176 REPORT — 1876. 



the year : thus the greatest daily tidal range in auy month yery rarely exceeds 3 

 feet ; but the high- and low-v/ater marks range dm-ing 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 Januarj^ as compared Avith the summer months, 

 was traced by the late Captain Beechey, E.N., at Holyhead (see Phil. Trans. 1848). 

 If this surface-osciUation is a general oceanic feature (and some further proofs in- 

 directly appear in the Reports of the Tidal Committee to this Association for 1868, 

 1870, 1872, 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 recognize this physical 

 condition — that the waters of the southern hemisphere attain a high level at the 

 period of the year when the sun 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 simimer months than in the opposite season ; 

 these difl'ereuces of level are conmionly assigned to the action of the winds. 



Renuell, 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. It is 

 possible that the cause, here and elsewhere, may in part be cosmical, and neither 

 meteorological nor astronomical in a tidal sense. 



These several facts in relation to the variations in level of the surface of the 

 ocean are interesting, and point to new fields of observation and research. 



Another physical feature connected \^^th the ocean-level is deserving considera- 

 tion ; 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 depression 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 •608 of an inch in the barometer produced a difference 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-barometer on 

 a vast scale of magnificence, and that the level of its surface is disturbed by every 

 variation of atmospheric pressure inversety as the mercm-y in the barometer, and 

 exactly in the ratio of the relative specific gravities of the water and the mercury." 

 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 pressiives cannot exist without presumably affecting the 

 surface-conditions of adjacent waters. 



There is yet one more point in connexion with the ocean-circulation which I 

 venture to think has not received the attention it demands ; this is, the economy of 

 those currents known as " counter-equatorial." Their limits are now fairly ascer- 

 tained, and are foimd 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 occasion- 



