8 SCIENCE PROGRESS 



the same thing, that the North and South Poles are not quite 

 fixed points on the Earth's surface. It has become known 

 more recently that the Poles move in irregular paths about 

 mean positions, round which they circulate in a period of 

 about fourteen months. The period which this movement 

 would have if the Earth were an absolutely rigid body is well 

 known to be about ten months; one reason why the actual 

 periodic movement, with a fourteen-months' period, remained so 

 long undiscovered was that observers sought in their records 

 for traces of a ten-months' period. The lengthening of the 

 period from ten months to fourteen is due to the yielding of the 

 Earth. A movement of the Poles means a change of the instan- 

 taneous axis of rotation ; this is necessarily accompanied by a 

 change in the so-called " centrifugal force." The adjustment 

 of the Earth to rotation about one axis after another involves 

 a deformation, in exactly the same way as if it were subjected 

 to forces which are the differences between the centrifugal force 

 referred to the actual axis and the centrifugal force referred 

 to an axis passing through the mean positions of the Poles. 

 Exactly as in the case of tidal forces, the deformation implies 

 a tilting of the surface and a " change of attraction." The 

 lengthening of the period has been proved to depend upon 

 the change of attraction not upon the tilting of the surface ; 

 and the law according to which the change of attraction is 

 connected with the force causing deformation, in the case of 

 variation of latitude the inequality of centrifugal force, has 

 been made out. Further, it has been proved that the law 

 connecting the change of attraction with the force causing 

 deformation must be exactly the same, whether the force in 

 question be an inequality in centrifugal force or a tide-raising 

 force. The result is that from the period of variation of 

 latitude we can infer the change of attraction due to the tide- 

 raising forces. 



To determine the actual height of earth-tides it only remains 

 to combine the results of observation in regard to variation of 

 latitude with those of horizontal pendulum experiments. The 

 change of attraction, the force due to tilting, the amount of 

 the deformation have all been determined. But from this 

 information we cannot infer much more about the rigidity of 

 the Earth than that on the whole it is great. It is impossible 

 to fit all the observations by treating the Earth as a body 



