August 2 2, 1878] 



NATURE 



449 



valuable. I must also mention the very important cruises in 

 connection with the Norwegian Department of Fisheries, which, 

 through the skilled labours of Prof. Mohn and Prof. G. O. Sars, 

 annually contribute largely to our knowledge of the distribution 

 of temperature, of the course of the ocean currents, and of the 

 range of animal life in the North Atlantic. I observe in a letter 

 from Prof. Mohn, dated from Hammerfest on the loth of last 

 month, that the expedition of the past year has had a successful 

 cruise to Bear Island, where she has left letters for the Dutch 

 Arctic schooner the IVillem Bareritz, and has made many im- 

 ixjrtant temperature obsen-ations. Prof. Mohn speaks highly of 

 the senice rendered by Negretti and Zambra's new reversing 

 thermometer. This is a most ingenious instrument, so con- 

 structed that by a simple mechanical arrangement the tempera- 

 ture may be registered at any given depth, iiTCspective of any 

 number of zones of temperature, higher or lower, through which 

 the instrument may have passed in descending. In the Challenger 

 we felt greatly the want of such a thermometer, for although 

 generally throughout the ocean the temperature of the water falls 

 steadily from a surface maximum to a minimum at the bottom, 

 in the Arctic and Antarctic Seas — where a special interest 

 attaches to the vertical distribution of temperatures — the coldest 

 layer is frequently, as in Prof. Mohn's observations, on the 

 surface ; and a warmer belt intervenes between it and a bottom- 

 stratum, probably in many cases of intermediate temperature. 

 With the ordinary deep-sea registering thermometer the tempe- 

 rature of the lowest layer cannot be ascertained with certainty. 

 We had Negretti and Zambra's earlier instrument on the reversing 

 principle on board during the latter part of our cruise, but 

 through some defect in construction we did not find its indica- 

 tions trustworthy for great depths. I always believed the plan 

 of construction of this instrument to be good, and I am very glad 

 to find from Prof, Mohn's report that this defect has now been 

 entirely overcome. 



It follows from the nature of these many and varied enterprises 

 that the department of geographical science to whose progress 

 they have most specially contributed is the physical geography 

 of the sea ; and the special appliances with which they have 

 been provided have been principally instruments for determining 

 the temperature of the water at different depths, the depth of the 

 sea and the nature of the sea-bottom, and, in some few cases, 

 the distribution of the deep-sea fauna. It is of covu-se impossible 

 for me in so short a time even to sketch their several lines of in- 

 vestigation, or to attempt to assign to each its share in the general 

 advance of knowledge ; I think it may be better that I should 

 give an outline of some of the conditions of the regions to which 

 they refer by the light of their combined results, I am aware 

 that in taking this course I shall be forced to face questions on 

 which there has been some controversy ; and I can only say that 

 I will avoid the controversial aspects of such questions as far as 

 possible, and merely describe as shortly as I can the condition of 

 things as they appear to me. 



The General Ocean Circulation. — It was pointed out long ago 

 by Sir Charles Lyell that many of the most marked phenomena 

 of the present physical condition of the globe depend upon the 

 fact that the surface of the world is divided into two hemi- 

 spheres, one of which contains nearly the whole of the dry land 

 of this world, while the other is almost entirely covered by 

 water. The centre of the land hemisphere is somewhere in 

 Great Britain, and the centre of the water hemisphere, which in- 

 cludes the southern sea, the South Pacific, w hatever antarctic land 

 there may be, Australia, and the southern point of South America, 

 is in this neighbourhood of New Zealand. With a full know- 

 ledge of the absolute continuity of the ocean we have hitherto 

 been too much in the habit of regarding it as composed of several 

 oceans, each possibly under special physical conditions. All 

 recent observations have, however, shown us that the vast 

 expanse of water which has its centre in the southern hemisphere 

 is the one great ocean of the world, of which the Atlantic with 

 the Arctic Sea and the North Pacific are merely northward 

 extending gulfs; and that any physical phenomena affecting 

 obviously one portion of its area must be regarded as one of an 

 interdependent system of phenomena affecting the ocean as a 

 whole. 



