180 SEE— FURTHER RESEARCHES ON [April 24, 



pressure be great enough to prevent fusion under the prevailing 

 temperature. In order to throw light upon this question, Hopkins 

 of Cambridge, England, took up the problem in 1839 {P^^H- Trans., 

 1839; " Researches in Physical Geology," 1839-1842), and sought to 

 prove from the observed phenomena of precession and nutation that 

 the earth could not be composed of a thin shell some tvventy miles 

 thick, filled with liquid. He concluded that the crust could not be 

 less than 800 to 1,000 miles thick, and that the globe might even be 

 solid to the center, except some small vesicular spaces here and 

 there filled with molten rock. 



In 1868 this subject was examined by the eminent French as- 

 tronomer, Delaunay, who published a paper on " The Hypothesis of 

 the Interior Fluidity of the Globe " (C. R. Acad, des Sci., Paris, 

 July 13, 1868), in which he threw doubt on the views of Hopkins, 

 and suggested that if the earth's nucleus were a mass of sufficient 

 viscosity it might behave as if it were solid, and hence concluded 

 that the observed phenomenon of precession and nutation did not 

 necessarily exclude a fluid nucleus. 



§ 13. Lord Kek'in's Earliest Studies on the Precession of a 

 Spheroid Containing Liquid. — Lord Kelvin had already taken up the 

 problem of the internal state of the earth in 1862, and considered the 

 efifects of a fluid nucleus enclosed in a thi^ shell when the whole 

 mass was subjected to tidal strains. As the shell must yield under 

 these strains the land would be carried up and down with the super- 

 jacent sea, and if such yielding occurred it ought to be sensible to 

 observation. But since the sensible obliteration of the tides had not 

 been observed, he naturally inclined to the view of Hopkins that the 

 earth is effectively rigid and behaves as a solid globe. 



In reply to Delaunay 's criticism Lord Kelvin pointed out that if 

 the French astronomer had worked out the problem mathematically 

 he could not fail to see that the hypothesis of a viscous and quasi- 

 rigid interior " breaks down when tested by a simple calculation of 

 the amount of tangential force required to give to any globular 

 portion of the interior mass the precessional and nutational motions 

 which, with other physical astronomers, he attributes to the earth 

 as a whole." {Nature, February i, 1872.) On making this calcula- 

 tion Lord Kelvin found that the earth's crust down to depths of 



