C74 Colonel Edmond H. Hills [May 1<), 



magnitude of this variation ; and, secondly, the fact that the earth 

 is by no means rigid, and hence that the true period of the preces- 

 sional rotation differs very substantially from the Eulerian period of 

 305 days. 



All the earlier attempts to find evidence of this variation were 

 in fact hampered by this preconceived notion of the ten month 

 period ; the observations were carefully scrutinized with a view to 

 detecting it, a process as we now see foredoomed to failure. It 

 would be a useless task to recount here the various attempts that 

 were made. Two of these, however, I should not like to pass over 

 without notice, those of C. A. F. Peters at Pulkowa and Clerk 

 Maxwell in this country. 



Peters in his great and classic memoir on the parallax of the 

 fixed stars devoted one section to a discussion on the variability of 

 the latitude in a ten month period. He found that the actual varia- 

 tion derived from the observations was of so minute a magnitude 

 that it was well within the limits of unavoidable sources of error 

 and he therefore concluded that if there was any separation of the 

 two poles it was too small to be detected by observation. 



Clerk Maxwell examined the Greenwich observations of Polaris 

 in 1851-54 and thought he found some small indications of maxima 

 at about ten month intervals, but he considered the results as very 

 doubtful and that more observations would be required to establish 

 the existence of so small a fluctuation. 



Substantially the same result was derived by other enquirers. 

 Astronomers were therefore satisfied, up to the year 1884, that the 

 earth's axis of figure was so nearly coincident with its axis of 

 rotation that the difference between the two was inappreciable to the 

 most refined observations. All methods of observation and all 

 principles of the reduction of observations, both of astronomers and 

 of geodesists, were tacitly based upon the idea of absolute coincidence 

 between the two axes. 



In 1884 the subject was independently reopened by two men — 

 Chandler in America, and Kiistner at Bonn — and entirely fresh light 

 was thrown upon it. Their work was simultaneous and quite 

 independent. I will take Chandler's first. 



In 1884-5 he took a thirteen month series of observations at 

 Harvard with an instrument of his own devising to which I will 

 revert later. These observations showed a progressive change in the 

 derived latitude, which appeared to him of a greater magnitude than 

 could be accounted for by any instrumental errors. He, however, 

 hesitated to ascribe it to a real change in the latitude without further 

 confirmatory observations, which he could not then make. He 

 therefore put these observations aside and was, six years later, 

 drawn to re-examine them by the publication of some of Kiistner's 

 results, which were also only explicable on the hypothesis of an 

 actual variation in the latitude of the place of observation. It was, 



