310 



The Harmonic Analysis of Tidal Observations 



although much weaker, but here there is a particularly evident period of 

 approximately 4 years with a large amplitude. Doodson proves that part 

 of this variation is due to the influence of the N 2 on M 2 , whereas the period 

 of 19 years can be eliminated, when the variation in the ascending node of 

 the moon's orbit is taken into account. If these two reductions are made, 



Fig. 124. Harmonic constant M 2 for St. John (Bay of Newfoundland). H 1 , original 



values; --•--•, reduced values; , average secular variation (Doodson). 



there remains in both cases under consideration a secular variation, which 

 for St. John is a decrease of the amplitude of about 3 cm and increase in 

 the phase of 1° over a period of 20 years. In Bombay, the decrease of the 

 amplitude is constantly 0085 cm per year, whereas in the period 1880-1900 the 

 phase increased from 329-2° to 331-5°, and then decreased again. The harmonic 

 constants of N 2 and S 2 show similar variations ; the amplitude of K x decreases 

 constantly since 1890. Karachi, on the contrary, shows variations in the 

 opposite direction (amplitude M 2 in 1870, 75-3 cm; in 1920, 79-3 cm), whereas 

 the phase does not show a steady variation. 



The variations of the mean sea level in Bombay, Karachi and Aden show 

 the following values: 



Decade ... 

 Bombay (cm) 

 Karachi (cm) 

 Aden (cm) 



Karachi shows the greatest variations with about 3-6 cm within 30 years; 

 however, the increase seems to be accelerated. These variations in the mean 

 sea level are not sufficiently important to explain the observed secular 



