414 



NATURE 



{Sept. 7, 1876 



similar one running parallel to the South African Continent, and 

 which extends to the parallel of the British Islands. It is 

 possible that certain tidal and, indeed, climatic conditions, 

 peculiar to the shores of the North Atlantic, may be traced to 

 this bottom conformation, which carries its deep, canal-like cha- 

 racter into Davis Strait, and between Greenland, Iceland, and 

 Spitzbergen, certainly to the 80th parallel. 



There is, however, one great feature common to all oceans, and 

 which may have some significance in the consideration of ocean 

 circulation, and as affecting the genesis and translation ol the 

 great tidal wave and other tidal phenomena, of which we know 

 so little ; namely, that the fringe of the seaboard of the great 

 continents and islands, from the depth of a few hundred feet 

 below the sea-level, is, as a rule, abruptly precipitous to depths 

 of 10,000 and 12,000 feet. This grand escarpment is typically 

 illustrated at the entrance of the British Channel, where the 

 distance between a depth of 600 feet and 12,000 feet is in places 

 only ten miles. Imagination can scarcely realise the stupendous 

 marginal features of this common surface depression. 



Vast in extent as are these depressed regions — for we must 

 recollect that they occupy an area three times greater than the 

 dry land of the globe, and that a temperature just above the 

 freezing-point of Fahrenheit prevails in the dense liquid layers 

 covering them — life is sustained even in the most depressed and 

 coldest parts ; while in those areas equivalent in depression below 

 the sea-level to that of European Alpine regions above it, animal 

 life abundantly prevails : structural forms complicated in arrange- 

 ment, elegant in appearance, and often lively in colour, clothe 

 extensive districts ; other regions apparently form the sepulchral 

 resting-place of organisms which when living existed near the 

 surface ; their skeletons, as it has been graphically put, thus, 

 " raining down in one continuous shower through the intervening 

 miles of sea-water." Geological formations, stamped with the 

 permanency of ages, common to us denizens of the dry land, 

 appear, in these regions, to be in course of evolution ; forces 

 involving the formation of mineral concretions on a grand scale 

 aie at work ; life is abundant everywhere in the surface and sub- 

 surface waters of the oceans ; in fine, life and death, reproduction 

 and decay, are active, in whatever depths have been attained. 



As a question of surpassing interest in the great scheme of 

 nature, the economy of ocean circulation, affecting as it does the 

 climatic conditions of countries, has of late attracted attention. 

 The general facts of this circulation in relation to climate have 

 been thus tersely summarised: "Cold climates follow polar 

 waters towards the equator, warm climates follow warm equato- 

 rial streams towards the poles." We can all appreciate the 

 geniality of our own climate, especially on the western shores of 

 the kingdom, as compared with the Arctic climate of the shores 

 of Labrador, situated on the same parallels of latitude ; or 

 indeed, with the vigorous winter climate of the adjacent North 

 American seaboard, even ten degrees farther to the south. 

 These, and kindred features in other parts of the globe, have led 

 to the summarised generalisation I have just referred to, but the 

 rationale of these movements of the waters is by no means 

 assured to us. 



That ocean currents were due primarily to the trade and other 

 prevailing winds, was the received opinion from the earliest in- 

 vestigation made by navigators of the constant surface movement 

 of the sea. Rennell's views are thus clearly stated — "The winds 

 are to be regarded as the prime movers of the currents of the 

 ocean, and of this agency the trade winds and monsoofts have by 

 far the greatest sliare, not only in operating on the larger half 

 of the whole extent of the circumambient ocean, but as possessing 

 greater power by their constancy and elevation to generate and 

 perpetuate currents " . , . "next to these, in degrees, are the 

 most prevalent winds, such as the westerly wind beyond, or to 

 the north and south, of the region of trade winds." 



Maury, so far as I am aware, was the first to record bis dissent 

 from these generally received views of surface currents being due 

 to the impulse of the winds, and assigned to differences of specific 

 gravity, combined with the earth's rotation on its axis, the move- 

 ment of the Gulf Stream, and other well defined ocean currents. 

 A writer of the present time, gifted with high inductive reason- 

 ing powers and with observed facts before him in wide extension 

 of those investigated by Rcnnell, regards the various ocean 

 currents as members of one grand system of circulation ; not 

 produced by the trade winds alone, nor by the prevailing winds 

 proper alone, but by the continued action of all the prevailing 

 winds of the globe regarded as one system of circulation ; and 

 that without exception, he finds the direction of the main cur- 



rents of the globe to agree exactly with the direction of the 

 prevailing winds. 



