684 The Movements of the Earth's Pole [May 19, 



The interpretation of such a diagram is a somewhat complex 

 matter and has not yet advanced far. The causes that seem to be 

 at work producing the irregular shift are either movements of the 

 earth's crust, slow or rapid as in an earthquake ; the transfer of arctic 

 ice from one point to another or its accumulation and disappearance 

 so far as this takes place unsymmetrically with respect to the earth's 

 axis ; and possibly extensive barometric changes extending over con- 

 siderable areas. 



Of these the transfer of ice is the largest factor and is probably 

 the one to which most of the irregular polar movement may be 

 ascribed. An earthquake, even of gigantic dimensions, would have 

 an almost negligible effect. The late Professor Milne estimated that 

 a very large earthquake might displace ten million cubic miles of 

 earth through a distance of ten feet horizontally or vertically. Such 

 a vast cataclysm would only change the position of the pole by a 

 feAV inches. 



In conclusion it will be an act of natural curiosity to enquire 

 whether there is any evidence of the amplitude of these polar w^ander- 

 ings having been greater in past times than at present, and whether 

 there is any likelihood of their being greater in the future. To both 

 these questions the answer is " No." The axis of rotation is always 

 kept near the axis of figure by internal friction, and it would require a 

 large change in the distribution of mass to move the axis of figure 

 very far. 



As regards the future the probabilities point still more strongly 

 in the same direction. Each shrinkage of the earth, whatever its 

 immediate effect on the position of the axis of rotation may be, tends 

 ultimately to bring it nearer to the axis of figure or principal axis of 

 inertia, and therefore tends to reduce the average amphtude of the 

 polar path. The distance of the pole of rotation from the mean 

 pole will therefore gradually decrease as the world grows older, 

 while at the same time as the earth cools and becomes less elastic 

 and more rigid the rate of rotation will quicken. 



[E. H. H.] 



