INVESTIGATION 



SOLAR-DIURNAL VARIATION OF THE MAGNETIC DECLINATION, 

 AND ITS ANNUAL INEQUALITY. 



HAVING discussed, in Part I, the eleven-year period in the amplitude of the 

 solar-diurnal variation, as well as in the disturbances of the magnetic declination, 

 I now proceed to the analysis of the annual inequality of the solar-diurnal 

 variation. 



To obviate the difficulty which would occur in cases of months of unusal dis- 

 turbance, if the crude observations were used, the normals or means freed from the 

 disturbances have been employed in the discussion. This mode of proceeding not 

 only obviates the necessity for rejecting the observations of particular months, but 

 brings out the most consistent results which the observations can furnish, for both 

 diurnal and annual variation. It is the course adopted by General Sabine in the 

 third volume of his discussion of the Toronto observations. 1 



Returning, then, to the hourly normals, they are rearranged in the tables which 

 follow, according to the different months of the year. The normals for 1840 are 

 corrected for the index error by the addition of 93.3 scale divisions. All correc- 

 tions for referring the partial monthly readings to the annual mean are, of course, 

 omitted. 



1 Table LXVI, of this volume, exhibits the solar-diurnal variation of the declination after the separa- 

 tion and omission of the larger disturbances ; whereas Table VII, of the preceding volume, similar in 

 form, differs from the latter, being derived from all the observations including the disturbances. 



