The Harmonic Analysis of Tidal Observations 



309 



amplitudes and in the phases, even when the above-mentioned reduction to 

 average conditions is made. As an example. Table 34 gives a summary of 

 the variations in amplitude and phase of the seven most important compo- 

 nents M 2 , S 2 , N 2 , K 2 and /v 1 ,0 1 ,P 1 , for Bombay for the years 1906-15. It 

 shows that the variations from one year to another are relatively small for am- 



Table 34. Variations in tidal constants of Bombay 1906-15 



plitudes and phases, but they, nevertheless, show that the differences increase 

 with decreasing amplitude of the component. Even more so, when one consid- 

 ers the other, less important constituents. For these Proudman (1920, p. 328) 

 has shown that in some years the amplitude is ten times larger than in other 

 years, whereas the phase values show great deviations from the average value. 



There is hardly any doubt that the yearly variations of the harmonic con- 

 stants are partly caused by the methods of analysis that are used; however, 

 it seems that there are also systematical variations attributable to specific 

 causes. Doodson (1924, p. 573) has closely checked the harmonic constants 

 of Bombay and St. John, Bay of Fundy, and has dealt probably for the first 

 time with such questions more thoroughly. In the case of the most important 

 semi-diurnal component M 2 , he found three definite kinds of disturbances. 

 The first one has a period of approximately 19 years, corresponding to the 

 regression of the node of the moon, the second one has a period of about 

 4 years and is supposed to be based chiefly upon insufficiencies of the analyt- 

 ical methods, whereas the third kind is of a secular, unknown origin. 



Figures 124 and 125 show the variations of the harmonic constants of 

 M 2 for St. John and Bombay,^ according to Dawson and Roberts. In both 

 harbours the variations are very large. In St. John H varies from 291-3 

 to 307-1 cm; if the reduction factor /is considered, these values are reduced 

 to 288-2 and 304-7 cm. It is obvious that the reduction factor /, which is 

 applied for the purpose of reducing H to average conditions, makes no sense, 

 for which reason Dawson wants it no more to be used. The main variation 

 in H and x is, at any rate, the period of 19 years. It also appears in Bombay, 



