the Cruise of H.M.S. ' Challenger. 3 469 



no free egress to the westward, being intercepted by the peninsula of 

 Malacca and the islands of the Malay archipelago ; but neither is it 

 completely arrested, as the equatorial current is in the Atlantic by the 

 unbroken coast of America ; consequently a return current, less perma- 

 nent and less defined than the return current in the Atlantic, finds its 

 way to the north-eastward along the coast of Japan. The course of the 

 Japan current is much the same as that of the G-ulf-stream, and is due, 

 as in the Atlantic return current, to the high initial velocity of the inter- 

 cepted water ; its influence on the temperature of the ocean is, however, 

 much sooner reduced and obliterated. 



The hot water of the Pacific equatorial current instead of being 

 gathered together and focused by the form o£ the land-barrier, as it is 

 in that of the Atlantic, spreads out in the Middle and "West Pacific in a 

 vast sheet of abnormally warm water, extending to a depth of nearly 

 100 fathoms ; thus the isobathytherm of 25° C. at 80 fathoms passes 

 near Hawaii and Tahiti, and near the parallel of 20° N. on sections 

 between the Admiralty Islands and Japan. The lower isotherm obaths 

 of the upper layer are a little nearer the surface in lat. 40° N. than in 

 lat. 40° S. ; and this I believe to be due to the banking of the Antarctic 

 indraught against the Arctic land-barrier, and to be the only case in 

 which the position of the lines of equal temperature in the upper layer 

 is not absolutely dependent upon the wind. 



The temperature of the underlying cold water is derived from another 

 source, and its distribution is governed by other laws. Throughout the 

 Pacific the isothermobath of 5° C. maintains on the whole a very even 

 course, oscillating between the 400- and 500-fathom lines. These oscil- 

 lations depend upon causes acting on the surface, for the line rises and 

 falls in harmony with the higher isothermobaths. The line of 5° O. 

 deviates sensibly on two occasions from its comparatively straight course. 

 In the equatorial region it sinks to a depth of 625 fathoms, probably 

 from the communication of heat from the upper layer of water by mixing; 

 and in lat. 40° it rises to 300 fathoms, probably, as I have already said, 

 from the accumulation of cold water against the Arctic barrier. The 

 next three degrees of temperature are lost with increasing slowness in 

 the next 700 fathoms, the line of 2° C. making a very even course at a 

 depth of 1100 fathoms, and the remaining degree or degree and a 

 fraction is lost between 1100 fathoms and the bottom. The depth of the 

 Pacific increases slowly from the south to the north, the mean difference 

 between the depth of the South Pacific and' that of the North being 

 perhaps as much as 1000 fathoms (see Plate 19, and the results of the 

 soundings of the ' Tuscarora '). Notwithstanding this increase in depth, 

 we have satisfied ourselves, although the determination is one of great 

 difficulty, that the bottom-temperature rises slightly from the south 

 northwards. We can scarcely say more than that it rises slightly ; for 

 the differences in the temperatures below 1500 fathoms are so small that 



