Part 46
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.
CONCEPcION.
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.
{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 Caille
1754
11. 10 N.
Lutke
1827
45. 33 S.
Cook
1775
8. 57 N.
King
1829
45. 10 S.
Sabine
1822
4. 30 N.
Fitz-Roy
1835
43. 15 S.
Duperrey
1825
1. 58 N.
Fitz-Roy
1836
1. 39 N.
KING GEORGE SOUND.
Vancouver
1791
64. 54 S.
ST. HELENA.
Flinders
1801
64. 01 S.
La Caille
1754
9. 0 S.
Fitz-Roy
1836
64. 41 S.
Cook
1775
11. 25 S.
Fitz-Roy
1836
18. 01 S.
VAN DIEMEN'S LAND.
Cook
1777
70. 15 S.
CAPE OF GOOD HOPE.
Bertrand
1792
70. 50 S.
La Caille
1751
43. 0 S.
Dentrecasteaux
1792
70. 30 S.
Bayley
1772
45. 37 S.
De Rossel
1793
70. 10 S.
Bayley
1775
45. 19 S.
Fitz-Roy
1836
70. 35 S.
Abercrombie
1775
46. 26 S.
Bayley
1776
46. 31 S.
SYDNEY.
Freycinet
1818
50. 47 S
Flinders
1803
62. 52 S.
Fitz-Roy
1836
52. 35 S.
Freycinet
1819
62. 47 S.
Brisbane
1821
62. 36 S.
MAURITIUS (PORT LOUIS)
Duperrey
1824
62. 20 S.
La Caille
1754
52. 17 S.
Fitz-Roy
1836
62. 49 S.
Duperrey
1824
53. 51 S.
Fitz-Roy
1836
54. 01 S.
N. ZEALAND (BAY OF ISLANDS).
Duperrey
1824
59. 45 S.
OTAHEITE.
Fitz-Roy
1835
59. 32 S.
Cook
1773
29. 43 S.
Cook
1774
29. 59 S.
TIERRA DEL FUEGO.
Bayley
1777
29. 47 S.
Bayley }
Duperrey
1823
30. 03 S.
Lat. 55.22'; }
1774
66. 54 S.
Erman
1830
30. 29.5 S
Long. 70. 03. }
Fitz-Roy
1835
30. 13.5 S
King }
Lat. 55.51'; }
1828
59. 44 S.
LIMA AND CALLAO.
Long. 67. 34. }
Feuillee
1710
10. 30 S.
Humboldt
1799
9. 59 S.
FALKLAND ISLANDS.
Duperrey
1823
8. 33 S.
Freycinet
1820
55. 20 S.
Fitz-Roy
1835
7. 03 S.
Duperrey
1822
54. 49 S.
Fitz-Roy
1833-4
53. 25 S.
VALPARAISO.
Malaspina
1793
44. 58 S.
STA CATHARINA.
Vancouver
1795
44. 15 S.
Duperrey
1822
22. 54 S.
Lutke
1827
39. 56 S.
King
1827
22. 12 S.
King
1829
40. 11 S.
Fitz-Roy
1835
38. 03 S.
RIO DE JANEIRO.
La Caille
1751
20. 0 S.
CONCEPcION.
Freycinet
1817
14. 42 S.
Feuillee
1710
55. 30 S.
King
1826
14. 00 S.
Perouse
1786
50. 00 S.
Lutke
1827
14. 35 S.
Duperrey
1823
44. 55 S.
Erman
1830
13. 31 S.
Fitz-Roy
1832
13. 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.
