Narrative of the surveying voyages of His Majesty's ships Adventure and Beagle, between the years 1826 and 1836. Volume I. Proceedings of the First Expedition, 1826-1830

by Robert Fitzroy; Philip Parker King

Otaheite, and the Bay of Islands in New Zealand, present examples 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 considerably 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 Mauritius; 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 discordance may be occasioned by the great prevalence of local disturbances at Ascension.]

II. Dip.

The following table exhibits the comparison of Captain Fitz-Roy's observations of the dip in the southern hemisphere, with those of earlier observers, at stations where the materials for such a comparison exist.

Ascension.					Observer.	Date.	Variation.
Observer.	Date.	Variation.					°	′
		°	′		Lütke	1827	45.	33	S.
La Caille	1754	11.	10	N.	King	1829	45.	10	S.
Cook	1775	8.	57	N.	Fitz-Roy	1835	43.	15	S.
Sabine	1822	4.	30	N.
Duperrey	1825	1.	58	N.	King George Sound.
Fitz-Roy	1836	1.	39	N.	Vancouver	1791	64.	54	S.
					Flinders	1801	64.	01	S.
St. Helena.					Fitz-Roy	1836	64.	41	S.
La Caille	1754	9.	0	S.
Cook	1775	11.	25	S.	Van Diemen's Land.
Fitz-Roy	1836	18.	01	S.	Cook	1777	70.	15	S.
					Bertrand	1792	70.	50	S.
Cape of Good Hope.					Dentrecasteaux	1792	70.	30	S.
La Caille	1751	43.	0	S.	De Rossel	1793	70.	10	S.
Bayley	1772	45.	37	S.	Fitz-Roy	1836	70.	35	S.
Bayley	1775	45.	19	S.
Abercrombie	1775	46.	26	S.	Sydney.
Bayley	1776	46.	31	S.	Flinders	1803	62.	52	S.
Freycinet	1818	50.	47	S	Freycinet	1819	62.	47	S.
Fitz-Roy	1836	52.	35	S.	Brisbane	1821	62.	36	S.
					Duperrey	1824	62.	20	S.
Mauritius (Port Louis)					Fitz-Roy	1836	62.	49	S.
La Caille	1754	52.	17	S.
Duperrey	1824	53.	51	S.	N. Zealand (Bay of Islands).
Fitz-Roy	1836	54.	01	S.	Duperrey	1824	59.	45	S.
					Fitz-Roy	1835	59.	32	S.
Otaheite.
Cook	1773	29.	43	S.	Tierra del Fuego.
Cook	1774	29.	59	S.	Bayley Lat. 55°. 22′; Long. 70. 03.
Bayley	1777	29.	47	S.		1774	66.	54	S.
Duperrey	1823	30.	03	S.
Erman	1830	30.29.5S.			King Lat. 55°. 51′; Long. 67. 34.
Fitz-Roy	1835	30.13.5S.				1828	59.	44	S.
Lima and Callao.
Feuillée	1710	10.	30	S.	Falkland Islands.
Humboldt	1799	9.	59	S.	Freycinet	1820	55.	20	S.
Duperrey	1823	8.	33	S.	Duperrey	1822	54.	49	S.
Fitz-Roy	1835	7.	03	S.	Fitz-Roy	1833-4	53.	25	S.
Valparaiso.					Sta Catharina.
Malaspina	1793	44.	58	S.	Duperrey	1822	22.	54	S.
Vancouver	1795	44.	15	S.	King	1827	22.	12	S.
Lütke	1827	39.	56	S.
King	1829	40.	11	S.	Rio de Janeiro.
Fitz-Roy	1835	38.	03	S.	La Caille	1751	20.	0	S.
					Freycinet	1817	14.	42	S.
Concepçion.					King	1826	14.	00	S.
Feuillée	1710	55.	30	S.	Lütke	1827	14.	35	S.
Perouse	1786	50.	00	S.	Erman	1830	13.	31	S.
Duperrey	1823	44.	55	S.	Fitz-Roy	1832	13.	37	S.

We may classify the changes which are taking place in the dip in the southern hemisphere in four divisions, characterised by an

alternate increase and decrease of dip. Commencing with the meridian of Greenwich, and proceeding eastwardly round the hemisphere, we may distinguish the divisions as follows, in the order of their geographical succession.