76 



farthest from the Equator and the diurnal tidal impulses are conse- 

 quently a maximum (paragraph 40) . In tides of the mixed type, the 

 two daily high waters and the two daily low waters become nearly 

 equal when the moon is near the Equator, and the diurnal tidal im- 

 pulses are a minimum. When the moon is at its maximum declina- 

 tion, near the celestial tropics, one of the two daily low waters or high 

 waters of tides of the mixed type occasionally may disappear, pro- 

 ducing a diurnal tide. On the other hand, tides of the diurnal type 

 usually break down into two daily tides during a part of the month, 

 although in the Gulf of Tongking in Indo-china, the tide remains 

 diurnal throughout the month. 



Obviously the types of tide merge into each other. The accepted 

 criterion distinguishing the types is the ratio (Ki + Oi)/(M2+S2), 

 derived from the harmonic components at the station. If this ratio 

 is less than 0.25, the tide is classed a semidirunal; if between 0.25 and 

 1.25 as mixed; and if over 1.25 the tide is classed as diurnal. 



THE EFFECT OF THE PRINCIPAL SEMIDIURNAL COMPONENTS 

 ON THE TIDES 



141. The M2 component, semidiurnal tides. — When the tide is of 

 the semidiurnal type, the M2 component, with rare exceptions, is the 

 dominant one, with an amplitude nearly but not quite one-half of the 

 mean tidal range. Generally, its amplitude may be taken as 0.47 

 times the mean range. 



142. Relation of epoch 0/M2 component to lunitidal intervals. — As has 

 been seen (paragraph 117), the expression for the M2 component of the 

 actual tide may be written in the form : 



y=M2 cos (m2t+Vo+u-M2°) (98) 



in which 



m2 is the speed of the component, and has the numerical value 

 of 28.984° per solar hour. 



Vq-^u is the value of the equilibrium argument at any arbi- 

 trarily chosen origin of time. 



M2° is the epoch of the component. 



The expression for the M2 equilibrium component is 



y=M2 cos im2t+Vo+u) (99) 



At the high water of the actual tide 



m2t+Vo+u-M2° = 

 whence 



t=[M2°-Vo-u]/m2 (100) 



