NOTES TO BOOK VII. 311 



assuming this form produce an effect nearly constant for 

 each place. Whatever be thought of this reasoning, the 

 conjecture is confirmed by observation with curious exact- 

 ness. The laws of a great number of the tidal phenomena, 

 namely of the Semi-mensual Inequality of the Heights, of 

 the Semi-mensual Inequality of the Times, of the Diurnal 

 Inequality, of the effect of the Moon's Declination, of 

 the effect of the Moon's Parallax ; are represented very 

 closely by formulae derived from the equilibrium theory. 

 The hydrodynamical mode of treating the subject has 

 not added anything to the knowledge of the laws of the 

 phenomena to which the other view had conducted us. 

 We may add, that Laplace's assumption that in the 

 moving fluid the motions must have a periodicity cor- 

 responding to that of the forces, is also a conjecture. 

 And though this conjecture may, in some cases of the 

 problem, be verified, by substituting the resulting expres- 

 sions in the equations of motion, this cannot be done in 

 the actual case, where the revolving motion of the ocean 

 is prevented by the intrusion of tracts of land running 

 nearly from pole to pole. I am not aware that for such a 

 case, anything has been done to bring the hydrodynamical 

 theory of oceanic tides into agreement with observation. 



With reference to the Maps of Cotidal lines, men- 

 tioned in the text, I may add, that we are as yet destitute 

 of observations which should supply the means of drawing 

 such lines on a large scale in the Pacific Ocean. Admiral 

 Liitke has however supplied us with some valuable mate- 

 rials and remarks on this subject in his Notice sur les 

 Marees Periodiques dans le grand Ocean Boreal et dans le 

 Ma- Glaciate; and has drawn them, apparently on suffi- 

 cient data, in the White Sea. 



