THE TIDES. (A PP. A) 217 



/. One of the most interesting of the questions 

 that can be proposed in reference to the tides is, 

 how much is the earth's angular velocity diminished 

 by them from century to century ? although the 

 direct determination of this amount, or even a 

 rough estimate of it, can scarcely be hoped for 

 from tidal observation, as the data for the quad- 

 rature required could not be had directly. But 

 accurate observation of amounts and times of the 

 tide on the shores of continents and islands of all 

 seas might, with the assistance of improved 

 dynamical theory, be fully expected to supply the 

 requisite data for at least a rough estimate. In 

 the mean time it may be remarked that one very 

 important point of the theory, discovered by Airy, 1 

 affords a ready means of disentangling some of 

 the complication presented by the distribution of 

 the times of high water in different places, and 

 will form a sure foundation for the practical estimate 

 of a definite part of the whole amount of retarda- 

 tion, when the times of spring tides and neap tides 

 are better known for all parts of the sea than they 

 are at present. To understand this, imagine a tidal 

 spheroid to be constructed by drawing an infinite 

 number of lines perpendicular to the actual mean 

 sea-level continued under the solid parts of the 

 earth which lie above the sea, and equal to the 

 spherical harmonic term or Laplace's function, of 



1 See Airy's Tides and Waves, 459. 



