I the Horizontal Component of Magnetic Force. 379 



LK ^.—Principal epochs of mean horizontal force. 



July." - - ? 57 o 53 



Au-ust, - - 7 38 o 44 



Ocl'.'.re'r/'^- ." 8 08 5 S 



N..ve.iil.er, - 8 4o 3 38 



December,- ■ ?_34__ 3 o3 



Siimn.er, - - 7 45 i 12 



tbe day and for each month of the year. The three vnriahles 

 are the hour of the day, the month of the year, and the difft-renee 

 of the horizontal force from the normal. The contour lines of 

 the magnetic surface differ 0-0005 of horizontal force in absohite 

 measure. Full lines indicate greater value, lines of dasiies le.ss 

 value than the mean; dotted lines represent the normal value. 

 Annual variation of the horizontal force. — For the discas.-<ion of 

 the annual variation we make use of monthly normal readings of 

 tue horizontal force as given in table No. IL If »» equals the 

 inonthly effect of the total progressive change, we obtain from 

 the twelve equations by the usual method the value ?n= +15-49, 

 and the correction for progressive change for July and June for 

 instance becomes -\-b'bm and -b-bm respectively. The fol- 

 lowing table contains the monthly normals uncorrected and cor- 

 J'ected for progressive change, also the differences from the mean 

 of each month constituting the annual variation. 



Table Xlt. 







CO... 



Corrected 





vaH..o„. 

















JdT 



676 3 





-fiTT 



+ .0-6 



-— 3i-- 



+.6 





t^:^.. 





T??^ 



IV,'. 



-6-7 





tTo 



October, 





+38 7 



7769 





-17 



















^' 



11 



tn 





- 40 





9 



Mirch, 



8o5- 



'542 









-o3 



illl 



856 8 



:j?; 









Vo\ 

















-__' 



2 I 











m-a,nre. 



