TRANSACTIO^'S OP THE SECTIOXS. 175 



origin and character of tlicsc ice-niasscs -wliich dot the southern seas. Each of 

 these voyagers was opposed in his progress southward (U'Ur^ille and Willies 

 on the G5th parallel, Ross on the 77th) by barrier-cliffs of ice. lloss traced this 

 barrier 250 miles in one unbroken line : he describes it as one continuous pei-peu- 

 dicular wall of ice, 200 to 100 feet high above the sea, with an unvarying level 

 outline, and probably more than 1000 feet thick — " a mighty and wonderful object." 

 Ross did not consider this ice-barrier as resting on the ground ; for there were 

 soundings in 2500 feet a few miles from the clitts ; Wilkes also sounded in over 

 5000 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 differences in character of Antarctic from 

 Arctic ice-formation, on the tabular form of the upper surface of the floating ice- 

 bergs and their striated appearance, on the extreme severity of the climate in 

 mid-summer, on the low barometric pressure experienced, and express equal 

 wonderment at the stupendous forces necessary to break awaj^ the face of these vast 

 ice barriers, and the atmospheric causes necessary for their reproduction. 



From the drift of this disrupted ice we have fair evidence of a great bodily move- 

 ment of the waters northward ; for it must be remembered that icebergs have been 

 fallen in with in the entire circumference of the southern seas, 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 SOtli 

 parallel. 



In the discussion of Ocean Circulation, it has been assumed that water flows 

 from Equatorial into Antarctic areas ; there is no evidence, so fiir as I am aware, 

 that warm surface-water in the sense implied is found south of the 45th parallel. 

 Surface stream movement northward and eastward appears to be that generally 

 experienced in the zone between the Antarctic circle and that parallel. AVith, then, 

 this great bodily movement northward of Antarctic waters included certainly 

 between the surf^ice and the base, or nearly so, of these tabular icebergs (and thus 

 representing a stratum certainlj' some thousand feet in thickness), the question 

 ai'ises, How and whence does the supply come to fill the created void ? Sir 

 Wyvillo Thomson, the leader of the ' Challenger ' scientific staff", in one of the 

 later cf 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 canses, the excess of evaporation over precipi- 

 tation 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 Antarctic waters,! would 

 draw attention to a physical feature in connexion with tidal movements, which 

 possibly may be one of the many links in the chain of causes affecting Ocean 

 Circulation. The mean tide-level (or that imaginary point equidistant from the 

 high- and low-water marks as observed throughout a whole lunation) has been 

 assumed as an irnariable quantity ; our Ordnance Survey adopts it as the zero 

 from which all elevations are given, the f/«^(»?i /eiW for Great Britain being the level 

 of mean tide at Liverpool. For practical piirposes, at least on our own shores, 

 this mean sea-level may be considered invariable, although recent investigations 

 of the tides at Liverpool and Ramsgate indicate changes in it to the extent of a 

 few inches, wJiich changes are embraced in an annual period, attaining the 

 maximum height in the later months of the year: these have been assumed as 

 possibljr 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 bo 

 remembered, to the wide southern seas — the sea-level -waries appreciably during 



16* ' 



