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

GENERAL REMARKS. 525 of the curves from all sides towards the centre, by which the areas severally comprehended by them become less ; and the lower varia- tions, as they successively reach the centre, disappear, and are replaced by the closing-in of those of next higher amount. The changes which have taken place at all Captain Fitz- Roy's stations comprised within the space referred to, are accordant with the systematic alteration thus described. In all other parts of the hemisphere the lines of variation have a progressive westerly movement, and to this also Captain Fitz- Roy's observations correspond. It follows, from what has been stated, that the lines on the western side of the concentric system in the South Pacific have an eastward movement, which presents an apparent anomaly to the general progress of the lines of variation in the southern hemis- phere, which is from east to west. Otaheite, and the Bay of Islands in New Zealand, present exam- ples of changes in the variation corresponding to this apparent anomaly. The consistency, however, both of the movement and of the configuration of the lines of variation in this quarter, with those in other parts of the southern hemisphere, and with the general system of the magnetic phenomena, has been shewn by Mr. Hansteen in the Magnetismus der Erde, and in the Annalen der Physik, vol. xxi. The annual amount of the change of the variation appears con- siderably greater at the Cape of Good Hope and Mauritius than at any other of Captain Fitz- Roy's stations, amounting to about eight minutes ; shewing that the variation lines in that quarter are changing their position more rapidly than elsewhere. The north pole of the needle is moving to the west at the Cape, and to the east at IVIauritius ; but it will be seen, by a reference to the map, that these opposite movements are in perfect correspondence with the uniform westerly progression of the variation lines, and result from their configuration. At the stations in the vicinity of the meridian of 65° west, the change appears to be very small. [The variation at Ascension (13° 30' W.) is correctly inserted in the tables : it is the only one of Captain Fitz-Roy's stations at which his observations are not accordant with those of other observers : the dis- cordance may be occasioned by the great prevalence of local disturbances .at Ascension.]