Dec. 7, 1876] 



NATURE 



121 



ach, and both branches acquiring a slight but decided 

 ieasterly set by their excess of initial velocity, pass 

 orthwards and southwards directed for a time by the 

 and coasts. But the fate of the southern is very different 

 rom that of the northern branches. Instead of accumu- 

 ting and " banking down " in the confined gulfs, the 

 Atlantic, the Pacific, and the Indian Oceans, they at once 

 pass into the waste of the " water hemisphere," and are 

 almost merged in the great drift current which sweeps 

 round the world occupying a belt from 600 to 1,000 miles 

 broad in the southern sea. But while the greater portions 

 of the Brazilian current, the East Australian current, and 

 the southern branch of the Agulhas current are thus 

 merged they are not entirely lost ; for at their points of 

 junction with the drift current of the westerlies the whole 

 belt of warm water is slightly deflected to the southward, 

 and it is oppi =;ite these points of junction that we have 

 the comparatively open sea and the penetrable notches in 

 the southern pack. 



Thus Ross, in 1841 and 1842, after forcing his wiy 

 through the moving pack, which he found sufficiently open 

 to allow of his doing so, passed between the meridians of 

 170° E., and 170° W,, to the latitude of 78" 11' S. 

 Weddell in 1823, and Ross in 1843, reached the parallels 

 of 74° 14' and 71° 30' S. respectively, between the meri- 

 dians of 15° and 30° W. The case of the Brazilian 

 current is, however, a little more complicated than that of 

 the others, for there is high and extensive land between 

 the meridians of 55° and 65° W., in 65° south latitude, 

 and the warm current already led fir to the southward by 

 the south-American coast appears to bifurcate upon 

 Graham Land, and to produce another bight, in 90° west 

 longitude, a little to the west of the southern point of the 

 South American Continent. In this bight. Cook, in 177 1, 

 and Bellingshausen in 1821, pushed nearly to the 70th 

 parallel of south latitude. 



The opening caused by the deflection of the southern 

 branch of the Agulhas current towards Kerguelen and 

 the Heard Islands, has not yet been fully explored, but 

 what has been already done in that direction seems to be 

 in striking confirmation of this view. We had indeed 

 arrived independently at the same conclusion before 

 reading Dr. Neumayer's paper. At the point where we 

 crossed the Antarctic Circle (Long. 78° o' E ), and for some 

 distance to the westward there were few icebergs, and the 

 sea was almost clear to the south-west. It was Capt. 

 Nares' opinion that had it been considered desirable and 

 had the attempt been made earlier in the season, it would 

 have been easy for us to have pushed southwards in that 

 direction — how far we had no means of ascertaining — but 

 the pack was moving about round us, and for the reasons 

 already given we believed the barrier to be at a consider- 

 able distance. 



But we not only observed the effect of the influx of 

 warmer water ; we were able to detect its presence by the 

 thermometer. Referring to the results of a serial tempe- 

 rature-sounding of February 14, with a surface-tempe- 

 rature of — r2 C, between 300 and 400 fathoms there 

 is a band rising to more than half a degree above the 

 freezing-point. That this warm layer is coming from the 

 north there is ample proof. We traced its continuity 

 with a band at the same depth gradually increasing in 

 warmth, to the northward ; and it is clear that its heat can 

 be derived from no other source, and that it must be con- 

 tinually receiving new supplies, for it is overlaid by a band 

 of colder water tending to mix with it by convection. 



It is of course possible that the three warm currents 

 may, by coincidence, be directed towards three notches 

 already existing in a continental mass of land ; but such a 

 coincidence would be remarkable, and there is certainly 

 a suggestion of the alternative, that the " continent " may 

 consist to so great an extent of ice as to be liable to have 

 its outline affected by warm currents. 



The second consideration is that during summer, the 



only time when these regions have been as yet visited, 

 the greater part of the outline of the area representing 

 the Antarctic continent has been found to consist of 

 moving ice-pack. The prevailing winds within the 

 Antarctic Circle are from the south-east, and as a rule 

 the pack and the icebergs are moving to the westward, 

 and fanning out from a centre. Almost all the navigators 

 who have passed the belt of pack have receivedthe im- 

 pression that there was open water within, that, in fact, 

 by that time, late in the summer, the pack of the year 

 had been drifted a considerable distance from its nucleus — 

 the land or the continuous ice-sheet. If this be so it 

 would at all events indicate that the "Antarctic continent" 

 does not extend nearly so far from the Pole as it has been 

 supposed to do. 



I conceive then that the upper part of one of these 

 tabular icebergs, including by far the greater part of its 

 bulk and culminating in the portion exposed above the 

 surface of the sea, was formed by the piling up of succes- 

 sive layers of snow during the period, amounting perhaps 

 to centuries, during which the ice-cap was slowly forcing 

 itself over the low land and out to sea, over a long extent 

 of gentle slope, until it reached a depth considerably 

 beyond 200 fathoms. The lower specific weight of the 

 ice then caused an upward strain which at length over- 

 came the cohesion of the mass, and portions were rent off 

 and floated away. If this be the true history of the for- 

 mation of these icebergs, the absence of all land debris 

 on the portion exposed above the surface of the sea is 

 readily understood. If any such exist it must be confined 

 to the lower part of the berg, to that part which has 

 moved upon the floor of the ice-sheet. 



The icebergs, when they are first dispersed, float in 

 from 200 to 250 fathoms ; when, therefore, they have been 

 drifted to latitudes of 65° or 64° S., the bottom of the 

 berg just reaches the layer at which the temperature of 

 the water is distinctly rising, and is rapidly melted, and 

 the mud and pebbles with which it is more or less charged 

 are precipitated. That this precipitation takes place all 

 over the area where the icebergs are breaking up, con- 

 stantly and to a considerable extent, is evident from the 

 fact that the matter brought up by the sounding-instru- 

 ment and the dredge is almost entirely composed of such 

 deposits from ice; for diatoms, Globigerin^., and radio- 

 larians are present on the surface in large numbers, and 

 unless the deposit from the ice were abundant, it would 

 soon be covered and masked by a layer of the exuviae of 

 surface organisms. 



There is one point in connection with the structure of 

 icebergs which is of great interest, but with regard to 

 which I do not feel in a position to form a definite judg- 

 ment. It lies, however, especially within the province of 

 a distinguished professor in the University of Glasgow, 

 Dr. James Thomson, and I hope he will find leisure to 

 bring that knowledge and experience to bear upon it 

 which have already thrown so much light upon some of 

 the more obscure phenomena of ice. I have mentioned 

 the gradual diminution in thickness of the strata of ice 

 in a berg from the top of the berg downwards. The re- 

 gularity of this diminution leaves it almost without a doubt 

 that the layers observed are in the same category, and 

 that therefore the diminution is due to subsequent pres- 

 sure or other action upon a series of beds which were 

 at the time of their deposition pretty nearly equally thick. 

 About 60 or 80 feet Jrom the top of an iceberg the strata 

 of ice a foot or so in thickness, although of a white 

 colour, and thus indicating that they contain a quantity 

 of air, and that the particles of ice are not inclose appo- 

 sition, are still very hard, and the specific gravity of the 

 ice is not very much lower than chat of layers not m^re 

 than 3 inches thick nearer the water-line of the berg. 

 Now it seems to me that this reduction cannot be due to 

 compression alone, and that a portion of the substance 

 of these lower layers must have been removed. 



G2 



