SECOND EDITION. xv 



(3). Upon a reconsideration of Mr. Airy's Trea- 

 tise On Tides and Waves, I am no longer disposed 

 to say, as I have said vol. n. p. 311, that for the 

 actual case of the distribution of land and water, 

 nothing has been done to bring the hydrody- 

 namical theory of oceanic tides into agreement 

 with observation. In this admirable work, Mr. Airy 

 has, by peculiar artifices, solved problems which 

 come so near the actual cases that they may repre- 

 sent them. He has, in this way, deduced the laws of 

 the semi-diurnal and the diurnal tide, and the other 

 features of the tides which the equilibrium theory 

 in some degree imitates ; but he has also, taking into 

 account the effect of friction, shown that the actual 

 tide may be represented as the tide of an earlier 

 epoch ; that the relative mass of the moon and sun, 

 as inferred from the tides, would depend upon the 

 depth of the ocean (Art. 455) ; with many other 

 results remarkably explaining the observed pheno- 

 mena. He has also shown that the relation of the 

 cotidal lines to the tide waves really propagated is, 

 in complex cases, very obscure, because different 

 waves of different magnitudes, travelling in differ- 

 ent directions, may coexist, and the cotidal line is 

 the compound result of all these. 



