Freycinet181826. 31 W.Fitz-Roy183628. 30 W.GALAPAGOS ISLANDS.
Vancouver17948. 00 E.
ST. HELENAB. Hall18218. 20 E.
Davis16107. 13 E.Fitz-Roy18359. 30 E.
Halley16770. 40 E.Halley16911. 0 W.OTAHEITE.
Mathews17247. 30 W.Cook17694. 45 E.
Wales177512. 18 W.Wales17735. 40 E.
Hunter178915. 30 W.Bayley17745. 49 E.
Macdonald179615. 48 W.Vancouver17946. 12 E.
Krusenstern180617. 18 W.Duperrey18236. 40 E.
Fitz-Roy183618. 00 W.Fitz-Roy18357. 34 E.
RIO DE JANEIRO.BAY OF ISLANDS, N. ZEALAND.
Cook17687. 34 E.Tasman16438. 40 E.
Hunter17876. 12 E.Cook176911. 25 E.
Freycinet18202. 54 E.Duperrey182413. 22 E.
Rumker18213. 21 E.Fitz-Roy183514. 00 E.
Erman18302. 10 E.Fitz-Roy18362. 00 E.SYDNEY.
Cook17708. 00 E.
FALKLANDS ISLANDS.Hunter17878. 30 E.
Freycinet182019. 26 E.Flinders18038. 51 E.
Duperrey182219. 07 E.Freycinet18199. 15 E.
Fitz-Roy183319. 00 E.Duperrey18248. 56 E.
Fitz-Roy183610. 24 E.
PORT FAMINEWallis176622. 30 E.VAN DIEMEN"S LAND.
Carteret176622. 22 E.Tasman16423. 00 E.
Fitz-Roy183123. 00 E.Bayley17777. 29 E.
Bligh17788. 33 E.
CONCEPcION.Fitz-Roy183611. 06 E.
La Perouse178615. 15 E.B. Hall182115. 30 E.MAURITIUS.
Duperrey182316. 16 E.Keeling160921. 0 W.
Fitz-Roy183516. 48 E.Mathews172219. 4 W.
Freycinet181812. 46 W.
Duperrey182413. 46 W.
Fitz-Roy183611. 18 W.
{524} 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 a.s.sociation; 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 s.p.a.ce between the coast of South America and the meridian of New Zealand. Throughout this s.p.a.ce 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 {525} 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 s.p.a.ce 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.
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.]
{526}
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.
La Caille175411. 10 N.Lutke182745. 33 S.
Cook17758. 57 N.King182945. 10 S.
Sabine18224. 30 N.Fitz-Roy183543. 15 S.
Duperrey18251. 58 N.Fitz-Roy18361. 39 N.KING GEORGE SOUND.
Vancouver179164. 54 S.
ST. HELENA.Flinders180164. 01 S.
La Caille17549. 0 S.Fitz-Roy183664. 41 S.
Cook177511. 25 S.Fitz-Roy183618. 01 S.VAN DIEMEN"S LAND.
Cook177770. 15 S.
CAPE OF GOOD HOPE.Bertrand179270. 50 S.
La Caille175143. 0 S.Dentrecasteaux179270. 30 S.
Bayley177245. 37 S.De Rossel179370. 10 S.
Bayley177545. 19 S.Fitz-Roy183670. 35 S.
Abercrombie177546. 26 S.Bayley177646. 31 S.SYDNEY.
Freycinet181850. 47 SFlinders180362. 52 S.
Fitz-Roy183652. 35 S.Freycinet181962. 47 S.
Brisbane182162. 36 S.
MAURITIUS (PORT LOUIS)Duperrey182462. 20 S.
La Caille175452. 17 S.Fitz-Roy183662. 49 S.
Duperrey182453. 51 S.Fitz-Roy183654. 01 S.N. ZEALAND (BAY OF ISLANDS).
Duperrey182459. 45 S.
OTAHEITE.Fitz-Roy183559. 32 S.
Cook177329. 43 S.Cook177429. 59 S.TIERRA DEL FUEGO.
Bayley177729. 47 S.Bayley }Duperrey182330. 03 S.Lat. 55.22"; }177466. 54 S.
Erman183030. 29.5 SLong. 70. 03. }Fitz-Roy183530. 13.5 SKing }Lat. 55.51"; }182859. 44 S.
LIMA AND CALLAO.Long. 67. 34. }Feuillee171010. 30 S.Humboldt17999. 59 S.FALKLAND ISLANDS.
Duperrey18238. 33 S.Freycinet182055. 20 S.
Fitz-Roy18357. 03 S.Duperrey182254. 49 S.
Fitz-Roy1833-453. 25 S.
VALPARAISO.Malaspina179344. 58 S.STA CATHARINA.
Vancouver179544. 15 S.Duperrey182222. 54 S.
Lutke182739. 56 S.King182722. 12 S.
King182940. 11 S.Fitz-Roy183538. 03 S.RIO DE JANEIRO.
La Caille175120. 0 S.
CONCEPcION.Freycinet181714. 42 S.
Feuillee171055. 30 S.King182614. 00 S.
Perouse178650. 00 S.Lutke182714. 35 S.
Duperrey182344. 55 S.Erman183013. 31 S.
Fitz-Roy183213. 37 S.
We may cla.s.sify the changes which are taking place in the dip in the southern hemisphere in four divisions, characterised by an {527} 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.