PERBING PROBLEMS OF ASTRONOMY. 51 



spending alterations in the position of the axis and in the places of 

 the poles — changes certainly very minute. The only question is, 

 Whether they are so minute as to defy detection. It is easy to see 

 that any such displacements of the earth's axis will be indicated by 

 changes in the latitudes of our observatories. If, for instance, the 

 pole were moved a hundred feet from its present position, toward the 

 Continent of Europe, the latitudes of European observatories would be 

 increased about one second, while in Asia and America the effects 

 would be trifling. The only observational evidence of such move- 

 ments of the pole, which thus far amounts to anything, is found in 

 the results obtained by Nyren in reducing the determinations of the 

 latitude of Pulkowa, made with the great vertical circle, during the 

 last twenty-five years. They seem to show a slow, steady diminution 

 of the latitude of this observatory, amounting to about a second in a 

 century ; as if the north pole were drifting away, and increasing its 

 distance from Pulkowa at the rate of about one foot a year. The 

 Greenwich and Paris observations do not show any such result ; but 

 they are not conclusive, on account of the difference of longitude, to 

 say nothing of their inferior precision. 



The question is certainly a doubtful one ; but it is considered of 

 so much importance that, at the meeting of the International Geodetic 

 Association in Rome last year, a resolution was adopted recommend- 

 ing observations specially designed to settle it. The plan of Signor 

 Fergola, who introduced the resolution, is to select pairs of stations, 

 having nearly the same latitude, but differing widely in longitude, and 

 to determine the difference of their latitudes by observations of the 

 same set of stars, observed with similar instruments, in the same man- 

 ner, and reduced by the same methods and formulae. So far as pos- 

 sible, the same observers are to be retained through a series of years, 

 and are frequently to exchange stations when practicable, so as to 

 eliminate personal equations. The main difficulty of the problem lies, 

 of course, in the minuteness of the effect to be detected ; and the only 

 hope of success lies in the most scrupulous care and precision in all 

 the operations involved. 



Other problems, relating to the rigidity of the earth and its internal 

 constitution and temperature, have, indeed, astronomical bearings, and 

 may be reached to some extent by astronomical methods and consider- 

 ations ; but they lie on the border of our science, and time forbids 

 anything more than their mere mention here. 



If we consider next the problems set us by the Moon, we find them 

 numerous, important, and difficult. A portion of them are purely 

 mathematical, relating to her orbital motion ; while others are physi- 

 cal, and have to do with her surface, atmosphere, heat, etc. As baa 

 been already intimated, the lunar theory is not in a satisfactory state. 

 I do not mean, of course, that the moon's deviations from the predicted 

 path are gross and palpable — such, for instance, as could be perceived 



