174 REPORT— 1876. 



and assigned to diflerences of specific gravity, combined with the earth's rotation 

 on its axis, tlie movement of the Gulf-stream and other well-defined ocean 

 currents. 



A writer of the present time, gifted with high inductive reasoning powers, and 

 with observed facts before him in wide extension of those investigated by Rennell, 

 regards the various ocean currents as members of one grand sj'stem of circulation, 

 3iot produced by tlie 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, without exception, he tinds the direction of 

 the main currents of the globe to agree exactly with the direction of the prevailing 

 winds. 



Another writer of the present day, distinguished for intellectual power, and 

 who personally' has devoted much time in the acquisition of exact ph3'sical facts 

 bearing on the question both in the ocean near our own shores and in the Mediter- 

 ranean sea, without denjdug the agency of the winds, so far as surface-drifts are 

 concerned, considers that general Ocean Circulation is dependent 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 towards the poles, the 

 " disturbance of hydrostatic equilibrium " being produced by the increase of density 

 occasioned by polar cold and the reduction of densit}' occasioned by equatorial 

 heat — and that polar cold rather than equatorial heat is t\ie primum mobile of the 

 circulation. Analogous views had also been entertained by Continental physicists 

 from sea-temperature results obtained in Russian and French voyages of research 

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

 consequence, "that every drop of water will thus [except in confined seas] bo 

 brought up from its gi-eatest depths to the surface." 



With these several hypotheses before us, it may be fairly considered that the 

 problem of "Ocean Circulation " is still unsolved. Possibly, too, the real solution 

 may require the consideration of phj'sical causes beyond those which have been 

 hitherto accepted. In attempting the solution, it appears to me impossible to deny 

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

 the surface-waters, the effects of the opposite monsoons in the India and China 

 seas furnishing corroborative proof. Again, the remarkable thermal condition of 

 the lower stratum of the water in enclosed seas, as the Mediterranean, and in those 

 basin-like areas of the Western Pacific cut off by encircling submarine ridges from 

 the sources of polar supplies, combined with the equallj' remarkable condition 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 Ijabrador and Gulf-streams, points 

 to the hypothesis of a vertical circulation as also commanding respect. 



The time may be considered, however, to have now arrived for gathering 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 briefl}' address myself. 



A grand feature in terrestrial physics, and one which I apprehend bears directly 

 on the subject before us, is that producing ice-movement in the 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'sm-rounded 

 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 vvhole 

 circumference of the globe south of the 60th parallel, icebergs, scattered more or 

 less, may be constantly fallen in with during the s oulhem summer. 



The Antarctic voyages of D'Urville, AVilkes, and James Ross asturo us of the 



