308 



The Harmonic Analysis of Tidal Observations 



of the principal tides of these four harbours. Immingham has a semi-diurnal 

 type of tides with a ratio F = 01 1 ; San Francisco has the mixed type, mainly 

 semi-diurnal tides with a ratio F = 0-90; Manila shows a mixed, but mainly 

 diurnal type of tides F = 215, whereas Don Son in the Gulf of Tonkin has 

 a pronounced diurnal tide with the very high ratio F = 18 9. These four 

 main tides (M 2 , S 2 , K t , O x ) are responsible for the tidal picture because they 

 account for about 70% of the total amplitude in all four harbours. When 

 we take into account the other important components N 2 , K 2 and P x this 

 percentage is increased to about 83%. 



San Franciso mixed, dominant semi diurnal type, ^"=090 



Manila mixed, dominant full diurnal type, ^"=2-15 



Do-Son full diurnal type, F-189 



8 10 

 OO 



16 18 20 22 24 26 28 30 



S( O • N l 



Fig. 123. Tide curves for March 1936 O ( ® ) > phases of the moon. N and S are the 

 largest northern and southern declination of the moon; Q passage of the moon through 



the Equator. 



3. Variations in the Harmonic Tidal Constants 



When tidal records extending over a number of years are available for 

 a locality the harmonic constants of the main constituants can be computed 

 for each period of one year. Contrary to what would be expected, they are 

 not constant, but show small variations from year to year, both in the 



