THE TIDES. 153 



ing an annual period in the amount of the daily 

 rise and fall of the sea, which seems to be not 

 altogether right, and is confessedly in part con- 

 jectural. He gave no theory, of course, and he 

 avoided the complication of referring to the sun. 

 But the mere mention of an annual period is 

 interesting in the history of tidal theory, as sug- 

 gesting that the rises and falls are due not to the 

 moon alone but to the sun also. The account 

 given by Posidonius is fairly descriptive of what 

 occurs at the present day at Cadiz. Exactly the 

 opposite would be true at many places ; but at 

 Cadiz the time of high water at new and full moon 

 is nearly twelve o'clock. Still, I say we have only 

 definition to keep us clear of ambiguities and 

 errors ; and yet, to say that those motions of the 

 sea which we call tides depend on the moon, was 

 considered, even by Galileo, to be a lamentable 

 piece of mysticism which he read with regret 

 in the writings of so renowned an author as 

 Kepler. 



It is indeed impossible to avoid theorising. The 

 first who gave a theory was Newton ; and I shall 



