SOLAR-DIURNAL VARIATION OF THE DECLINATION. 503 



described in this section has the effect in the northern 

 hemisphere of increasing both the forenoon easterly and 

 the afternoon westerly deflections, and in the southern 

 hemisphere of decreasing both the forenoon westerly 

 and the afternoon easterly deflections and during the 

 greater part of the other half year in which the sun has 

 south declination, the semi-annual inequality has the 

 opposite effects, i. e. in the northern hemisphere, it 

 decreases both the morning easterly and the afternoon 

 westerly deflections, and in the southern hemisphere, it 

 increases both the morning westerly and the afternoon 

 easterly deflections. The apparent complexity of the 

 phenomena which we have described is occasioned by 

 the remarkable difference which takes place in the mode 

 in which the sun's influence is exercised in producing, 

 1. the mean diurnal variation, and 2. the semi-annual 

 inequality of the same. In the first the directions of 

 the deflection are uniform throughout the year in the 

 middle latitudes of the one hemisphere, and (although 

 opposite) are also uniform throughout the year in the 

 middle latitudes of the other hemisphere ; whilst in the 

 semi-annual inequality the directions of the deflection 

 are uniform in the two hemispheres, but opposite in the 

 two half years. Thus in each hemisphere the semi- 

 annual deflections concur with those of the mean di- 

 urnal variation for half the year, and consequently 

 augment them ; and oppose, and consequently diminish 

 them in the other half year. 



We have next to consider the effect of the super- 

 position of the semi-annual inequality at the magnetic 

 equator and adjacent stations, using the term "mag- 



