Narrative of the surveying voyages of His Majesty's ships Adventure and Beagle Part 45

Hansteen, from whom he received the apparatus, has now given permission to Captain Fitz-Roy to publish them with his own. I have already noticed the great loss of magnetism which took place in this cylinder during Captain King's voyage, and the care with which that officer availed himself of every opportunity of ascertaining, by direct observation, the proportion of the loss sustained in separate portions of the voyage. There are twelve stations of observation on the east and west coasts of South America, besides three stations in ports of the Atlantic on the outward voyage. By the practice of repeating observations at the same station at distant intervals, the South American stations are so linked together and connected, that by adopting a method similar to that used in determining longitudes by means of chronometers, we may compute the intensity at all the South American stations referred to and dependent on the force at Rio de Janeiro; regarding Rio in the same light as a first meridian is considered in determinations of longitude. We may then make Rio the means of connecting the whole series with Europe; for which it is remarkably well suited, the intensity there having been determined, independently of Captain King, by four observers of different nations, whose results are extremely accordant.

The dip observations of Captain King were communicated, in occasional correspondence during the voyage, to M. Hansteen, who computed them by Mayer's formula, and arranged them in a table, of which a copy was given by Captain King to Captain Fitz-Roy, and is printed in the next page. At some of the stations Captain Fitz-Roy also observed the dip in the subsequent voyage, and, as will be seen, the results of the two observers sometimes differ considerably. This may have been caused, either by instrumental or other error of observation, or by actual differences of dip existing in different localities at the same station.

{516}

DIPS, _observed by_ Capt. P. P. King, 1826 to 1831.

+------------------+-------+---------------------+---------------------+ Marked end a N. Pole. Marked end a S. Pole. ----------+----------+----------+----------+ Station. Needle. a a' a" a"' +------------------+-------+----------+----------+----------+----------+ ' ' ' ' Rio de Janeiro { 1 15 58,5 13 02,4 15 05,7 13 18,0 { 2 14 39,4 12 30,4 14 49,0 13 07,3 { 1 22 54,1 24 04,7 21 55,6 22 46,0 Sta. Catharina { 2 23 22,8 20 32,3 23 34,2 21 00,7 { 3 21 17,8 21 08,5 23 26,2 22 57,2 { 1 36 48,4 37 49,3 35 25,4 37 26,9 Monte Video { 2 37 17,2 34 53,8 37 17,1 34 37,4 { 3 36 36,1 35 10,3 37 12,4 36 13,2 { 1 64 41,7 52 42,6 65 51,2 54 54,0 Port Famine { 2 60 41,9 59 32,2 60 32,1 59 33,6 { 3 60 03,9 58 43,4 60 35,5 59 37,5 { 1 41 09,9 30 50,9 41 04,6 31 32,6 Gorriti { 2 36 29,2 33 26,4 35 17,1 34 02,7 { 3 34 52,3 34 18,5 36 06,9 35 01,3 Sea Bear Bay 1 58 26,5 47 53,8 60 34,8 47 53,4 { 1 65 24,7 54 23,8 65 47,7 54 43,6 St. Martin Cove { 2 60 25,6 59 28,3 59 49,9 58 48,6 { 3 59 48,9 58 40,7 60 43,4 59 53,3 { 1 54 23,9 45 38,3 56 19,2 45 11,1 Chiloe { 2 50 24,0 49 12,9 50 28,4 48 29,6 { 3 49 43,9 48 48,3 51 24,5 50 02,2 { 1 43 04,5 33 59,9 44 38,8 34 54,6 Valparaiso { 2 40 48,2 40 49,7 40 53,7 41 09,7 { 3 40 54,9 40 45,4 40 49,8 41 16,7 { 1 50 22,7 39 16,0 51 08,2 40 03,2 Juan Fernandez { 2 44 19,3 45 57,6 43 45,8 46 13,7 { 3 45 50,4 45 07,9 44 34,8 43 25,7 Talcahuano -- -- -- -- -- +------------------+-------+----------+----------+----------+----------+

