34 Fisher// Board for Scotland. 



On the other hand, the 19-year (or, rather, 18-6 year) tide, which 

 follows the regression of the modes of the moon's orbit, and which we 

 have just seen to appear so plainly in our curves for the mean inter- 

 tidal ranjie, is not to be detected in our values of Mean 8ea Level. 

 As Darwin has shown,* the ampUtude of this tide must be very small. 

 Its total range amounts to about -12 of a foot at the poles, and about 

 half of that amount at the equator. About latitude 35° it vanishes 

 alt(jgether, and in the latitude of Aberdeen (57°) its total amplitude 

 should be about the same as at the equator. Viz., -06 ft., or say, 

 1-8 cm. ; that is to say the average change of level from one year to 



FEET 1870 

 15-68 



■66 



64 



75 



80 



85 



90 



95 



1900 



05 



62 



60 



■SB 



■56 



•54 



1552 



Fig. 15. — Values of Mean Sea Level at Aberdeen after smoothing in 

 successive groups of 19 years. 



another, due to this tide, is only about -006 ft., or less than (>2 cm. It 

 is not surprising that so small a fluctuation should fail to manifest 

 itself, masked as it evidently is by many and much larger perturbations. 



I have tried to explore, by various " periodogram " methods, our 

 series of values of Mean Sea Level, in the hope of discovering other 

 long-period fluctuations, but as yet with very little, or no, success, 

 save perhaps for the 14-monthly period already alluded to. 



I have some reason to suspect a small fluctuation having a period 

 * Scientific Papers, vol. i. pp. 116, 371, 1907. 



