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 precession 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.[1] 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 equatorial 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,"[2] in which this illustrious physicist gives to Mr. Hopkins's views all the weight of his authority. "Whatever objection may be made to the mathematical 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. Hopkins 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 superannuated prejudices, when the lamented M. Delaunay undertook to demolish the principal argument, and declared that in his opinion Mr. Hopkins's reasoning had no real foundation.[3] "To make clear our idea," said M. Delaunay, "let us take a glass globe filled with water.
