IX RELATION TO T II E EARTH'S INTERNAL STRUCTURE. 41 



(Kali.), for past periods of only 1250, 5000, and 20,000 years, and authorize him 

 to pronounce Poisson's hypothesis impossible, without destruction of life, or relega- 

 tion of the date to so remote an era as to demand an intensely heated stellar region. 



A reheating after solidification which should again fuse the surface and extend 

 to great depths, would be, it seems to me, as " inadmissible" for the origin of ob- 

 served .subterranean temperatures, as the heat originally "developed by the solidi- 

 tication of the earth." Of course the hypothesis of a reheating to fusion of the 

 surface by impact of meteoric bodies would be likewise excluded by Poisson's 

 theory of internal temperatures. 



Efenoe, notwithstanding the high rigidity demanded, the foregoing determina- 

 tion of Sir Wm. Thomson seems to represent the most probable thermal condition 

 of the earth's interior. 



ADDENDUM. s*~* ) 



THE note, written two years ago, to page 39, concerning M. Delaunay's experi- 

 ment (then and since so often referred to as a conclusive refutation of the argu- 

 ments of the opponents of the " thin shell" hypothesis) had for purpose not to 

 deny the reality of " viscosity" as a force which claimed attention in the discussion 

 of the Hopkins problem (for I presume the notion of it has presented itself to 

 every one who has looked over that analysis) but to show the un-crucial cha- 

 racter of the experiment. Just at this moment comes to my notice the letter of 

 Sir Wm. Thomson to Mr. G. Poulett Scrope (" Nature," February 1st, 1872), in 

 which what is known about " viscosity," and how great that of the fluid contents 

 of the earth must be to produce the effect attributed to it by Delaunay, is set 

 forth. 



The remaining portion of the letter relates to the effects of imperfect rigidity of 

 /// in/' rnal contents upon precession, on the supposition of a "thin preternaturally 

 rigid crust;" i. c., in which the internal substance (e. g., an "ordinary elastic jelly") 

 i> yielding, but for which the envelope is unchangeable in form; and are intended 

 to show that the precession would be considerably more rapid than if the rigidity 

 were infinite throughout. 



A few additional words are in place here concerning the results of Prof. Hop- 

 kins' investigation (against which M. Delaunay's objections, note, page 38, are 

 especially directed), briefly stated, pages 34, 35. They are as follows: First FOB 

 HOMOGENEOUSNESS. " Supposing the earth to consist of a homogeneous spheroidal 

 shell (the ellipticities of the outer and inner surfaces being the same) filled with a 

 fluid mass of the same uniform density as the shell;" then, "the precession will 

 be the same, whatever be the thickness of the shell, as if the whole earth were 

 homogeneous and solid." 



