i 4 A TEXTBOOK OF OCEANOGRAPHY 



irregular near the land, but also in the open ocean a corre- 

 sponding irregularity is met with, since each depression of the 

 ocean bottom is correlated with a depression of the sea-level. 

 These irregularities of sea-level can be determined by experi- 

 ment or can be calculated from theoretical considerations. The 

 pendulum is used for this determination. If a pendulum be 

 swung on the same parallel of latitude over the land and then 

 over the sea where there is a marked depression of the ocean 

 floor, it should swing more quickly in the latter position, where 

 it is nearer the earth's centre. Records of the seconds pendulum 

 taken off the Bonin Islands (Japan) and the north coast of 

 Brazil showed, according to Listing, a difference in level and 

 consequently a height for the continental wave of 2 kilometres. 

 This estimate is almost certainly erroneous and far too high. 



More modern attempts with Sterneck's half-second pen- 

 dulum are certainly more accurate, and the best of these were 

 recorded by Scott Hansen of the Fram when Nansen's ship 

 was frozen in in the Arctic (1894-95). Scott Hansen found in 

 areas of the sea where the depth was 1,640 fathoms (3,000 

 metres) that the periodicity of Sterneck's pendulum only 

 departed very little from the normal, and it is very probable 

 that this is true, not only for the Arctic " Ocean," but also for 

 the larger oceans. 



Comparisons of the pressure by the mercurial barometer 

 and observations made with delicate boiling-point thermo- 

 meters on the South Atlantic between Lisbon and Bahia show 

 the gravity on this route is nearly normal. 



Temporary differences in sea-level are due to a number of 

 causes, among which may be mentioned the sun's rays; the 

 influence of large volumes of \vater derived from land drainage, 

 very noticeable at Kronstadt (Gulf of Finland) and Odessa; 

 prevailing winds ; and the barometric pressure. 



The level of the sea varies with the pressure of the air; 

 every movement of the mercurial barometer is correlated with 

 a change in the sea-level. Sir James Ross, who wintered at 

 Port Leopold in 1848, made hourly observations of the height 



