SOLAR-DIURNAL VARIATION OF THE DECLINATION. 509 



character of a middle latitude station, and the Cape, that 

 of an equatorial station. 



The facts thus cited seem to be quite irreconcilable 

 to the thermic or thermo-electric- current hypotheses ; 

 and not less so to M. Arago's belief that the diurnal 

 variation would be found to have changed completely 

 " du moment ou le soleil a passe d'un cote du zenith a 

 1'autre : " nor do they sanction the opinion that the line 

 of no dip should be regarded as the magnetic equator. 



It seems to follow from what has been said, that a 

 physical explanation of the facts of the semi-annual 

 inequality will have to account for the epochs at which 

 the half-yearly phases pass into each other, which 

 epochs appear to respect the sun's passage of the geo- 

 graphical equator, and also for the terrestrial distri- 

 bution of the phenomena in question, which appears to 

 respect the magnetical equator, or the line of least 

 magnetic force. 



It has been stated above, that the turning hours, or 

 hours of greatest easterly and greatest westerly deflection 

 of the semi-annual portion of the diurnal variation, are 

 from 7 to 9 A.M., and from noon to 2 P.M. On pursuing 

 this correct general statement into still more precise 

 details, we find, within the above-named limits, a re- 

 markable difference between the two half-years, strik- 

 ingly analogous to a similar difference pointed out in 

 the preceding section, as existing between the exact 

 turning hours of the regular solar-diurnal variation in 

 the two hemispheres. When the sun is in the northern 

 signs, and the direction of the movement pertaining to 

 the semi-annual inequality agrees with that of the 

 regular solar-diurnal movement throughout the year in 



