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direction as the earth's rotation. In consequence of this motion the 

 huiar day, that is to say, the interval between two successive transits 

 of the moon across the meridian, is longer than the mean solar day of 

 24 hours of the clock. But the lunar day is evidently the interval 

 between two corresponding high waters ; therefore, high water is later 

 by several minutes of the clock each day, 



The action of the sun is similar to that of the moon, but much 

 less in amount, owing to its greater distance. Were there no moon, 

 the sun would raise a water spheroid and cause three tides, viz. , a 

 semi-diurnal, a diurnal, and a long period tide, called the solar 

 semi-annual tide. 



Since the oscillation of the ocean surface due to either luminary is 

 very small in comparison with the earth's radius, the whole tidal oscil- 

 lation is the sum of those due to the sun and moon separately. 



The amount of the lunar tide is about 2.1 times that of the solar. 



At new and full moon, lunar and solar high waters occur together, 

 and the highest high waters occur ; similarly the lowest low waters 

 occur at this time : these are the spring tides. 



When the moon is in quadrature, that is, when it is half moon, 

 solar low water occurs at the same time as lunar high water, thus at 

 this time high water is less than at any other ; whilst since solar high 

 water synchronizes with lunar low water, the highest low waters occur 

 at this time : these are the tieap tides. Hence the spring tides at 

 open coast stations are 3.1, and the neap tides only i.i, each reckoned 

 in terms of the solar tide. 



At new and full moon, when the sun and moon arises syzygj', 

 actual high water occurs at the time of lunar high water, as we have 

 just seen, that is when the moon is on the meridian. But in other 

 positions of the luminaries, the compound luni-solar high water will 

 not occur when the moon souths, but at some time intermediate 

 between those of the lunar and solar tides : this deviation being in 

 advance or in arrear of the lunar tide according as the crown of the 

 solar tide precedes or follows the crown of the lunar tide : the 

 maximum advance in the time of high water takes place 4^ days after, 

 and the maximum retardation the same number of days before spring 

 tide. This is known as the priming and lagging of the tides. 



We have hitherto spoken as if the high water due to the sua or 



