168 EECENT PKOGEESS IN ASTRONOMTCAL RESEARCH. 



feet, measiirecl at the earth's surface. Assuming provisionally, for the piirpose 

 of statement, that this is a motion of the north pole of the principal axis of 

 inertia about that of the axis of rotation, the direction of the former from the 

 latter lay toward the Greenwich meridian about the beginning of the year 

 1890. This, with the period of 427 days, will serve to fix approximately the 

 relative positions of these axes at any other time for any given meridian. It is 

 not possible at this stage of the investigation to be more precise, as there are 

 facts which appear to show that the rotation is not a perfectly uniform one, 

 but is subject to secular change, and perhaps irregularities, within brief spaces 

 of time. 



These results of observation fell squarely across the long-accepted 

 theory of Enler, according to which the variation of latitude, if any, 

 must be a uniform one of ten months, and also contradicted the 

 result of careful reduction of excellent observations from which it 

 had been concluded that no sensible wandering of the pole did in 

 fact take place. The theoretical difficulty was, indeed, soon partly 

 removed by Professor Newcomb, who pointed out that the fluidity 

 of the ocean and the elasticity of the earth had been neglected in 

 deriving ten months as the theoretical period of a possible rotation 

 of the earth's pole, but at the same time he professed himself iniable 

 to accoimt on the principles of dynamics for a variation of the period 

 of the inequality, unattended l)y an alteration of its amplitude dur- 

 ing the preceding half century, conclusions which had come from 

 recomputations which Mr. Chandler had in the meantime made of 

 the classic observations of Bradley in 1728 and others of dift'erent 

 periods of the nineteenth century 



Under the spur of the theoretical difficulties stated by Professor 

 Newcomb, Mr. Chandler collected an inunense mass of evidence on 

 the subject, involving the reduction of more than thirty-three thou- 

 sand observations made b}" nine different methods, comprising the 

 Avork of seventeen observatories, distributed over both the northern 

 and southern hemispheres, and covering the interval of time from 

 lT28 to 1890. P'rom this great array of evidence the fact of the 

 wandering of the pole was not only clearly confirmed, but also the 

 variation of its period and amplitude came out without question, 

 and an insight was gained as to the causes of this baffiing phenomenon 

 as follows : 



Tlie observed variation of the latitude is the resultant curve arising from two 

 periodic fluctuations superposed upon each other. The first of these, and in 

 general the more considerable, has a period of about 427 days, and a semiampli- 

 tude of about 0.12 second. The second has an annual period with a range 

 variable between 0.04 and 0.20 seconds during the last half century. During the 

 middle portion of this interval, roughly characterized as betw eenlSGO and ISSO, 

 the value represented by the lower limit has prevailed, but before and after 

 these dates, the higher one. * * * 



As the resultant of these two motions, the effective variation of latitude is 

 subject to a systematic alternation in a cycle of seven years' duration, resulting 

 from the commeusurability of the two terms. According as they conspire or 



