80 REPORT— 1898. 



Comparing and Reducing Magnetic Ohservations. — Report of the Com- 

 mittee, consisting of Professor W. G. Adams (Chairman), Dr. 

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

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

 Creak, the Astronomer Royal, Mr. William Ellis, and Professor 

 A. W. Rlcker. 



rAOB 



I. Magnetic Remits at Greenivich and Kent Discussed and Compared, 1889 to 



1896. J3y William Ellis, F.R.S. 80 



II. Account of the Late Professor John CovcJi Adams's Determination of the 



Gaussian Mapnetic Constants. By Professor W. G. Adams, F.R.S. . . 109 



The Committee report that they have received the following two com- 

 munications bearing on the comparison and reduction of magnetic 

 observations : — ■ 



I. JIagneiic Eesults at Greenwich and Kew, discussed and compared, 

 1889 to 1896. Bij William Ellis, F.R.S. 



It is known that, in tabulating the photographic records of magnetic 

 declination and horizontal and vertical forces at the Royal Observatory, 

 Greenwich, it has been the custom to include all days, except those of 

 occasional extreme disturbance, so that many days of considerable dis- 

 turbance become included. It is also known that the diurnal inequality 

 so calculated differs from that obtained by the use of methods such as 

 those of Sabine and Wild. Theoretically, it is questionable whether it be 

 right arbitrarily to reject days of one particular character. The point 

 is one that occurs also in meteorology, and it is not usual in meteorological 

 tabulations to omit days of abnormal character. In dealing with mag- 

 netic records there is, however, this difficulty — that the abnormal variations 

 are at times of such magnitude as to defy treatment in the ordinary way. 

 And between these occasional outbursts and the days of quiet magnetism 

 intermediate degrees of intensity at different times occur. The arbitrary 

 exclusion, to any considerable extent, of daj's other than those of 

 extreme disturbance necessarily omits a certain portion of the diurnal 

 phenomena, since the diurnal movement as a whole is a combination of 

 the ordinary diurnal march with disturbance occurring apparently in an 

 unexpected or accidental manner that appears to some extent to have also 

 a local character. It having, however, become desirable to frame some 

 method of comparing together the diurnal inequalities of magnetism at 

 the different British magnetic observatories, at some of which it was 

 inconvenient to undertake any great amount of tabulation of records, this 

 circumstance led to the proposal to tabulate the curves for five days of quiet 

 magnetism only in each month, the selection of days being made by the 

 Astronomer Royal. This convention excludes entirely the influence of 

 seemingly irregular magnetic disturbance. But it furnishes diurnal 

 inequalities for the different observatories that are strictly comparable, 

 and, depending entirely on days of quiet magnetism, they represent the 

 undisturbed local diurnal variation. 



In the Report of the Committee for the year 1895, Dr. Chree discussed 

 at some length the results for Kew for the years 1890 to 1894, as found 

 from the five selected quiet days in each month. In the same Report he 

 drew attention also to an effect which, influencing only to a small 



