ON COMPARING AND REDUCING MAGNETIC OBSERVATIONS. 



83 



taken without regard to sign, are in Spring and Autumn 13' -92 and 

 12' -97 respectively, and in Summer and Winter 7'-54: and 9'-34. The 

 larger values thus occur in Spring and Autumn, periods of the year 

 at which magnetic disturbance might more actively influence the 

 all day values. In horizontal force the values of excess in the difierent 

 seasons do not show such similarity as in declination ; indeed, there is 

 much discordance. Here the Spring and Autumn sums of excess are 

 247 eg s. and 280 c.g.s. i-espectively, and the Summer and Winter sums 

 210 c.g.s. and 260 c.g.s. In vertical force there is greater similarity in 

 the values of excess at different seasons than in horizontal force : the sums 



Diurnal Inequality at Greenwich : Excess of quiet days ordinate above all days 



ordinate, 1890-1894. 



'20 



-to 





 ■ ZP 



to 



- ao 



*Z0 



J/ .Force 



of excess of the Spring and Autumn values are 623 c.g.s. and 760 c.g.s. 

 respectively, and of the Summer and Winter values 688 c.g.s. and 

 408 c.g.s. The differences between the quiet day and all day diurnal 

 inequalities for the year are shown in the annexed diagram, in which, 

 for better estimation of comparative magnitude, the variations of declina- 

 tion are converted into variations of westerly force. It is curious to see 

 how in horizontal force the opposite seasonal effects (in part probably 

 accidental, rather than real) have tended so to neutralise each other that 

 the resulting yearly curve appears to be an insignificant accidental 

 residual. 



The years 1890 to 1894 were selected for comparison of the Greenwich 



