464 THE POPULAR SCIENCE MONTHLY. 



ment of rotation ; the axis of the earth would always remain parallel 

 to itself i. e., always point to the same place in the heavens ; but the 

 sun's action on the equatorial protuberance gradually effects a change 

 of direction in the earth's axis, and the moon produces an analogous 

 effect. These perturbations constitute the phenomena of the preces- 

 sion of the equinoxes and nutation, by virtue of which the celestial 

 pole is continually displaced among the stars. 



It is from such considerations as these that Mr. Hopkins has drawn 

 a serious argument against the fluidity of the earth's interior.* In 

 considering the effect of the sun's and moon's action on the equatorial 

 swelling, says Mr. Hopkins, we look upon the earth as a solid body, 

 with all of its parts joined together, which ought to experience in its 

 entirety the effects of these perturbing causes. But, if the earth is a 

 liquid mass covered by a solid shell, these effects will be exerted only 

 on the solid portion, which will in a manner slide on the liquid nucleus. 

 As the perturbing forces will thus act on so small a portion of the* 

 globe, the effect on the rotary movement of the crust ought to be 

 much greater than if the earth were viewed as a solid mass, and these 

 forces will be the more intense in proportion as the crust is thin. In 

 order to reconcile the possible effect of luni-solar action on the equa- 

 torial protuberance with the known amount of precession and nutation, 

 Mr. Hopkins calculates the requisite thickness of the crust at not less 

 than thirteen hundred to sixteen hundred kilometres, or from a fifth to 

 a quarter of the earth's radius. 



Mr. Hopkins's calculations were revised twenty years later by Sir 

 W. Thomson in his " Memoir on the Rigidity of the Earth," f in which 

 this illustrious physicist gives to Mr. Hopkins's views all the weight of 

 his authority. " Whatever objection may be made to the mathemati- 

 cal portion of Mr. Hopkins's work," he says, " I can see no force in the 

 reasoning employed to refute his conclusions, and I am happy to see 

 my opinion in the matter confirmed by such an eminent authority as 

 Archdeacon Pratt. It has, indeed, always seemed to me that Mr. Hop- 

 kins might have carried his argument further, and concluded that no 

 completely liquid mass, approximating to a spheroid six thousand 

 miles in diameter, can exist in the interior of the earth without being 

 accompanied by a very different rate of precession and nutation from 

 that which actually exists." 



These arguments grew in favor with geologists, and the hypothesis 

 of a liquid nucleus was gradually relegated to the limbo of superan- 

 nuated prejudices, when the lamented M. Delaunay undertook to de- 

 molish the principal argument, and declared that in his opinion Mr. 

 Hopkins's reasoning had no real foundation. \ " To make clear our 

 idea," said M. Delaunay, " let us take a glass globe filled with water. 



* "Philosophical Transactions of the Royal Society," London, 1839-1842. 



f Ibid., 1863. 



" Comptes rendus dc l'Academie des Sciences," July, 1808. 



