22 



O so- SBO" 8,10" 



Lunar hour angle 



Figure 13.— Diurnal, semidiurnal, and resultant tides. 



It will be noted that the semidiurnal part of the lunar equilibrium 

 tide follows the sinusoidal curve of the cosine function, with a very 

 slight skew because of the change in the moon's declination during the 

 day. The skew of the diurnal part of the tide because of this change 

 is more pronounced. The combination of these two parts produces 

 the inequalities of the daily tides inferred in paragraph 34. The 

 ranges of the two parts of the equilibrium tide are affected differently 

 by the latitude of the tidal station. At high latitudes the diurnal 

 part may mask the semidiurnal fluctuations and produce an equili- 

 brium tide which is predominantly or wholly diurnal. 



40. Variations in the amplitude of the diurnal and semidiurnal parts 

 of the lunar equilibrium tides with the moon's declination. — The ampli- 

 tude of the fluctuations of the semidiurnal part of the lunar equilib- 

 rium tide evidently varies with the coefficient of cos 2H in the first 

 term of equation (24) . At a tidal station whose latitude is X, it con- 

 sequently varies with cos^ 5. It is therefore a maximum when the 



