SOLAR-DIURNAL VARIATION OF THE DECLINATION. 501 



2. The semi-annual inequality. 



The diurnal variation described in the preceding sec- 

 tion is that which we find when we take an average of the 

 whole year, and corresponds with what we may suppose 

 would actually take place throughout the year if the 

 sun's path were always in the plane of the equator. 

 But observation has made known to us the existence of 

 a semi-annual inequality in the diurnal variation, 

 having opposite phases according as the sun has north 

 or south declination; having consequently its turning 

 epochs about the times of the solstices, and disap- 

 pearing, as one phase passes into the other, about the 

 times of the equinoxes. As far as can be judged 

 from the stations where the facts have been most satis- 

 factorily examined, which are chiefly equatorial and 

 middle latitude stations, the semi-annual inequality is a 

 general phenomenon, and nearly uniform in character 

 and amount at all stations. Its principal effect takes 

 place during the hours of the day, the extreme deflec- 

 tions occurring about 7 or 8 A.M., and from 1 to 2 P.M. ; 

 the north end of the magnet being deflected in the 

 forenoon to the east when the sun is north of the 

 equinoctial, and to the west when he is south of 

 it; and, conversely, in the afternoon to the west 

 when the sun is north, and to the east when he is 

 south of the equinoctial. In other words its effect 

 is, when the sun has north decimation to cause the 

 north end of the magnet in both hemispheres to be 

 more to the east in the forenoon, and more to the west 

 in the afternoon, than it would otherwise be, and 



