May 4, 1876] 



NATURE 



15 



The section from Papeete to Valparaiso (Plate III.) is about 

 5,000 miles in length, and is naturally divided into two parts, the 

 run southwards to the parallel of 40° S. , and the course along that 

 parallel towards Valparaiso. 



Setting aside Station 279 in 680 fathoms close to Tahiti, the 

 mean depth throughout the section was 2,139 fathoms, con- 

 siderably less than that of the meridional section from Honolulu 

 to Tahiti, and very much less than that of the section in the 

 North Pacific, between Japan and San Francisco. The nature 

 of the bottom is very much the same as in the meridional section, 

 red clay imbedding nodules, and lumps of various sizes of man- 

 ganese peroxide, and passing in the shallower soundings into 

 more or less pure Globigerina ooze, and as in the section between 

 Hawaii and Tahiti the fauna is generally meagre. The trawling 

 between Juan Fernandez and Valparaiso (Station 298) was par- 

 ticularly interesting ; animal forms were much more abundant 

 than they usually are in the Pacific ; and the general character 

 of the assemblage resembled in a remarkable degree that of the 

 fauna of the Southern Sea in the neighbourhood of the Crozets 

 and Kerguelen, many of the species, including some singular 

 Urchin of the family Ananchytidse, being identical. The 

 bottom at this station was a bluish mud, the surface layer con- 

 taining little or no carbonate of lime, and curiously enough a 

 deeper layer, with a considerable proportion of Globigerina 

 shells. There was no considerable quantity of manganese in the 

 sounding. Notwithstanding the considerable depth of 2,225 

 fathoms, the conditions in ihis locality seem much more favour- 

 able to animal life than even the manganese area ; and I am 

 inclined to think that we had struck upon one of the highways 

 by which migration takes place to the northward from the 

 Southern Sea. 



Although there are certain points which have yet to be 

 worked out in detail, the general distribution of temperature 

 in the Pacific seems sufficiently simple. In the first place, 

 the whole mass of water consists of two well-marked divisions, 

 an upper layer of no great depth, in which there is rapid 

 cooliui^ from the surface downwards, and considerable vari- 

 ation in temperature in different localities ; and a mass 

 of water of incomparably greater amount, which extends 

 to the bottom, and which may be said to have nearly the same 

 temperature throughout. These two divisions shade into one 

 another, but the isothermobath of 5° C. may be taken as indi- 

 cating generally the limit between them ; below this Ime the 

 isotherniobaths are still affected by surface thermal conditions, 

 but comparatively slightly. Above the line of 5° C. the course 

 of the isothermobaths is to all appearance entirely regulated by 

 causes affectmg the surface-temperature, that is to say directly 

 or indirectly by surface currents produced by permament, 

 periodic, or variable winds. The equatorial current occupies the 

 region of the trade-winds, approximately from lat. 20° N. to 

 20" S., and there is a strong but narrow counter current entirely 

 comparable with the counter current in the Atlantic between the 

 parallels of 5" and 8° N. The water of the equatorial current has 

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

 of Malacca and the islands of tfie 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 permanent and less defined than the return 

 current in the Atlantic finds its way to the north-eastward along 

 the coast of Japan. The course ot the Japan current is much 

 the same as that of the Gulf-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 focussed by the form of 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 isobathy- 

 iherm of 25° C. at 80 fathoms passes near Hawaii and Tahiti, 

 and near the parallel of 20* N. on sections between the Admi- 

 ralty Islands and Japan, The lower isothermobaths 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 tempera- 

 ture 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 oscillations depend upon causes acting 

 on the surface, for the line rises and falls in harmony with the 

 higher isothermobaths. The line of 5° '_'. 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 l,loo fathoms, 

 and the remaining degree or degrees and a fraction is lost 

 between 1,100 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 1,000 fathoms. Notwith- 

 standing this increase in depth, we have satisfied ourselves, 

 although the determination is one of great difficulty, that the 

 bottom temperature rises sljghtly from the south northwards. 

 We can scarcely say more than that it rises slightly, for the 

 differences in the temperatures below 1,500 fathoms are so small 

 that a result can only be arrived at by a careful combination 

 and comparison of many observations, taking into full considera- 

 tion the errors of the thermometers arising from all sources. 

 There is a like very slight decrease in the bottom-temperatures 

 from east to west. 



I think we can scarcely doubt that like the similar mass of 

 cold bottom water in the Atlantic, the bottom water of the 

 Pacific is an extremely slow indraught from the Southern Sea. 

 That it is moving, and moving from a cold source, is evident 

 from the fact that it is much colder than «he mean winter tem- 

 perature of the area which it occupies, and colder than the meaia 

 winter temperature of the crust of the earth ; that it is moving in 

 one mass from the southward is .«hown by the uniformity of its 

 conditions, by the gradual rise of the bottom-temperatures to the 

 northward, and by the fact that there is no adequate northern 

 source of such a body of water, Behring's Strait being only forty 

 fathoms deep, and a considerable part of that area bting occu- 

 pied by a warm current from the Pacific into the Arctic Sea, and 

 by our knowledge fropi observations that one or two trifling 

 currents from the Sea of Okotsk and the Behring Sea, which are 

 readily detected and localised, and are quite independent of the 

 main mass of cold water, represent the only Arctic influx. 

 During its progress northwards the upper portion of the mass 

 becomes slightly raised in temperature by mixture with, and pos- 

 sibly by slow conduction from, the upper layers which are 

 affected by solar heat. At the end of the Gulf, that is to say in 

 the extreme north, furthest from the cold source, the temperature 

 is, as I have already pointed out, influenced to the very bottom ; 

 and the isothermobaths between 8" and 5° C. are obviously 

 raised and pressed together, probably by the accumulation of the 

 cold water against the land. The colder bottom-water to the 

 westward might be expected from the lower initial velocity of 

 the Antarctic water causing it to drag against the west coast. 



I am every day more lully satisfied that this influx of cold 

 water into the Pacific and Atlantic oceans from the southward 

 is to be referred to the simplest and most obvious of all causes, 

 the excess of evaporation over precipitation in the northern 

 portion of the land hemisphere, and the excess of precipitation 

 over evaporation in the middle and southern part of the water- 

 hemisphere. 



After what I have already said I need scarcely add that I have 

 never seen, whether in the Atlantic, the Southern Sea, or the 

 Pacific, the slightest ground for supposing that such a thing 

 exists as a general vertical circulation of the water of the ocean 

 depending upon differences of specific gravity. 



NOTES 



The forty-seventh anniversary of the Zoological Society was 

 held on Saturday last. Viscount Walden, F.R.S., the Pre- 

 sident, being in the chair. Mr. P. L. fc'clater, F.R.S., the 

 Secretary, read the report, which showed that the income 

 (28,738/.) was greater than it had been in any previous year 

 since the foundation of the Society. The total number of 

 visitors in 1875 had been 699,918. The new lion house had 

 been, as far as its main portions were concerned, completed and 

 opened to the public. The building contains fourteen dens, the 



