in estimating the Earth's Age. 465 



covered may be accounted for in that manner, if the earth is 

 slightly more rigid than steel ; but he does not stop at this ex- 

 planation. He goes on to say, " we have next to consider the 

 effect of viscosity of the earth. Those geologists, who have 

 given special attention to the subject, regard it as well estab- 

 lished, that the earth yields under the weight of deposits, as if 

 it were a thin crust floating upon a liquid interior, and must 

 therefore be a viscous solid, if a solid at all." He then states, 

 that the phenomena observed might in such a case be pro- 

 duced, if some disturbing cause acted, adding, " a vera causa 

 was pointed out some years ago by Sir William Thomson in 

 the motions of the winds and oceans, and especially in changes 

 in the polar ice cap." Thus it appears that Professor New- 

 comb does not consider that there may not be a possible alter- 

 native to the hypothesis of steel-like rigidity ; and evidently 

 he does not regard the question as settled, for the concluding 

 words of his paper are, " but under the actual circumstances 

 we must await the results of further investigation into the 

 whole subject." 



Professor Newcomb tells us that, in the case of a viscous 

 earth, k 'the poles [of rotation and figure] would eventually ap- 

 pear to meet, unless separated from time to time by [disturb- 

 ing] causes changing one or both of them." A geologist 

 acquainted with the flow of solid rock would hardly recognize 

 a condition of the interior so elastic, and so devoid of viscosity, 

 that during geological ages the elasticity would be constantly 

 maintained, so that this coincidence of the poles should not by 

 this time have been accomplished. If this be so, then the 

 action of disturbing causes is still needful to account for their 

 separation at present, and there is no stronger argument from 

 variation of latitude for an elastic earth than for a viscous or 

 liquid interior. 



While referring to authorities it is worth while to mention 

 that Professor Harkness, in his exhaustive work "On the Solar 

 Parallax and its related Constants including the Figure and 

 Density of the earth," remarks, that, " Notwithstanding the 

 difficulties which arise in connection with the rigidity of the 

 earth under the action of the forces which generate precession, 

 nutation, and the tides, the theory of a comparatively thin 

 crust resting in approximate hydrostatic equilibrium upon a 

 denser substratum is favored by enough facts to render it very 

 plausible."* 



Let us now turn to what Mr. King describes as " the hitherto 

 unshaken results of Ld. Kelvin and Professor G. H. Darwin" 

 as to the tidal rigidity of the earth. It is no doubt a very 

 serious matter to hold an opinion opposed to such high author- 



* Washington, Government printing office, 1S91, p. 143. 



