position of this Pole and about its movement that we demand a knowledge of its whereabouts with an accuracy which at first appears wholly unattainable. It sounds almost incredible when we are told that a shift in the position of the North Pole to the extent of twenty yards, or even of twenty feet, is appreciable, notwithstanding that we have never been able to get nearer to it than from one end of England to the other. Indeed, as a matter of fact, our knowledge of the movements of the Pole are derived from observations made not alone hundreds but even many thousands of miles distant. It is in such observatories as those at Greenwich or Berlin, Pulkova or Washington, that the determinations have been made by which changes in the position of the Pole can be ascertained with a delicacy and precision for which those who were not aware of the refinement of modern astronomical methods would hardly be prepared. I do not, however, imply that the observations conducting to the discoveries now about to be considered have been exclusively obtained at the observatories I have named. There is a large number of similar institutions over the globe which have been made to bear their testimony. Tens of thousands of different observations have been brought together, and by discussing them it has been found possible to remove a large part of the errors by which such work is necessarily affected, and to elicit from the vast mass those grains of truth which could not have been discovered had it not been for the enormous amount of material that was available. Mr. Chandler has discussed these matters in a remarkable series of papers, and it will be necessary for me now to enter into some little detail, both as regards the kind of observations that have
