Sept. 7, 1876] 



NA TURE 



415 



character of Antarctic from Arctic ice formation, on the tabular 

 form of the upper surface of the floating icebergs, and their 

 striated appearance ; on the extreme severity of the climate in 

 midsummer ; of the low barometric pressure experienced — and 

 express equal wonderment at the stupendous forces necessary to 

 break away the face of these vast ice barriers, and the atmo- 

 spheric causes necessary for their reproduction. 



From the drift of this disrupted ice we have fair evidence of a 

 great bodily movement of the waters northward ; for it must be 

 remembered that icebergs have been fallen in with in the entire 

 circumference of the southern sea?, and that they are pushed in 

 the South Atlantic Ocean as far as the 40th parallel of latitude ; 

 in the South Indian to the 45th parallel ; and in the South 

 Pacific to the 50th parallel. 



In the discussion of ocean circulation, it has been assumed 

 that water flows from Equatorial into Antarctic areas ; there is 

 no evidence, so far as I am aware, that warm surface water in 

 the sense implied is found south of the 55th parallel. Surface 

 stream movement northward and eastward appears to be that 

 generally experienced in the zone between the Antarctic circle 

 and that parallel. With, then, this great bodily movement north- 

 ward of Antarctic waters included certainly between the surface 

 and the base, or nearly so, of these tabular icebergs (and thus 

 representing a stratum certainly some thousand feet in thickness), 

 the question arises. How and from whence does the supply come 

 to fill the created void ? Sir Wyville Thomson, the leader of 

 the Challenger scientific staff", in one of the later of the many 

 able reports he has forwarded to the Admiralty, furnishes, I 

 think, a reasonable answer. Stating first his views as derived 

 from study of the bottom temperature of the Pacific Ocean 

 generally, he writes : — "We can scarcely doubt that, like the 

 similar mass of cold bottom-water in the Atlantic, the bottom- 

 water of the Pacific is an extremely slow indraught from the 

 Southern Sea." He then gives the reason. " I am every day 

 more fully satisfied that this influx of cold water into the Pacific 

 and Atlantic Oceans from the southward is to be referred to the 

 simplest and most obvious of all causes, the excess of evaporation 

 over precipitation of the land-hemisphere ; and the excess of 

 precipitation over evaporation in the middle and southern parts 

 of the water-hemisphere." 



Before following up the great northward movement of Ant- 

 arctic waters, I would draw attention to a physical feature in 

 connection with tidal movements, which possibly may be one 

 of the many links in the chain of causes affecting ocean circu- 

 lation. The mean tide level (or that imaginary point equi- 

 distant from the high and low water-marks as observed through- 

 out a whole lunation) has bten assumed as an invariable 

 quantity ; our Ordnance Survey adopts it as the zero from whence 

 all elevations are given : the datum level for Great Britain being 

 the level of mean tide at Liverpool. For practical purposes, at 

 least on our own shores, this mean sea -level may be considered 

 invariable, although recent investigations of the tides at Liver- 

 pool and Ramsgate indicate changes in it to the extent of a few 

 inches, and which changes are embraced in an annual period, 

 attaining the maximum height in the later months of the year ; 

 these have been assumed as possibly due to meteorological 

 rather than to the astronomical causes involved by tidal theory. 



From an examination of some tidal observations recently made 

 near the mouth of Swan River, in Western Australia, during the 

 progress of the Admiralty survey of that coast, there appears to me 

 evidence that in this locality — open, it will be remembered, to the 

 wide southern seas — the sea-level varies appreciably during the 

 year : thus, the greatest daily tidal range in any month very rarely 

 exceeds 3 feet, but the high and low water-marks range during 

 the year 5 feet. The higher level is attained in June, and 

 exceeds the lower level, which is reached in November, by one 

 foot or more. At Esquimalt in Vancouver Island, fairly open 

 to the North Pacific Ocean, there are indications of the sea-level 

 being higher in January than it is in June ; and a distinct excess 

 of the mean level of the tide by several inches in December and 

 January, as compared with the summer months, was traced 

 by the late Captain Beechy, R.N., at Holyhead (see FMl. Trans. 

 1848). If this surface oscillation is a general oceanic feature, 

 and some further proofs indirectly appear in the Reports of the 

 Tidal Committee to this Association for 1868, '70, '72, to which 

 I have just referred — for mention is also made of a large annual 

 tide of over three inches, reaching its maximum in August, 

 having been observed at Cat Island, in the Gulf of Mexico ; — 

 we may have to recognise this physical condition, that the waters 

 of the southern hemisphere attain a high level at the period of 

 the year wjien the smi is to the north of the equator, and that 



the northern waters are highest at the period when the sun is to 

 the south of the equator. This is a question of so much interest 

 that I propose again to revert to it. 



