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

General Remarks.

1. Variation.

Captain Fitz-Roy's observations are so well distributed over the southern hemisphere, that a good view of the changes which the variation is undergoing throughout its meridians may be obtained by comparing his determinations with those of earlier observers at the same stations. The following table has been formed for the purpose of exhibiting such a comparison at all those stations where materials for it exist; and I may here remark how much such comparisons are facilitated by the valuable collection of early observations contained in the Appendix to the Magnetismus der Erde.

Cape of Good Hope.					Valparaiso.
Observer.	Date.	Variation.			Observer.	Date.	Variation.
		°	′				°	′
Davis	1605	0.	30	E.	Don G. Juan	1744	12.	30	E.
Keeling	1609	0.	12	W.	Vancouver	1793	14.	49	E.
Leydecker	1675	8.	28	W.	Lütke	1827	15.	00	E.
Mathews	1724	16.	22	W.	Fitz-Roy	1835	15.	18	E.
La Caille	1752	19.	0	W.
Wales	1772	20.	26	W.	Callao.
Wales	1775	21.	14	W.	Ulloa	1740	9.	02	E.
Bligh	1788	23.	16	W.	Duperrey	1823	9.	30	E.
Dentrecasteaux	1792	24.	30	W.	Fitz-Roy	1835	10.	36	E.
Freycinet	1818	26.	31	W.
Fitz-Roy	1836	28.	30	W.	Galapagos Islands.
					Vancouver	1794	8.	00	E.
St. Helena					B. Hall	1821	8.	20	E.
Davis	1610	7.	13	E.	Fitz-Roy	1835	9.	30	E.
Halley	1677	0.	40	E.
Halley	1691	1.	0	W.	Otaheite.
Mathews	1724	7.	30	W.	Cook	1769	4.	45	E.
Wales	1775	12.	18	W.	Wales	1773	5.	40	E.
Hunter	1789	15.	30	W.	Bayley	1774	5.	49	E.
Macdonald	1796	15.	48	W.	Vancouver	1794	6.	12	E.
Krusenstern	1806	17.	18	W.	Duperrey	1823	6.	40	E.
Fitz-Roy	1836	18.	00	W.	Fitz-Roy	1835	7.	34	E.
Rio de Janeiro.					Bay of Islands, N. Zealand.
Cook	1768	7.	34	E.	Tasman	1643	8.	40	E.
Hunter	1787	6.	12	E.	Cook	1769	11.	25	E.
Freycinet	1820	2.	54	E.	Duperrey	1824	13.	22	E.
Rumker	1821	3.	21	E.	Fitz-Roy	1835	14.	00	E.
Erman	1830	2.	10	E.
Fitz-Roy	1836	2.	00	E.	Sydney.
					Cook	1770	8.	00	E.
Falklands Islands.					Hunter	1787	8.	30	E.
Freycinet	1820	19.	26	E.	Flinders	1803	8.	51	E.
Duperrey	1822	19.	07	E.	Freycinet	1819	9.	15	E.
Fitz-Roy	1833	19.	00	E.	Duperrey	1824	8.	56	E.
					Fitz-Roy	1836	10.	24	E.
Port Famine
Wallis	1766	22.	30	E.	Van Diemen's Land.
Carteret	1766	22.	22	E.	Tasman	1642	3.	00	E.
Fitz-Roy	1831	23.	00	E.	Bayley	1777	7.	29	E.
					Bligh	1778	8.	33	E.
Concepçion.					Fitz-Roy	1836	11.	06	E.
La Perouse	1786	15.	15	E.
B. Hall	1821	15.	30	E.	Mauritius.
Duperrey	1823	16.	16	E.	Keeling	1609	21.	0	W.
Fitz-Roy	1835	16.	48	E.	Mathews	1722	19.	4	W.
					Freycinet	1818	12.	46	W.
					Duperrey	1824	13.	46	W.
					Fitz-Roy	1836	11.	18	W.

We may derive from the facts in the above table the following general and easily remembered conclusion in regard to the changes of the variation in the southern hemisphere; namely, that taking for our point of departure the meridian of 65° west in South America, we find that at all the stations east of that meridian to the Cape of Good Hope inclusive, the north pole of the needle has moved towards the west; and that all the stations west of the same meridian to Mauritius inclusive, the north pole of the needle has moved towards the east.

An almost equally simple generalization may be drawn in respect to the changes of situation of the lines of equal variation in the southern hemisphere: but here it must be supposed either that the reader is thoroughly familiar with the general arrangement of these lines, or that he has a map of them before him. He will find such a map of the variation lines in 1787 in the Fifth Report of the British Association; but any other map, corresponding to any epoch within the last hundred years, will equally serve the purpose. Referring to such a map, it will be seen that the lines of variation in the South Pacific, form a system of nearly concentric curves, of an oval, or pear-shaped form, the outside curves having a higher variation, which progressively diminishes to the centre. We may regard this system as comprehending the whole of the geographical space between the coast of South America and the meridian of New Zealand. Throughout this space the variation is easterly, and increases: we may consequently characterize the change in the situation of the lines of equal variation as a progressive closing-in

of the curves from all sides towards the centre, by which the areas severally comprehended by them become less; and the lower variations, 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 hemisphere, which is from east to west.