32 Fishery Board for Scotland. 



and as we see from Table VII., it shows us a semi-amiiial wave, whose 

 maxima correspond to the spring tides of spring and autumn. 



When we deal with it over a long period of years, we see at once 

 that it varies with very remarkable regularity. As wall be seen from 

 Table VII. and Fig. 13, the Mean Amiual Intertidal Kange varies in 

 very close approximation to a sine curve, having a total amplitude 

 of about 07 ft., with maxima in or about the years 1866, 1885, 1904. 

 This is in precise correspondence with the variation in the moon's 

 inclination to the ecliptic, which, in its period of ISJ years, has its 

 minima at these identical epochs. Of the three maxima shown by 

 our curve, that of 1866 is considerab],v the lowest, and that of 1885 

 is the highest of the three. If, for the purpose of eliminating this 

 19-year tide, we smooth the annual readings in successive groups 



FEET 1871 

 •06 r 



1901 04 



■06 



Fig. 14. — The Mean Intertidal Range at Alierdeen, after elimination 

 of the 19-year wave by smoothing in successive groups of 19 years. 



of 19 years, we find that there remains a residual tide of very long 

 period, so long that the whole extent of our observations does not 

 give us a complete wave (Fig. 14). The maximum of this wave lies 

 somewhere about 1886, that is to say, it approximately coincides 

 with one of the 18j-year maxima. The total fall, from 1886 to 1904, 

 in which year the curve is still a descending one, is just over '11 ft., 

 or say 3"3 cm. The form of the curve suggests that its period is 

 somewhere about 50 years. In spite of the very small amplitude of 

 this fluctuation, it will be seen that the successive points of the curve 

 come out with very great regularity. I have no hypothesis to offer 

 as to the cause of this phenomenon. 



The 19-year tide is equally well-marked in our records from 

 Dundee, as will be seen from the following Table : — 



