294 KELVIN 



quoting Posidonius "This is the daily circuit of the sea. 

 Moreover, there is a regular monthly course, according to 

 which the greatest rise and fall takes place about new 

 moon, then diminishing rise and fall till half moon, and 

 again increasing till full moon." And lastly he refers to a 

 hearsay report of the Gaditani (Cadizians) regarding an 

 annual period in the amount of the daily rise and fall of 

 the sea, which seems to be not altogether right, and is con- 

 fessedly in part conjectural. 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 suggesting that the rises and falls 

 are due not to the moon alone but to the sun also. The ac- 

 count 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 am- 

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

 the sea which we call tides depend on the moon, was con- 

 sidered, even by Galileo, to be a lamentable piece of mysti- 

 cism which he read with regret in the writings of so re- 

 nowned an author as Kepler. 



It is indeed impossible to avoid theorising. The first who 

 gave a theory was Newton ; and I shall now attempt to speak 

 of it sufficiently to allow us to have it as a foundation for 

 estimating the forces with which we are concerned, in deal- 

 ing with some of the very perplexing questions which tidal 

 phenomena present. 



We are to imagine the moon as attracting the earth, sub- 

 ject to the forces that the different bodies exert upon each 

 other. We are not to take Hegel's theory that the Earth 

 and the Planets do not move like stones, but move along 

 like blessed gods, each an independent being. If Hegel had 

 any grain of philosophy in his ideas of the solar system, 

 Newton is all wrong in his theory of the tides. Newton 

 considered the attraction of the sun upon the earth and the 

 moon, of the earth upon the moon, and the mutual attrac- 

 tions of different parts of the earth; and left it for Caven- 

 dish to complete the discovery of gravitation, by exhibiting 



