4i6 



NATURE 



\Sept. 7, 1876 



In all seas within the torrid and temperate zones, provided 

 any given area is not cut off by submarine barriers from a sup- 

 ply of polar or glacial water, the sea bed is covered by a thick 

 stratum of water, the temperature of which is confined between 

 32° and 35° F. In the Pacific Ocean this cold stratum must be 

 derived from Antarctic sources, for the opening of Behring 

 Strait is too small to admit of an appreciable efflux of Arctic 

 waters. In this ocean the cold stratum obtains generally at 

 depths below 9,000 feet from the surface, with an almost invari- 

 able isothermal line of 40° F., at from 2,500 to 3,000 feet from 

 the surface. Similarly, in the Indian Ocean basin, the cold 

 stratum at the bottom is derived from Antarctic sources, for the 

 temperature of 33°'5 F. underlies the hot surface waters of the 

 Arabian Gulf. 



In the South Atlantic, Antarctic waters, with a bottom tem- 

 perature of 31° to 33°"5 F., certainly cross the equator ; the bed 

 of the North Atlantic basin then warms up to 35° — marked 

 diversities in both the temperatures and thickness of the succes- 

 sive layers of water from the surface downwards are found — 

 and in the central parts of the basin it is not until the vicinity of 

 the Faroe Islands is reached that Arctic waters of an equivalent 

 emperature to those from Antarctic sources are experienced. 



Turning now to the scientific aspect of the question : — 



The doctrine of a general oceanic thermal circulation assumes 

 two general propositions — i, the existence of a deep under-flow 

 of glacial water from each pole to the equator ; and 2, the 

 movement of the upper stratum of oceanic water from the equa- 

 torial region towards each pole, as the necessary complement of 

 the deep polar under-flow — this double movement being de- 

 pendent "upon the disturbance of hydrostatic equilibrium con- 

 stantly maintained by polar cold and equatorial heat," 



Proposition 2, in its general application as to the movement of 

 surface waters, is unquestionable ; but that of a deep under-flow 

 from the poles, as a necessary complement, remains open to 

 doubt. Proposition i, in its wide generality, must, from what 

 we know of the Pacific, be confined to the Atlantic Ocean ; and 

 it appears to me that it is on the interpretation of the movement 

 of the waters in its northern basin that the hypothesis of a ver- 

 tical circulation and the potency of thermal agency in bringing it 

 about must be judged. 



We have followed the movements of Antarctic waters in the 

 Atlantic to the 40th parallel, as illustrated by the progress of 

 icebergs ; we know that the movement deflects the strong 

 Agulhas current, and that the cold waters well up on the western 

 shore of the South African continent, cooling the equatorial 

 current near its presumed source ; the thrusting power of this 

 body of water is therefore great. About the equator it rises 

 comparatively near to the surface. But we now come to another 

 and distinct movement — the equatorial current — and on this, I 

 apprehend, the material agency of the winds cannot be denied, 

 in forcing an enormous mass of surface-water from east to west 

 across the ocean. The Gulf Stream results, and the compara- 

 tive powers of this stream, as especially influencing the climate 

 of our own and neighbouring countries, together with the forces 

 at work to propel its warm waters across the Atlantic, has be- 

 come the controversial field for the upholders of horizontal and 

 vertical circulation. The one hypothesis assigns to the Gulf 

 Stream all the beneficent powers of its genial warmth — extending 

 even beyond the North Cape of Europe — which has teen con- 

 ceded to it from the time of Franklin. The other hypothesis 

 reduces its capacity and power, considers that it is disintegrated 

 in mid- Atlantic, and that the modified climate we enjoy is 

 brought by prevailing winds from the warm area surrounding 

 the stream ; and to this has been more recently added, "by the 

 heating power of a warm sub-surface stratum, whose slow north- 

 ward movement arises from a constantly renewed disturbance of 

 thermal equilibrium between the polar and equatorial portions 

 of the oceanic area." 



Without denying the active powers of this disturbed thermal 

 equilibrium — although in this special case it is an abstraction 

 difficult to follow — and giving due weight to the many cogent 

 facts which have been brought forward in support of both views, 

 there appears to be still a connecting link or links wanting to 

 account for the southern movements of Arctic waters ; which 

 movements to me are even more remarkable as physical phe- 

 nomena than the translation of the warm waters from the Gulf 

 Stream area to a high northern latitude. 