Shallow as the stratum of water forming the ocean is— a mere 

 filna in proportion to the radius of the earth— it is very definitely 

 split up into two layers, which, so far as all questions concern- 

 ing ocean movements and the distribution of temperature is con- 

 cerned, are under very different conditions. At a depth varying 

 in different parts of the world, but averaging perhaps 500 I 



fathoms, we arrive at a layer of water at 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 sometimes toler- 

 ably 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. 



The causes of natural phenomena, such "as the movements of 

 great masses of water, or the existence over large areas of 

 abnormal temperature conditions, are always more or less com- 

 plex, but in almost all cases one cause appears to be so very 

 much the most efficient that in taking a general view all others 

 may be practically disregarded ; and speaking in this sense it 

 may be said that the trade-winds and their modifications and 

 counter-currents are the cause of all movements in the stratum 

 of the ocean above the neutral layer. This system of horizontal 

 circulation, although so enormously important in its influences 

 upon the distribution of climate is sufficiently simple. Disre- 

 garding minor details, the great equatorial current driven from 

 east to west across the northerly extensions of the ocean by the 

 trade-winds, impinges upon the eastern coasts of the continents, 

 A branch turns northwards and circles round the closed end of 

 the Pacific, tending to curl back to the North American coast 

 from its excess of initial velocity ; and in the Atlantic, following 

 a corresponding course, the Gulf Stream bathes the shores of 

 Northern Europe, and a branch of it forces its way into the 

 Arctic basin, and battling against the palaeocrystic ice, keeps im- 

 perfectly open the water-way by which Nordenskjold hopes to 

 work his course to Behring's Strait, The southern deflections 

 are practically lost, being to a great extent, though not entirely 

 dissipated in the great westerly current of the southern anti- 

 trades. 



One of the most singular results of these later investigations is 

 the establishment of the fact that all the vast mass of water, 

 often upwards of 2,000 fathoms in thickness, below the neutral 

 band, is moving slowly to the northward ; that in fact the depths 

 of the Atlantic, the Pacific, and the Indian Oceans are occupied 

 by tongues of the Antarctic Sea, preserving in the main its cha- 

 racteristic temperatures. The maintenance of a low tempera- 

 ture while the temperature of the floor of the ocean must be 

 higher, and that of the upper layers of the sea greatly higher, 

 is in itself a conclusive proof of steady movement of the water 

 from a cold source ; and the fact that the temperature of the 

 lower layers of water, both in the Atlantic and the Pacific, is 

 slightly but perceptibly raised to the northward, while the con- 

 tinuity of every layer with a corresponding layer in the southern 

 sea can be clearly traced, indicates the southern position of that 

 source. 



The immediate explanation of this very unexpected phenome- 

 non seems simple. For some cause or other, as yet not fully 

 understood, evaporation is greatly in excess of precipitation 

 over the northern portion of the land-hemisphere, while over 

 the water-hemisphere, and particularly over its southern portion, 

 the reverse is the case ; thus one part of the general circulation 

 of the ocean is carried on through the atmosphere, the water 

 being raised in vapour in the northern hemisphere, hurried by 

 upper wind currents to the zone of low barometric pressure in 

 the south, where it is precipitated in the form of snow or rain, 

 and welling thence northwards in the deepest channels on 

 account of the high specific gravity dependent on its low 

 temperature, it supplies the place of Uie water which has been 

 removed. 



The cold water wells northwards, but it meets with some 

 obstructions on its way, and these obstructions, while they prove 

 the northward movement, if further proof was needed, bring out 

 another law by which the distribution of ocean ternperature is 

 regulated. The deeper water sinks slowly to a minimum at the 

 bottom, so that if we suppose the temperature at a depth of 

 2,000 fathoms to be 36° F., the temperature at a depth of 3,000 

 may be, say, 32°. Now, if in this case the slow cmxent meet on 

 its northward path a continuous barrier in the form of a sub- 

 marine mountain ridge rising to within 2,000 fathoms of the 

 sea-surface, it is clear that all the water below a temperature 

 of 36° will be arrested, and, however deep the basin beyond 

 the ridge may be, the water w ill maintain a minimum of 36° 

 from a depth of 2,000 fathoms to the bottom. In many parts 

 of the ocean we have most remarkable examples of the effect 

 upon deep-sea temperature of such barriers intersecting cold in- 

 draughts, the most marked instance, perhaps, a singular chain of 



