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Y. Discussion of Kew Magnetic Data , especially the Diurnal Inequalities of 
Horizontal Force and Vertical Force , from Ordinary Days of the 
Eleven Years 1890 to 1900. 
By C. Chree, Sc.D., LL.D., F.R.S., Superintendent , Kew Observatory. 
Received November 25, 1915—Read January 27, 1916. 
Contents. 
§§ Page 
1. Introduction. 187 
2. Description of material. Days not used.189 
3. Methods of tabulation employed.192 
4. Non-cyclic change. 194 
5-11. Diurnal inequalities. Tables and curves. 197 
12-14. „ „ Vector diagrams.225 
15-18. Annual inequality.234 
19-22. Fourier coefficients.240 
23, 24. Sunspot relations. Wolf’s formula. 252 
25, 26. Daily range. Tables . 256 
27. ,, ,, Sunspot relations. 262 
28-30. ,, ,, Frequency of occurrence.267 
§ !• In 1901 the development of electrical traction in West London rendered it 
clear that unless conditions altered in an unexpected direction, no further magnetic 
records from Kew Observatory would be sufficiently free from artificial disturbances 
to be an altogether satisfactory medium for the minute study of phenomena such as 
the regular diurnal variation. The time seemed to have come for taking stock of the 
records obtained. The measurement of magnetic curves formed no regular part of 
the work at Kew Observatory until 1890. Prior to that date the Annual Reports 
contained only a summary of the results of the absolute observations. Since 1890 
the programme of work has included the measurement of the magnetic curves for 
5 “quiet” days a month. Until a few years ago these days were selected at 
Greenwich by the Astronomer Royal. Now they are selected under international 
auspices at De Bilt, the central station of the Meteorological Institute of the 
Netherlands. Since Kew Observatory was transferred to the Meteorological Office, 
diurnal inequalities have been published for each month of the year for I) 
(declination) and TI (horizontal force). Previously the inequalities published were 
VOL. CCXVI.- A 542. 2 D [Published Maroli 2S, 1916. 
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