86 REPORT — 1898, 



diurnal curve, the values of non-cyclic increment of Table VII. would 

 produce. 



Table VIII. gives further information as to the non-cyclic increment on 

 quiet days in declination and horizontal and vertical force, as derived for 

 Greenwich from the records of the eight years 1889 to 1896, combining 

 not only the months of the different years to form mean monthly values, 

 but giving also values for the separate years. The mean values for 

 months, except in January and February for declination, are on the 

 whole very similar to those of Table VII., which includes six of the years 

 here employed. The greatest and least mean values of non-cyclic in- 

 crement in individual months are here added, both for the monthly group- 

 ing and the yearly grouping. The mean of the differences between the 

 greatest and least values differs for the mean of months from that for the 

 mean of years in each element, since in the two groupings the extreme 

 values combine in a diffei-ent way. The mean for years is the larger, but 

 this may have no significance, since the monthly differences depend on 

 extremes selected from eight months only, whilst the yearly differences 

 depend on selection from twelve months. 



Table IX. contains a comparison of the annual values of the non-cyclic 

 increment at Greenwich for declination and horizontal and vertical force, 

 as appearing in Table VIII., with the corresponding annual values for 

 Kew, as given in Table III. of the B.A. Report for 1896. The years 

 are those on which the Kew values for months given in Table VII. are 

 founded. 



At the conclusion of the Report for the year 1896, Dr. Chree sug- 

 gested that considerable light might be thrown on the question of the 

 uniformity or variability of the non-cyclic element throughout the day by 

 measurement of the curves for the noons preceding and succeeding each 

 selected quiet day. As such measurement of the Greenwich curves had 

 been made in the ordinary Greenwich tabulation, it appeared to be desir- 

 able to employ them for the suggested purpose, for which the Astronomer 

 Royal kindly gave me the necessary permission. The grouping of these 

 noon values, and, in the case of horizontal and vertical force, their cor- 

 rection for temperature, proved, however, to be a much heavier work than 

 I had anticipated. An abstract of the results arrived at is given in 

 Table X. The succeeding epochs alternately of noon and midnight 

 being : — 



midnight noon midnight noon 



P f 



the middle midnight to midnight increment =m in Table X. is that of 

 the selected quiet days as appearing in Table VIII. ; 7^= the increment 

 from the preceding noon to the middle noon, and/= that from the middle 

 noon to the following noon. The last twelve hours of p and the first 

 twelve hours of /thus together comprise the middle interval from midnight 

 to midnight of the selected quiet day. Care was taken to see that the 

 added initial and terminal half days introduced no vitiating element of 

 disturbance. For uniform variation of the non- cyclic increment, m would 

 be equal to the mean of 2^ and/^w in the table, as, for instance, in decli- 

 nation in October. Thus, positive values of m — n indicate excess of the 



