1868.] 



Inequalities of Terrestrial Magnetism. 



165 



itself in the discussion of the solar inequalities, it appeared to the author 

 very desirable to examine whether there is any discoverable difference in the 

 lunar inequalities for the same years. The years were accordingly thus di- 

 vided : — 



Large solar curves. . 1848 to 1852, 1859, 1860, 1862, 1863. " 

 Small solar curves.. 1853 to 1858, 1861. 



On discussing these, it was found that in all cases the lunar horary epoch 

 for the inequality was sensibly the same for years of large solar curves and 

 for years of small solar curves ; but the coefficient was different. The 

 value of the fraction 



lunar semidiurnal inequality in years of large solar curves 

 lunar semidiurnal inequality in years of small solar curves 



is 



For declination 1*35 



For horizontal force T25 



The author remarks that it would seem possible to suggest two conjec- 

 tural reasons for this remarkable association in the time-law of changes of 

 solar effect and lunar effect. One is, that the moon's magnetic action is 

 really produced by the sun's magnetic action ; and a failure in the sun's 

 magnetic power will make itself sensible, both in its direct effect on our 

 magnets and in its indirect effect through the intermediation of the moon's 

 excited magnetism. The other is that, assuming both actions (solar and 

 lunar) to act on our magnets indirectly by exciting magnetic powers in the 

 earth, which alone or principally are felt by the magnets, the earth itself 

 may have gone through different stages of magnetic excitability, increasing 

 or diminishing its competency to receive both the solar and the lunar action. 



The epochs of lunar inequality in western declination from north and in 

 horizontal force to magnetic north are sensibly the same ; and the coeffi- 

 cients expressed in terms of horizontal force on the mean of all the years 

 are sensibly the same, and equal to 0'000061. The direction of the com- 

 posite disturbing force is therefore sensibly N."W. and S.E. magnetic, or 

 (roughly) in the direction of a line from the Red Sea to the south of 

 Hudson's Bay. It may be remarked in opposition to this that the solar 

 diurnal action is mainly in the S.AV. direction. 



The luno- diurnal inequality of vertical force on the mean of all the years 

 appears to consist of a luno-diurnal and a luno-semidiurnal term. 



December 17, 1868. 

 Capt. KICHAKDS, R.N., Vice-President, in the Chair. 

 The following communications were read : — 



