Narrative of the surveying voyages of His Majesty's ships Adventure and Beagle 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.