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they represent substantially the same phenomenon. The magni- 

 tude and inflexions of the curves are not indeed identical, but they 

 approach so near to it that we may well suppose the small differ- 

 ences to be very minor modifications which will some day receive 

 their explanation, It will be remarked that during the hours when 

 the sun is above the horizon and the effects are greatest, the corre- 

 spondence of the phenomena at the three stations is most striking, 

 and that there is no inversion of the phenomena in the opposite hemi- 

 spheres ; in both (as well as at St. Helena, in the tropics), the De- 

 clination is easterly of the mean in the forenoon and westerly in the 

 afternoon when the sun is north of the equator, and the reverse 

 when the sun is south of the equator. The effects are the same at 

 the three stations, though in the one hemisphere the sun being 

 north of the equator corresponds to summer, and in the other hemi- 

 sphere to winter ; whilst in the tropics this distinction of seasons 

 almost ceases to be sensible, and the epochs of maximum and mini- 

 mum of temperature do not correspond with either of those of the 

 extra-tropical stations. The phenomena thus represented embrace 

 above 86 of latitude, presenting not only almost extreme contem- 

 poraneous diversities of climate, but also not less remarkable diver- 

 sities of absolute dip, declination and magnetic force. 



" No doubt can, I apprehend, be entertained that the annual 

 variation which is here represented, is attributable, primarily, to the 

 earth's revolution round the sun in a period of the same duration 

 and in an orbit inclined to the equator. But in what way, it may be 

 asked, does the sun superimpose upon the earth's magnetism this 

 comparatively small but systematic magnetic variation ? The simi- 

 larity of effect, amounting almost indeed to identity at the hours 

 when the sun is above the horizon of the station, taking place at 

 stations where both the climatic and the terrestrial magnetic con- 

 ditions are so dissimilar, seems to remove it altogether from those 

 physical connexions, which have so often and in so many various 

 ways been referred to as affording possible explanations of the mag- 

 netic variations. In this difficulty some assistance may perhaps be 

 afforded by examining more closely, by means of the St. Helena 

 observations, the epochs when the phenomena of one of the semi- 

 annual groups passes into the very dissimilar phenomena of the 

 other semiannual group. This has been stated to take place 





