Magnetic Declination and Horizontal Force and Solar Spots. 73 



Fia. 3. Sun-spot Frequency and Diurnal Eange of Decimation and 

 Horizontal Force compared. 



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The upper curve is that of 

 sun-spot frequency. The middle 

 and lower curves are those of 

 diurnal range of declination and 

 horizontal force respectively ; 

 the thick line is that resulting 

 from the employment of all 

 days, and the thin line that 

 found from five selected quiet 

 days in each month. 



(thin line) in a small degree only as compared with its variation 

 with sun spots, from which it follows that the diurnal range as found 

 from employing all days (thick line), as in the 1841 1896 series, may 

 be taken to represent the solar diurnal range (thin line) the varia- 

 tion of the former being essentially that of the solar diurnal range. 



The explanation of the circumstance that the difference in the 

 amplitude of the magnetic range, as found from five selected quiet 

 days, and from all days, is so small as compared with the variation 

 in amplitude of both with variation of sun spots, would seem to be 

 that the effect of irregular magnetic disturbance is such as sometimes 

 to increase the value of the magnetic element and sometimes to de- 

 crease it, these opposite effects combining to neutralise each other to 

 such an extent as to influence the solar diurnal range in an unim- 

 portant degree only, as compared with its considerable variation 

 with that of sun spots, which is the point that is here material. 



Although the change produced in the character of the diurnal 

 magnetic movement by including disturbed days is not a matter that 

 possesses significance as regards the purposes of this paper, it be- 

 comes in other respects one for consideration. It is, however, a 

 question related toothers that would be better discussed in a separate 

 communication. 