Variations in the sea level have been observed, notably in the 

 central parts of the Red Sea, where the surface water, as shown 

 by the exposure of coral reefs, is said to be fully two feet lower 

 in the summer months than in the opposite season j these differ- 

 ences of level are commonly assigned to the action of the winds. 



Rennell, in his " Investigation of the Currents of the Atlantic 

 Ocean," states, on what would appear reliable authority, that on 

 the African Guinea coast the level of the sea is higher by at 

 least six feet perpendicular in the season of the strong S.W. and 

 southerly winds — which winds blow obliquely into the Bay of 

 Benin between April and September, the rainy season also — than 

 during the more serene weather of the opposite season ; the 

 proof being that the tides ebb and flow regularly in the several 

 rivers during the period of strong S. W. winds, but that in the 

 other season the same rivers run ebb constantly, the level of the 

 sea being then too low to allow the tide waters to enter the 

 mouths of the rivers. Ic is possible the cause, here and elsewhere, 

 may, in part be cosmical, and neither meteorological nor astro- 

 nomical in a tidal sense. 



These several facts in relation to the variations in levels 

 of the surface of the ocean are interesting, and point to new fields 

 of observation and research. 



Another physical feature connected with the ocean level is 

 deserving consideration ; I refer to the effect of the pressure of 

 the atmosphere. On good authority we know that the height 

 of high water in the English Channel varies inversely as the 

 height of the barometer ; the late Sir John Lubbock laid it down 

 as a rule that a rise of one inch in the barometer causes a depres- 

 sion in the height of high water amounting to seven inches at 

 London and to eleven inches at Liverpool. Sir James Ross 

 when at Port Leopold, in the Arctic seas, found that a difference 

 of pressure of "668 of an inch in the barometer produced a differ- 

 ence of 9 inches in the mean level of the sea, the greatest 

 pressure corresponding to the lowest level. These results 

 appeared to him to indicate " that the ocean is a water-baro- 

 meter on a vast scale of magnificence, and that the level of its 

 surface is disturbed by every variation of atmospheric pressure 

 inversely as the mercury in the barometer, and exactly in the 

 ratio of the relative specific gravities of the water and the mer- 

 cury." When we consider the exceptionally low barometric 

 pressure prevailing in the southern seas, and the comparatively 

 low pressure of the Equatorial Ocean zones as compared with 

 the areas of high pressure in the oceans north and south of the 

 Equator — the latter features a late development by Mr. Buchan 

 — these characteristic conditions of atmospheric pressures cannot 

 exist without presumably affecting the surface conditions of 

 adjacent waters. 



There is yet one more point in connection with the ocean 

 circulation which I venture to think has not received the atten- 

 tion it demands ; this is the economy of those currents known 

 as "counter equatorial." Their limits are now fairly ascertained, 

 and are found to be confined to a narrow zone ; they run in a 

 direction directly opposite to, and yet side by side with, the 

 equatorial streams of both the Atlantic and Pacific Oceans. We 

 know that they run at times with great velocity (the Challenger 

 experienced fifty miles in a day in the Pacific Ocean), and occa- 

 sionally in the face of the trade wind ; and that they are not 

 merely local, stretching as they do across the wide extent of the 

 Pacific ; and in the Atlantic, during the summer months of our 

 hemisphere, extending nearly across from the Guinea Coast to 

 the West India Islands. They have too this significant feature 

 that their narrow zone is confined to the northern side alone of 

 the great wect-going equatorial currents ; this zone is approxi- 

 mately between the parallels of 7° and 10° N., and thus corre- 

 sponds with the belt of greatest atmospherical heat on the earth's 

 surface. 



That the functions of the counter currents in the physics of the 

 ocean are important must, I think, be conceded. They appear 

 to act on their eastern limits as feeders to the equatorial cur- 

 rents ; and from the seasonal expansion, which has been well 

 traced in the Atlantic, are probably more immediately associated 

 with some oscillatory movement of the waters following, though 

 perhaps only remotely connected with, the sun's movements in 

 declination. 



A brief summary of the thermal conditions of the oceanic 

 basins will now enable us to review the salient features of ocean 

 circulaiion, and the more immediate scientific position the ques- 

 tion has assigned. 