Another writer of the present day, distinguished for intellec- 

 tual power, and who personally has devoted much time to the 

 acquisition of exact physical facts bearing on the question both 

 in the ocean near our own shores and in the Mediterranean sea, 

 without denying the agency of the winds, so far as surface drifts 

 are concerned, considers that general ocean circulation is de- 

 pendent on thermal agency alone ; resulting in the movement of 

 a deep stratum of polar waters to the equator, and the movement 

 of an upper stratum from the equator lowards the poles : the 

 "disturbance of hydrostatic equilibrium" being produced by the 

 increase of density occasioned by polar cold and the reduc- 

 tion of density occasioned by equatorial heat ; and that polar 

 cold rather than equatorial heat is the primum mobile of the cir- 

 culation. Analogous views had also be(n entertained by Conti- 

 nental physicists from sea temperature results obtained in Russian 

 and French voyages of lesearch in the early part of this 

 century. 



We have here presented to us two distinct conceptions of 

 ocean circulation — the one to a great extent confined to the 

 surface and horizontal in its movements, the other vertical ex- 

 tending from the ocean surface to its bed, and involving, as a 

 consequence, " that every drop of water will thus [except in 

 confined seas] be brought up from its greatest depths to the 

 surface." 



With these several hypotheses before us, it may be fairly con- 

 sidered that the problem of "ocean circulation " is still unsolved. 

 Possibly, the real solution may require the consideration of 

 physical causes beyond those which have been hitherto accepted. 

 In attempting the st lution, it appears to me impossible to deny 

 that the agency of the winds is most active in bringing about 

 great movements on the surface waters : the effects of the 

 opposite monsoons in the India and China seas furnishing corro- 

 borative proof. Again, the remarkable thermal condition of 

 the lower stratum of the water in enclosed seas, as the Mediter- 

 ranean, and in those basin-like areas of the Western Pacific cut 

 off by encircling submarine ridges from the sources of polar sup- 

 plies, combined with the equally remarkable conditions of cold 

 water from a polar source flowing side by side or interlacing 

 with warm water from equatorial regions — as in the action of 

 the Labrador and Gulf Streams— points to the h)poihesis of a 

 vertical circulation as also commanding respect. 



The time may be considered, however, to have now arrived 

 forgathering up the many threads of information at our disposal; 

 and by fresh combinations to enlarge at least our conceptions, 

 even if we fail in satisfying all the conditions of solution. To 

 this task I will briefly address myself. 



A grand feature in terrestrial physics, and one which I appre- 

 hend bears directly on the subject before us, is that producing 

 ice movement in Antarctic seas. We know from the experience 

 gained in ships — which, to shorten the passages to and from this 

 country, Australia and New Zealand, have followed the great 

 circle route, and thus attained high southern latitudes — that vast 

 tracts of ice from time to time become disrupted from the fringe 

 of southern lands. Reliable accounts have reached us of vessels 

 frequently running down several degrees of longitude, sadly 

 hampered by meeting islands of ice ; and especially of one ship 

 being constantly surrounded with icebergs in the corresponding 

 latitudes to those of London and Liverpool, extending nearly 

 the whole distance between the meridians of New Zealand and 

 Cape Horn. Indeed, accumulated records point to the conclusion 

 that on the whole circumfeience of the globe south of the 50th 

 parallel, icebergs, scattered more or less, may be constantly 

 fallen in with during the southern summer. 



The Antarctic voyages of D'Urville, Wilkes, and James Ross 

 assure us of the origin and character of these ice masses which 

 dot the Southern seas. Each of these voyagers were opposed in 

 their progress southward — D'Urville and Wilkes on the 65'.h 

 parallel, Ross on the 77ih, by barrier cliffs of ice. Ross traced 

 this barrier 250 miles in one unbroken line ; he describes it as 

 one continuous perpendicular wall of ice, 200 to 100 feet high 

 above the sea, with an unvarying level outline, and probably 

 more than 1,000 feet thick — "a mighty and wonderful object." 

 Ross did not consider this ice barrier as resting on the ground, 

 for there were soundings in 2,500 feet a few miles from the 

 cliffs ; Wilkes also sounded in over 5, coo feet, only a short 

 distance from the barrier. 



There is singular accord in the descriptive accounts by Wilkes 

 and Ross of this ice region ; they both dwell on the difference in 



