of the n. Atlantic is covered with a slimy " ooze," composed for the most part of the chalk-producing globigerina; in very deep parts this is replaced by a brown, clay-like mud, with few traces of animal forms.
Regarding the depth of the A., it is only recently that reliable data have been obtained; along certain tracts, especially those of the Challenger, the profile of the bot tom can now be laid down with considerable certainty. The deepest sounding made by the Challenger with its improved method of sounding (see SouNnixes), is 3875 fathoms, or 28,250 ft., at a point about 90 m. off St. Thomas, West Indies. A remarkable ridge, about 400 in. wide, and 10,000 to 12,000 ft., or 2 to 2} m., below the surface of the sea, extends along the bottom of the A. from cape Clear in Ireland to cape Race in New foundland, a distance of 1640 miles. Along this, which is known as the " telegraph pla teau," the Atlantic cables are laid.