470 0. FISHER ON MR. MALLET'S 



described. If it be so, the fact will be betrayed in two ways ; but 

 the evidences which they will give of it will be totally distinct in 

 kind. 



One of these is, that the earth and the ocean being drawn up at 

 the same place, the apparent ocean tide, or the increased distance 

 between the bed of the ocean and its surface, will be smaller than 

 if the solid earth was not so drawn up. Hence the observed ocean 

 tide would be very considerably less than the calculated tide. ISTow 

 he is of opinion that, if this were so, it might be observed. But he 

 hesitates to speak very decidedly on this point, because great diffi- 

 culties are introduced through the effect produced on the tides by the 

 "irregular distribution of land and water, and of depth where there 

 is water"*. Besides this, itseems to me that, in order to suit the 

 argument in the form in which he puts it, it ought also to be shown 

 that the crest of the tidal wave, if any exists in the solid earth, will 

 occur at the same place as that of the ocean, which would, I conceive, 

 not be the case, because the crest of the wave formed in the solid 

 earth, consisting of such materials as he supposes, would occur 

 immediately opposite to the attracting bodyf, while with respect to 

 the ocean tide that would not happen J. 



The other manner in which the tidal deformation of the earth 

 would be betrayed would be by the effect produced by it upon the 

 phenomenon of precession. If it were fluid or elastic, and rendered 

 oblate by its rotation, the same capacity for yielding which admits 

 of the oblateness being produced, would also admit of the formation 

 of tidal waves in the body of the earth by the attraction of the 

 disturbing body. And their magnitude and position would be such 

 as exactly to balance by their centrifugal action the precession-pro- 

 ducing action of the disturbing body. In other words the disturbing 

 body would produce two simultaneous effects upon the earth which 

 would neutralize each other. In this case there would be no preces- 

 sion. If it yielded tidally to a certain extent, the precession would 

 be reduced. Sir W. Thomson says§ that if it were no more rigid 

 than glass, the precession would be reduced to two ninths of what 

 it would be if it did not yield at all ; and if it were no more rigid 

 than steel, to three fifths. Observation, however, shows that preces- 

 sion is not thus reduced, and therefore proves that no such yielding 

 as this can exist, and, consequently, that if the materials of the 

 earth are of such a nature as to be liable to behave in the manner 

 supposed under the attracting force, it must be rigid. 



To the abo^e argument the following objection occurs to my mind. 

 The equatorial protuberance is such as the present rate of rotation 

 would produce ; and not only is there this protuberance at the sur- 



* Thomson, loc. cit. ; also Natural Phil. §§ 843, 844. 



t Phil. Trans., loc. cit. § 4. Compare with his paper " On Geological Time," 

 Trans. Geol. Soc. of Glasgow, 1868, p. 6. 



\ Herschel, Phys. Geography, 2nd ed. § 66. Tf there were no friction it 

 would occur 90° from the attracting body. See Trans. Geol. Soc. Glasgow, 

 loc. cit. p. 6, note. 



§ Loc. cit. § 26. See also General Barnard's " Problems of Eotary Motion/' 

 Smithsonian Contribution^, No. 240,- p. 37. 



