THE DEEP SEA AND ITS CONTENTS. 335 



uppermost layer of about four hundred fathoms (less than one- 

 seventh of the whole) that the temperature exceeds 40° ; and the 

 regularity of the rise of the thermometer, from 40° at its base to 

 the summer surface-temperature of 70°, at the rate of about "^^^ 

 for every 100 fathoms, justifies our regarding the plane of 40° as 

 the limit of the depth at which the solar rays here exert any direct 

 heating influence. 



On her passage southwards towards the Antarctic ice-barrier, 

 the Challenger found the progressive reduction of surface-tempera- 

 ture to correspond with the progressive thinning of the warm 

 superficial layer in a manner which clearly showed that the 

 thermal condition of the southern ocean is entirely dominated 

 by the flow into it of the great mass of glacial water which has 

 been cooled down in the Antarctic area, and that it is, so to speak, 

 a vast reservoir of cold, the outflow from which keeps down the 

 temperature of every part of the oceanic area in free communi- 

 cation with it. This we see best in the Pacific, whose vast basin 

 is almost entirely filled by water of glacial or sub-glacial coldness, 

 on the surface of which in the intertropical region there floats a 

 layer whose temperature rises rapidly from its lower limit of 40° 

 to 80° at the surface, and whose thickness is nowhere more than 

 one fifth of the whole depth. This exceptional stratum, which 

 clearly derives its heat from the direct action of the solar rays 

 upon its surface, progressively thins away in either hemisphere 

 as it is traced from the tropic to the parallel of 55°, where it 

 disappears altogether, except in the course of the Kuro Siwo, or 

 gulf-stream of the Pacific, which slants northwards from Japan 

 towards Behring's Strait. That the cold of the great mass of 

 glacial and sub-glacial water which everywhere underlies it, and 

 which rises to the surface beyond its northern and southern 

 borders, is due to an underflow from the Antarctic area, is dis- 

 tinctly indicated by the absolute continuity of the same glacial 

 temperature throughout the deepest stratum — all the way from 

 the southern ocean to the Aleutian Islands, the bottom-tempera- 

 tures at depths of 2000 fathoms or more not differing as much 

 as \° Fahr., whilst the thermal stratification of the whole super- 

 incumbent mass up to within 500 fathoms of the surface shows 

 a similar uniformity. 



