1879.] 



Relation between Declination and Force. 



43 



IV. " On the Relation between the Diurnal Range of Magnetic 

 Declination and Horizontal Force, as observed, at the Royal 

 Observatory, Greenwich, during the years 1841 to 1877, 

 and the Period of Solar Spot Frequency." By William 

 Ellis, F.R.A.S., Superintendent of the Magnetical and 

 Meteorological Department, Royal Observatory, Greenwich. 

 Communicated by Sir George Atry, K.C.B., F.R.S., Astro- 

 nomer Royal. Received April 23, 1879. 



(Abstract.) 



In this paper the author draws attention to the long- series of mag- 

 netical observations which have been made at the Royal Observatory,, 

 under the direction of Sir George B. Airy, K.C.B., Astronomer Royal. 

 Commencing in the year 1841, the observations for a few years con- 

 sisted of eye readings of the various instruments, made every two 

 hours ; since the year 1848 the motions of the magnets have been 

 registered by photography, according to a plan arranged by Mr; 

 Charles Brooke. 



Attention is briefly drawn to the question of magnetic variations, 

 and to the circumstance that examination of the Greenwich records 

 shows that, in addition to the ordinary diurnal and annual changes, 

 there appears to exist, in the magnetic diurnal ranges, an- inequality of 

 marked character, and of longer period, resembling in its features the 

 well-established eleven-year sun-spot period. 



It is remarked that this is not, by any means, the first time that 

 such relation has been discussed, it beingj by some investigators, 

 considered to be already sufficiently well proved. But it appeared 

 to the author that the long- series of Greenwich observations might be 

 well applied as an independent test of the accuracy of the supposed 

 relation. For (as regards the results actually employed) the observa- 

 tions have been throughout made on the same general plan, and with 

 the same instruments. The conclusion arrived at should therefore 

 be one worthy of some confidence. 



The results for declination and horizontal force only are used, 

 because, the vertical force instrument having been more than once 

 changed, the strict continuity- of the record by the latter instrument is 

 somewhat broken. 



The monthly mean diurnal range of declination, or of horizontal 

 force, is taken to represent the magnetic energy of the month rela- 

 tively to other months. Two series of numbers are thus formed, each 

 series being treated independently. Days of unusual magnetic dis- 

 turbance are omitted. 



Before the numbers can be compared with sun-spot numbers, it is 



