SOLAR-DIURNAL VARIATION OF THE DECLINATION. 505 



mena of the semi-annual inequality is in no way in- 

 consistent with the ingenious hypothesis suggested by 

 the late Dr. Langberg for its explanation (Proc. Eoyal 

 Society, vol. vii. p. 434). The epoch of the transition from 

 the one semi-annual phase to the other certainly takes 

 place about the time of the equinoxes, but it cannot be 

 said that observation has yet denned the precise day of 

 the transition at either equinox. 



The important question next presents itself, by 

 what other geographical or magnetical peculiarities is 

 the line distinguished, which we may regard as the 

 zero-line of the mean diurnal variation, and where 

 consequently the semi-annual inequality constitutes the 

 sole subsisting diurnal variation? The places which 

 first made known to us these characteristic phenomena 

 of a magnetically-equatorial station were St. Helena 

 and the Cape of Grood Hope. Both are situated near 

 the line of least magnetic force on the surface of the 

 globe. This is a line encircling the globe, passing 

 through and connecting the points in each terrestrial 

 meridian where the magnetic force is least in that meri- 

 dian : it is not therefore itself an isodynamic line, but 

 divides the surface of the globe into two hemispheres, in 

 each of which the magnetic force progressively increases 

 on either side of the dividing line, until the isodynamics 

 form lemniscates around the two foci of maximum 

 force in each hemisphere. In this respect it has a 

 claim to be considered as a magnetic equator ; and its 

 claim will be greatly strengthened if it be identified, as 

 seems probable, with the zero -line of the sun's mean 

 diurnal influence on the globe. It would be super- 

 fluous to dwell on the obvious theoretical importance of 



