in estimating the Earth? s Age. 467 



may be taken at the equilibrium value, which for a large body 

 like the earth is very much more suitable than it is to a layer 

 of water like the ocean. The height of such a lunar tide is 

 estimated to be about If feet from highest to lowest,* and the 

 lag would be small. The diminution of the tide from its nor- 

 mal amount is caused by the attraction of the two bodily pro- 

 tuberances, and these must be taken as of half the mean 

 density of the earth. Introducing these values into Darwin's 

 formula I have obtained the resultf that the tide would be 

 diminished by only one-fifth of what its height would be on a 

 rigid earth. If for instance the height would be fifty inches 

 on a rigid earth, it would still be forty inches if the earth was 

 liquid. !Now, since we do not know what the precise height 

 of a tide on a rigid earth would be, it is quite possible that the 

 tides we actually experience may be of the height appropriate 

 to a liquid interior, seeing that the diminution would be so 

 small. 



If there is no flaw in the above reasoning, the rigidity of 

 the earth has not been established by the argument from the 

 tides of short period, and no estimate of the earth's age can be 

 based upon a belief in such a condition of the interior derived 

 from their existence. If, however, the argument is trans- 

 ferred to the fortnightly tide, the reply may be made that 

 there is no certain evidence from observation of the existence 

 of such a tide ; for " it is certain that, if at a given port a tide 

 exists, the average height of that tide ought to be always the 

 same year by year, and its lag ought to be the same. But in 

 fact the average annual height of the fortnightly disturbances 

 of the sea at Karachi, in India, which is the port where the 

 necessary observations have been carried out for fifteen years, 

 does not maintain anything like a constant value. This ap- 

 pears to show that no conclusion can be drawn from the obser- 

 vations to decide the question whether there is such a tide or 

 not. The disturbances may be wholly due to meteorological 

 causes. At any rate, if such a tide exists, it is so masked by 

 meteorological disturbances as to be unrecognizable. The 

 irregularity of the times of disturbances is equally noticeable 



* Thomson and Tait's Nat. Phil., 2d Ed. § 804. Mr. King, p. 17, writes as if 

 the tidal deformation of the earth would be five feet if it is not rigid, but does 

 not give his authority. Possibly he may be referring to the observed height of 

 the tide at oceanic islands, which on the assumption that the tide would be ob- 

 literated by a yielding earth would afford a measure of the yielding. The only 

 numerical estimate I have been able to find is in "Thomson and Tait," and 

 is that which I have given above. If any larger one is mentioned I have over- 

 looked it. 



f See a paper by the writer on "the hypothesis of a liquid condition of the 

 earth's interior considered in connection with Professor Darwin's theory of the 

 Genesis of the moon." Proc. Cam. (England) Phil. Soc, May 30, 1892. 



