Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.1): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

528 GENERAL REMARKS. stations in the second of these localities only, between Africa and New Zealand. At Hobart Town, Sydney, and King George Sound, there appears to have been little or no change in the dip since the commencement of the present century. The arrangement of the changes of dip in the southern hemis- phere in four divisions, characterised by an alternate increase and decrease of dip, is in correspondence with the double flexure of the lines of dip ; and is a consequence of the western motion of the two southern magnetic poles. Careful observations made at St. Petersburgh, have shewn that the annual change of the dip in the northern hemisphere takes place altogether between the months of May and December ; there being in fact a small movement in an opposite direction between December and May. This fact is of great interest in its bearing on the study of the causes of the magnetic phenomena. We have as yet no corresponding knowledge in regard to the southern hemis- phere. The magnitude of the annual change which Captain Fitz- Roy's observations show is now taking place at the Cape of Good Hope, is deserving of attention in this respect. A large amount of annual change is obviously highly favourable for a determination of all the circumstances belonging to it ; and its existence at the Cape, where there is already a fixed observatory, points to that station as most eligible for this investigation. The observations at Ascension shew that the epoch is fast approaching when the needle will pass from north to south dip at that island : it is extremely desirable that the period at which this change takes place should be determined with as much precision as possible. III. Intensity. 1 have discussed in the Seventh Report of the British Association, the very important inferences in regard to the general distribution of magnetism in the southern hemisphere, afforded by Captains King and Fitz-Roy's most valuable series of intensity observa- tions ; but no inferences in regard to the changes which this phe- nomenon may be supposed to undergo can be drawn, as has been done in the cases of the variation and dip, because we possess no observations of the intensity made at a sufficiently early period to afford good materials for such a comparison, EDWARD SABINE.