MAGNETIC VARIATION. '125 



to August, since the intermediate months of March and Sep- 

 tember present, as it were, phenomena of transition. At 

 Hobarton the extremity of the needle, which points north- 

 ward, exhibits two eastern and two western maxima of elon- 

 gation,* so that in the period of the year from October to 

 February it moves eastward from 8 or 9 o'clock A.M. till 2 

 P.M., and then from 2 till 11 P.M., somewhat to the west; 

 from 11 P.M. to 3 A.M. it again turns eastward, and from 

 3 to 8 A.M. it goes back to the west. In the period between 

 April and August the eastern turning hours are later, oc- 

 curring at '3 P.M. and 4 A.M. ; while the western turning 

 hours fall earlier, namely, at 10 A.M. and at 11 P.M. In 

 the northern magnetic hemisphere the motion of the needle 

 westward from 8 A.M. till 1 P.M. is greater in the summer 

 than in the winter ; while in the southern magnetic hemis- 

 phere, where the motion has an opposite direction between 

 the above-named turning hours, the quantity of the elonga- 

 tion is greater when the sun is in the southern than when it 

 is in the northern signs. 



The question which I discussed seven years ago in the 

 Picture of Nature,f whether there may not be a region of 

 the earth, probably between the geographical and magnetic 

 equators, in which there is no horary variation (before tha 

 return of the northern extremity of the needle to an oppo- 

 site direction of variation in the same hours), is one which, 

 it would seem, from recent experiments, and more especially 

 since Sabine's ingenious discussions of the observations made 

 at Singapore (1 17' N. lat.), at St. Helena (15 56' S. lat.), 

 and at the Cape of Good Hope (33 5G / S. lat.), must be an- 

 swered in the negative. No point has hitherto been discov- 

 ered at which the needle does not exhibit a horary motion, 

 and since the erection of magnetic stations the important and 

 very unexpected fact has been evolved that there are places 

 in the southern magnetic hemisphere at which the horary 

 variations of the dipping-needle alternately participate in the 

 phenomena (types) of both hemispheres. The island of St. 

 Helena lies very near the line of weakest magnetic intensity, 

 in a region where this line divaricates very widely from the 

 geographical equator and from the line of no inclination. 



* Sabinc, Maqn. Observ. made at Hobarton, vol. i. (1841 and 1842), 

 p. xxxv.; 2, 148; vol. ii. (1843-1845), p. iii.-xxxv., 172-344. See 

 also Sabiue, Obs. made at St. Helena, and in Phil. Transact, for 1847, 

 pt. i., 55, pi. iv., and Phil. Transact, for 1851, pt. ii., p. 36, pi. xxvii. 



t Cosmos, vol. i., p. 183. 