This movement of Arctic waters is forcibly illustrated by the 

 winter drifts down Davis Strait of the ships Resolute, Fox, Ad- 

 vance, and part of the crew of the Polaris, when enclosed in pack 

 ice, exceeding in some cases a thousand miles j similarly of tjie 



winter drift of a part of the German expedition of 1870 down 

 the east side of Greenland, from the latitude of 72° to Cape Fare- 

 well. If to these examples we add the experience of Parry in 

 his memorable attempt to reach the North Pole from Spitz- 

 bergen in the summer of 1827, it must be inferred that a perennial 

 flow of surface water from the polar area into the Atlantic 

 obtains ; and, judging from the strength of the winter northerly 

 winds, that the outflow is probably at its maximum strength in 

 the early months of the year. 



When we further know that the northern movement of warm 

 waters gives in winter a large accession of temperature to the 

 west coast of Scotland, to the Faroe Islands, and extending to 

 the coasts of Norway as far as the North Cape ; the conside- 

 ration arises whether this onward movement of waters from 

 southern sources is not the immediate cause of displacement ofth 

 water in the polar area, and its forced return along the channels 

 indicated by those winter drifts to which I have referred. 



That some hitherto unlooked-for and unsuspected cause is the 

 great agent in forcing southern waters into the Atlantic polar 

 basin has long forced itself on my conviction, and I now suspect 

 it is to the cause producing the annual variations in ihe sea 

 level — for, as I have mentioned, indications exist of the seas of 

 the northern hemisphere having a higher level in winter than in 

 summer, — that we must direct our attention before the full solu- 

 tion of ocean circulation is accepted. 



The facts of the annual changes of sea level, whatever they 

 may ultimately prove, have hitherto ranged themselves as a part 

 of tidal action, and so escaped general attention. Physicists 

 well know the complication of tidal phenomena, and if one may 

 be permitted to say, the imperfection of our tidal theory ; certain 

 it is that the tides on the European coasts of the Atlantic are so 

 far abnormal that one of our best authorities on the subject (Sir 

 William Thomson) describes them, in relation, I assume, to 

 tidal theory, as "irregularly simple," while the tides in all other 

 seas are comparatively complicated, but " regular and explicable." 

 However this may be, specialists should direct their attention to 

 the disentanglement of the variations in the sea level from tidal 

 action simple ; and our colonies, especially those in the southern 

 hemisphere, would be excellent fields for the gathering in of reli- 

 able observations. 



I am unwilling to leave the subject without tracing some of the 

 consequences that might be fairly considered to follow this 

 assumed change of level in the North Atlantic basin. We can 

 by it conceive the gradual working up of the warmed water from 

 southern sources as the winter season approaches, including the 

 expansion of the Gulf Stream in the autumn months ; the con- 

 sequent welling up of a head of water in the enclosed and com- 

 paratively limited area northward of Spitzbergen, Greenland, 

 and the broken land westward of Smith Sound ; the forced 

 return of these glacial waters, their greatest volume seeking the 

 most direct course, and thus working down the Labrador coast, 

 charged with ice, and passing the American coast inside the Gull 

 Stream ; while the smaller volume, reaching the higher latitudes 

 in mid-Atlantic, interlaces with the warm barrier waters, causing 

 those alternating bands of cold and warm areas familiar to us 

 from the Lightning and Porcupine observations, and which are 

 now being worked out by the Norwegian exploring expedition 

 in the government ship Voringen. 



We can further conceive that the larger function of the 

 " counter currents " on the north margin of the great equatorial 

 streams is to act as conduits for the surcharged waters of the 

 Northern Oceans consequent on the gradual changes of level. 

 The Atlantic counter-current we know expands markedly in the 

 autumnal season, and there may be some connection between 

 this expansion and the high level of the waters said to exist in 

 the Gold Coast and Guinea bights at the same season. 



We are thus, as it appears to me, now only on the threshold of 

 a large field of inquiry bearing on the physical geography of 

 the sea ; but we have this advantage, — the admirable discussions 

 which have taken place in the past few years, productive as they 

 have been of the marshalling hosts of valuable facts, will lighten 

 the labours of those who engage in its prosecution. Science is 

 deeply indebted to, and I am sure honours, those who have so 

 earnestly worked on the opening pages of the coming chapter on 

 Ocean Circulation. 



Unwillingly I turn from this interesting subject ; but the 

 demands on my time and your patience are imperative : as, fol- 

 lowing precedent, it is incumbent on me briefly to bring under 

 the review of the Association the latest unrecorded incidents in 

 geographical progress or research. 



There is one absorbing topic which, in the potirse of a fe\7 



