igos.] THE PHYSICS OF THE EARTH. 185 



Observed Period of the Polar Motion Arising in the Variation of 

 Latitude linth the Theoretical Eulerian Period Calculated for a Rigid 

 Earth. — The detection of the variation of latitude by Kiistner at 

 Berhn in 1890-91 and the subsequent discussion by Chandler of long 

 series of observations showing that the movement of the pole in the 

 body of the earth has a period of some 427 days, instead of the 305 

 days long ago inferred from Euler's theory of the rotation of a 

 rigid spheroid, led Professor Newcomb to point out that this ob- 

 served prolongation of the theoretical Eulerian period indicates some 

 yielding of the matter of the globe under the stresses to which it 

 is subjected by the movement of the pole, and would afi'ord a new 

 method of evaluating the earth's rigidity. In his well-known paper 

 on the " Dynamics of the Earth's Rotation " (Monthly Notices, R. 

 A. S., March, 1892) Newcomb showed that the results already ob- 

 tained decidedly confirmed Darwin's conclusion that the rigidity of 

 the globe as a wdiole is comparable to that of steel. 



The essential point in Newcomb's explanation is that when the 

 pole changes its position in the body of the globe, the distribution 

 of centrifugal force shifts with respect to the solid earth, which is 

 thus put into a state of stress and must yield to the forces acting 

 upon it, like any other elastic solid body ; the periodic deformation 

 of the earth's figure operating to lengthen the period of the free 

 nutation, by an amount depending on the average rigidity of the 

 whole earth. 



The continued investigation of the variation of latitude carried 

 out at the various international latitude observatories by Albrecht 

 and others confirms this observational result, and the subject has 

 also been examined theoretically by Darwin. Hough, Larmor and 

 others ; so that the validity of the method suggested by Newcomb 

 is generally recognized. 



In 1896 J\Ir. S. S. Hough treated of the problem in a very thor- 

 ough manner in his well-known paper, " On the Rotation of an 

 Elastic Spheroid" (Phil. Trans., A, 1896). He considered chiefly 

 the case of an incompressible homogeneous spheroid, and was en- 

 abled to show by rigorous methods that the rigidity of the earth in 

 all probability slightly exceeds that of steel. 



In a remarkable paper " On the Period of the Earth's Free 



