ON COMPARISON AND REDUCTION OF MAGNETIC OBSERVATIONS. 209 



Comparison and Reduction of Magnetic Observations. — Report of the 

 Committee, consisting of Professor W. G. Adams (Chairman), Mr. 

 C. Chree (Secretary), Lord Kelvin, Professor G. H. Darwin, 

 Professor G. Chrystal, Professor A. ScilUSTER, Captain E. W. 

 Creak, The Astronomer Royal, Mr. William Ellis, and Pro- 

 fessor A. W. RuCKER. (Draicn tip hij the Secretary.) 



[PLATES V. and VI. Thirteen Curves illtistratinff Results in §§ 7-8 and §§ 11-12.] 



Analysis of the Results from, the Kew Declination and Horizontal Force 

 Maqnetographs during the selected ' Quiet ' Days of the Five Years 

 1890-94. By C. Curee, Sc.D. 



Contents. 



SECTIONS r.VGK 



1-3 Introductory 209 



4-6 Non-cijcllo Nature of B^snlts oMained from ^ Quiet' Bays . . . . 210 

 7-8 Tables of Diurnal Inequalities for each Month of Year, for Quarters, 



Halves, and Whole Year 213 



9-10 Harmonic Analysis of Biui-nal Inequalities ; Times of Maxima, ^w . . 216 



11-12 liesulta^it of Horizontal Forces to which Biitrnal Inequality is due . 218 



13-15 Variation if Ranges and Sums of Bepartures from Mean for Bay 



tliroiiyhout the Yea.r, with Harmonic Analysis of Ranges . . . 221 



16-20 Annual Ineqitalities (or Cyclic Fart of Yearly Variations') . . . 223 



Introduction. 



§ 1. The hourly measurement of the Kew magnetic curves on five 

 quiet ' clays a month, selected annually by the Astronomer Royal, has 

 •^een in operation since the beginning of 1890. Tables of the mean 

 "curly values for each month of the declination, inclination, horizontal 

 and vertical forces, based exclusively on these quiet days, have been 

 published annually in the ' Report ' of the Kew Committee to the Royal 

 Society. Tables have also been given of the mean diurnal variations of 

 the several elements for the six winter months, the six summer months, 

 and the whole year. 



With the consent of the Kew Committee I now propose to give a 

 general resume of the results deducible fi'om the declination and hori- 

 zontal force records on the selected quiet days of the five years 1890-94. 



For some reasons it would have been desirable to allow a larger 

 number of years' records to accumulate before entering on a general 

 discussion ; to have included, for instance, the complete cycle of ten or 

 eleven years believed to occur in magnetic phenomena would have 

 possessed obvious advantages. On the other hand twenty-five quiet 

 days for each month of the year seem a sufficient number for a com- 

 parison of the different months. Further, the frequent occurrence of 

 certain phenomena, depending apparently on the limitation of the inquiry 

 to ' quiet ' days, which cause an appreciable amount of indeterminateness 

 in the results, has led me to think an early survey of the situation 

 desirable. 



§ 2. The first thing to consider is the distribution of the selected 



' quiet ' days. The aim of the Astronomer Royal, he informs me, has been 



to ensure, first, that the five days selected each month are good specimens 



of ' quiet ' days ; and, secondly, that their mean comes near the middle of 



loJo. p 



