212 POPULAR LECTURES AND ADDRESSES. 



revolution round the earth, of the earth's round the 

 sun, and of the progression of the moon's perigee. 

 The motion of the first point of Aries, and of the 

 earth's perihelion, are neglected. It is almost 

 certain that the slow variation of the lunar 

 declinational tides due to the retrogression of the 

 nodes of the moon's orbit, may be dealt with with 

 sufficient accuracy according to the equilibrium 

 method ; and the inequalities produced by the 

 perturbations of the moon's motion are probably 

 insensible. But each one of the twenty-three tides 

 enumerated above is certainly sensible on our 

 coasts. And there are besides, as Laplace has 

 shown, very sensible tides depending on the fourth 

 power of the moon's parallax, 1 the investigation of 

 which must be included in the complete analysis 

 now suggested, although for simplicity they have 

 been left out of the preceding schedule. The 

 amplitude and the epoch of each tidal constituent 

 for any part of the sea is to be determined by 

 observation, and cannot be determined except by 

 observation. But it is to be remarked that the 

 period of one of the lunar diurnal tides agrees with 

 that of one of the solar diurnal tides, being 

 twenty-four sidereal hours ; and that the period of 

 one of the semidiurnal lunar declinational tides 

 agrees with that of one of the semidiurnal solar de- 

 clinational tides, being twelve sidereal hours. Also 



1 The chief effect of this at any one station is a ter-diurnal lunar 

 tide, or one whose period is eight lunar hours. 



