On Mntii Sea Level and tVs Flnctuationa. '■' 



Table 0. — Interva! between Mean lliuh and JjOW Water Levels 

 (in feet) at Dundee. 



Just as in the case of Aberdeen, we see that the largest interval 

 was in 1904, and that the height of this interval fell away in a nearly 

 regular curve from this year of maximum to the beginning of our 

 series in 1897, and again towards the end. The whole period of 19 

 years is not represented ; but the amplitude within the years available 

 is about 0"63 ft., which agrees very well with the total amplitude which 

 we have found at Aberdeen, viz., 0"70 ft. 



While the curve for the Intertidal Range is a very smooth one, the 

 curves for Mean High and Mean Low Water, from which it is derived, 

 are not nearly so smooth. On the contrary, they both show minor 

 undulations, which to a large extent cancel one another out when 

 the one set are subtracted from the other. In the main, the curves 

 for Mean High and Mean Low Water are each made up of a succession 

 of sine cm'ves, corresponding to the 19-year period, but they apparently 

 contain other secondary harmonics which I have not elucidated. 



When smoothed in successive periods of 19 years, the values both 

 for Mean High and Mean Low AVater show a long-period curve, de- 

 scending from the beginning of the period, and again ascending towards 

 its end. But the two curves differ in amplitude and apparently 

 also in phase. The amplitude of the Mean High Water curve, so far 

 as it goes, i.e. from 1871 to the minimum about 1893, is about "10 ft. ; 

 that of the Mean Low Water curve, in the same period, is about 

 •19 ft. Owang to the fact that both curves are incomplete, showing 

 only part of an entire vibration, it is impossible to determine the 

 pha^e even approximately ; but it looks as though the minimuin were 

 somewhat later in the case of Mean High Water than for Mean Low 

 Water. 



Returning now to the curve of Mean Sea Level, it is obvious that 

 it must show a long period fluctuation, similar to tliose of Mean High 

 and Mean Low Water. This fluctuation is illustrated in Fig l.j. 

 From 1871 (which is the middle of our first group of 19 years) the 

 Mean Sea Level falls away, on the whole smoothly, till 1893. and again 

 rises till 1904 (which again is the middle of our last gioup of 19 years). 

 The difference of level between 1871 and 1893 amounts to just about 

 •15 ft, or 1*8 in., or, say, 45 cm. Small as the amplitude of this 

 fluctuation is, and spread over some 22 years, it is remarkable 

 how clearly and regularly it is brought out by the observations at 

 hand. 



