the constellations. In all this there is no novelty; these movements of the Pole are so conspicuous that they were detected long before the introduction of accurate instruments. They were discovered so far back as the time of Hipparchus, and the exposition of their cause by Newton was one of the triumphs of his doctrine of universal gravitation.
In giving the title of "The Wanderings of the North Pole" to this chapter I did not, however, intend to discuss the movements to which I have hitherto referred. They are so familiar that every astronomer has to attend to them practically in the reduction of almost every observation of the place of a celestial body. It was, however, necessary to make the reference which I have done to this subject in order that the argument on which we are presently to enter should be made sufficiently clear. It must be noted that the expression, "the North Pole," is ambiguous. It may mean either of two things, which are quite distinct. In the case we have already spoken of, I understand by the North Pole that point on the celestial sphere which is the centre of the system of concentric circles described by the circumpolar stars. The other sense in which the North Pole is used is the terrestrial one; it denotes that point on this earth which has been the goal of so many expeditions, and to reach which has been the ambition of so many illustrious navigators.
We have a general notion that the terrestrial North Pole lies in a desolate region of eternal ice, somewhat relieved by the circumstance that, for six months of the year, the frozen prospect is brightened by perpetual day, though on the other hand, during the remaining six months of the year this region is the abode of perpetual night. The
