oiTAmJoaa t ^o mn^tiir jtdii^y^t. Am 



Royal Society. 149 



and that there is no inversion of the phanomena 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 pha^nomena 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 

 l^hysical 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, theepochs when the phaenomena of one of the semi- 

 annual groups passes into the very dissimilar phsenomena of the 

 other semiannual group. This has been stated to take place 

 approximately at the equinoxes. The approximation, particularly 

 at the September equinox, is very distinctly and definitely marked. 

 The day of the equinox is the 21st of September; if a mean be 

 taken of the diurnal march in the three weeks from the ist to the 

 21st of September, the line which represents it scarcely differs 

 sensibly at any hour of the twenty-four from the mean line of the pre- 

 ceding half-year, taken from the 22nd of March to the 20th of Sep- 

 tember ; thus showing that the phaenomena of that semiannual group 

 are unchanged up to the time of the equinox. If in the same way a 

 mean be taken of the diurnal march in the three weeks following the 

 21st of September, the line which represents them shows that the 

 passage from the phsenomena of one semiannual group to those of the 

 other has not only commenced, but that in half ths period of three 

 weeks, i. e. within eleven days of the equinox, the change has already 

 advanced very far towards its completion ; and by the middle of Oc- ? 



tober it is found to be quite complete, the mean ni October retaining 

 no trace of those semiannual characters which had undergone no '.. 



modification ten days before the equinox." The facts thus stated ,| 



were illustrated by diagrams. h'i 



