676 Colonel Edmond H. Hills [May 19, 



shall be on safe ground if we say that the observed polar motion is 

 compounded of a precessional rotation in a period of something very 

 near 428 days at an average distance of 20 feet from the mean pole, 

 with an irregular movement superimposed on it ; this irregular 

 movement having sometimes the effect of modifying the rate of 

 precessional rotation and sometimes of changing its amplitude — that 

 is to say altering the distance between the pole of rotation and the 

 mean pole — according as it is acting parallel to, perpendicular to, or 

 at any intermediate angle to the direction of the precessional rota- 

 tion. I shall revert to this question later and show how it is possible 

 by a simple graphical construction to separate out this irregular 

 motion and construct a diagram of it which should be helpful in 

 elucidating its cause. 



While it is thus to Chandler that the credit of discovering the 

 428 day period should be ascribed it is to Kiistner that we owe the 

 first real proof that there is an actual variation in the latitude of 

 a point upon the earth. 



Kiistner's observations were made in the same years as Chandler's, 

 1884-5, and were designed to determine the constant of aberration, 

 a class of observation identical with those which would be used to 

 determine the latitude of the place. Upon reducing these observa- 

 tions the results were at first sight anomalous in that they 

 gave an impossibly small value of the aberration constant. The 

 anomaly was not due to any instrumental cause : it could not be due 

 to any seasonal change in the refraction, as the morning observations 

 of 1884 were not accordant with the morning observations of 1885, 

 nor could it be explained by any possible error in the proper motions 

 of the stars. Kiistner was thus enabled to state positively that the 

 latitude of the place of observation had actually changed. It must 

 be admitted that the years 1884-5 were particularly favourable ones, 

 and that both these astronomers were in a sense lucky in having 

 chanced upon them. The movement of the pole happened at that 

 time to be exceptionally rapid. I do not, however, mention this as 

 detracting in any way from the merit of their achievements ; they 

 deserve to be remembered as simultaneous but independent dis- 

 coverers of this important and interesting phenomenon, and should be 

 honoured, Chandler especially for his courageous rejection of mathe- 

 matical theory, and Kustner for the very high skill and exquisite 

 refinement of his observational work. 



The importance of Kiistner's discovery was at once recognized 

 upon the Continent and a proposal was made to the International 

 Geodetic Conference to establish a chain of stations for carrying on a 

 series of simultaneous observations and thus deducing the true law 

 of this latitude variation. This suggestion was soon carried into 

 effect. Six stations were chosen, all at the same latitude, 39° * 1 X. — 

 Carloforte, in an island close to Sardinia : Mizusawa, in Japan ; 

 Oaithersburg in Maryland, and Ukiah in California ; all new stations, 



