460 HISTORY OF PHYSICAL ASTRONOMY. 



Celeste (in which the ratio of the heights of spring-tides and neap- 

 tides was computed on an assumed mass of the moon) without an 

 alteration of level which was, in fact, equivalent to an alteration of the 

 moon's mass. Thus all things appeared to tend to show that the 

 Equilibrium-theory would give the formulae for the inequalities of the 

 tides, but that the magnitudes which enter into these formulae must be 

 sought from observation. 



Whether this result is consistent with theory, is a question not so 

 much of Physical Astronomy as of Hydrodynamics, and has not yet 

 been solved. A Theory of the Tides which should include in its con- 

 ditions the phenomena of Derivative Tides, and of their combinations, 

 will probably require all the resources of the mathematical mechan- 

 ician. 



As a contribution of empirical materials to the treatment of this 

 hydrodynamical problem, it may be allowable to mention here Mr. 

 Whewell's attempts to trace the progress of the tide into all the seas 

 of the globe, by drawing on maps of the ocean what he calls Cotidal 

 Lines ; — lines marking the contemporaneous position of the various 

 points of the great wave which carries high water from shore to shore.'' 6 

 This is necessarily a task of labor and difficulty, since it requires us to 

 know the time of high water on the same day in every part of the 

 world ; but in proportion as it is completed, it supplies steps between 

 our general view of the movements of the ocean and the phenomena 

 of particular ports. 



Looking at this subject by the light which the example of the his- 

 tory of astronomy affords, we may venture to repeat, that it will never 

 have justice done it till it is treated as other parts of astronomy are 

 treated ; that is, till Tables of all the phenomena which can be observed, 

 are calculated by means of the best knowledge which we at present 

 possess, and till these tables are constantly improved by a comparison 

 of the predicted with the observed fact. A set of Tide-observations 

 and Tide-ephemerides of this kind, would soon give to this subject that 

 precision which marks the other parts of astronomy; and would leave 

 an assemblage of unexplained residual phenomena, in which a careful 

 research might find the materials of other truths as yet unsuspected. 



[2d Ed.] [That there would be, in the tidal movements of the ocean, 

 inequalities of the heights and times of high and low water corres- 



« Essay towards a First Approximation to a Map of Cotidal Lines. Phil. Tram. 

 1833, 1836. 



