On Mean Sea Level and Us Fluctuations. 21 



phenomenon sini[)ly to tlic greater effect of the wind in clistnrbint!, the 

 compaiatively small bulk of water to be moved at low tide than the 

 •greater bulk which has to be moved at high. 



In the case of Mr. Ortt and Mr. Wheeler's work, they have dealt 

 with the effect of winds and of barometric gradient, as the case may 

 be, upon individual tides, and have not attempted to trace the average 

 periodic fluctuation which may be ascribed to the same cause. Mr. 

 I), la Cour, in a recent paper dealing with Mean Sea Level in 

 Danish waters, has dealt at considerable length wdth this problem. 

 He shows, on the one hand, that there is a marked correlation 

 between the observed heights, monthly and annual, of Mean Sea Level, 

 and the barometric gradient as determined by a comparison of the 

 barometric heights at the four angles of a quadrilateral extending 

 between Shetland, Calais, Cracow, and the Aland Islands. But 

 after making all due allowance for the barometric variation, the 

 corrected curve of Mean Sea Level is still, very approximately, a sine 

 ciu-ve, with a minimum about the month of March and a maximum 

 about the month of August. Its total amplitude (at Frederikshaven) 

 would seem to be, as far as I can judge from Mr. la Cour's ciu'ves, about 

 12 or 13 cm. The correction for barometric gradient, however, has 

 a very marked effect in depressing the water level as a whole ; the 

 corrected Mean Sea Level for the year being 11*1 cm. at Frederikshaven, 

 and 19' 1 cm. at Esbjerg, below the actual observed mean level. The 

 cause of the under-hdng annual tide is still to .seek. 



There are other points of considerable interest in Mr. la Coiu-'s 

 paper. He shows, for instance, that the mean level of the sea at 

 different places in the Danish Sounds (taking 5-day means) is very 

 closely correlated : in such a way that the difference in level between 

 any two stations can be almost precisely equated with the difference 

 between two others ; or, in other words, that the level at three stations 

 being known, that at a fourth can be at once determined. He shows, 

 in the next place, that the difference in level (5-day or monthly means) 

 between two neighbouring stations is very nearly proportional to, 

 and is therefore the chief cause of, the velocity of the sm-face current 

 in the adjacent channel : with this further refinement, that a given 

 difference of level gives rise to a velocity which differs at different 

 seasons of the year, this difference being apparently due to a diminished 

 internal friction of the water with the rise of temperatui'e in summer. 

 Again, Mr. la Cour has many inteiesting observations i-egarding 

 inaccuracies in. and variations in, the levels of the Danish Survey, 

 and he concludes that, both by the study of the currents and the 

 actual levels, something like an annual movement of the earth's 

 surface is capable of detection. 



The present paper was already WTitten. and in proof, when I received 

 from General V. H. 0. Madsen of Copenhagen an extremely important 

 paper by himself and Lieut. N-. M. Petersen on the Mean Sea Level of 

 the Danish Coasts.* 



These authors work out, by very careful methods, the harmonic 

 formulae for the annual variation of Mean Sea Level at the various 

 Danish stations, and arrive at results very simjlar to those which I 

 have given above. The amplitudes which they find for the various 



* De Danske Kysters Middelvandstaiid. Den Danskr Gradmaaling (Nv 

 Raekke), Heft Nr. 13. Copenhagen, 19U. 