+------------------+---------------------+-------------------------+ Station. Dip deduced. Remarks. +------------------+---------------------+-------------------------+ ' ' Rio de Janeiro { 14 16,2 } 14 00,1 S. { 13 44,1 } { 22 49,6 } Sta. Catharina { 22 03,6 } 22 12,4 S. { 21 44,0 } { 37 00,7 } Monte Video { 35 59,3 } 36 28,4 S. { 36 25,3 } { 59 44,9 } Port Famine { 60 04,5 } 59 52,6 S. { 59 48,5 } { 35 38,3 } Gorriti { 34 43,8 } 35 05,9 S. { 34 55,6 } Sea Bear Bay 53 13,5 53 13,5 S. { 59 46,4 } St. Martin Cove { 59 38,4 } 59 43,8 S. { 59 46,6 } { 49 59,4 } Chiloe { 49 38,7 } 49 52,6 S. { 49 59,7 } { 38 40,1 } Valparaiso { 40 55,4 } 40 10,7 S. { 40 56,7 } { 44 40,5 } Juan Fernandez { 45 04,6 } 53 13,5 S. { 44 44,7 } Talcahuano -- 45 10,0 S. The particulars of this observation are wanting. +------------------+---------------------+-------------------------+

{517}

The following TABLE contains Captain KING'S Observations of Intensity, with the times of vibration corrected for the arc and reduced to a standard temperature of 60.

MAGNETIC INTENSITY, _observed by_ Capt. P. P. King _between 1826 and 1831._

----------------+------------+------+------+-----------+----------------- Corrected Station. Date. Time. Therm. Time. Particular place.

----------------+------------+------+------+-----------+----------------- s. Greenwich { 1826 Mar. 22 734,45 55,0 733,97 } Observatory.

{ 1831 Jan. 24 775,80 41,6 776,01 } Madeira 1826 May 31 628,68 66,0 627,79 Consul's Garden.

Teneriffe -- June 12 607,96 75,0 606,73 Fort St. Pedro.

Port Praya { -- -- 22 559,53 81,5 } 557,08 { Landing Place, { -- -- 24 557,58 85,0 } { Quail Island.

{ -- Aug. 29 546,36 77,0 545,16 } { 1827 Sept.12 552,06 75,5 } Rio de Janeiro { -- -- 15 552,95 80,0 } } Rat Island.

{ -- -- 15 553,73 83,0 } 551,70 } { -- -- 15 553,48 85,0 } } { 1828 Dec. 21 562,55 84,0 561,05 } Sta Catharina 1827 Nov. 3 554,42 67,5 553,58 Anhatomirim.

{ 1826 Oct. 29 550,68 63,0 } 549,44 } Gorriti { -- Nov. 6 549,63 66,0 } } The Well.

{ 1829 Jan. 10 564,12 80,0 562,78 } { 1827 Dec. 18 555,12 78,3 553,87 } South point near Monte Video { 1828 Oct. 8 562,00 72,5 560,95 } the sea.

{ 1830 June 1 565,41 59,0 564,89 } Port Sta Elena 1826 Dec. 4 557,26 68,0 556,42 Landing-place.

Sea Bear Bay 1829 Mar. 20 577,36 70,0 576,37 Landing-place.

{ 1827 Jan. 15 584,88 56,0 } St. Martin Cove{ -- -- 15 583,48 58,0 } 584,29 } Head of the { -- -- 22 585,82 55,0 } } Cove.

{ -- Mar. 27 585,82 64,0 585,08 } { 1828 Jan. 28 589,75 55,0 589,36 } { -- May 8 596,49 43,8 596,54 } Port Famine { -- June 18 595,86 32,7 } 595,81 } Observatory.

{ -- July 30 594,85 36,2 } } { 1830 Apr. 30 598,95 45,0 598,97 } Chiloe { 1829 Sept. 1 565,40 54,0 } 565,23 San Carlos.

{ -- Dec. 15 565,69 53,0 } Juan Fernandez 1830 Feb. 19 552,77 70,0 551,83 Landing-place.

Talcahuano { 1829 Dec. 18 556,42 67,0 555,59 } Fort Galvez.

{ 1830 May 12 557,18 60,5 557,18 } { 1829 Aug. 4 549,10 59,0 548,59 } Valparaiso { 1830 Jan. 11 551,77 77,0 } 551,60 } Almendral.

{ -- Feb. 1 553,50 68,5 } } ----------------+------------+------+------+-----------+-----------------

{518}

At Rio de Janeiro, which was the first station observed at in South America, the cylinder was vibrated in August 1826, September 1827, and December 1828; in the intervals between these dates are comprised the greater part of the observations on the east side of South America. There is no direct observation at Rio subsequently to December 1828; but we are enabled to supply the time of vibration, which would have been observed had the cylinder been employed at Rio on June 1, 1830, in the following manner.

We have seen that on the 15th September 1827 the time of vibration was observed at Rio; on the 18th December following it was observed at Monte Video. These observations give the intensity at Monte Video relatively to that at Rio, subject to whatever change of magnetism the cylinder may have undergone in the interval of three months. This comparison was repeated in the following year, on Captain King's return from Monte Video to Rio, the interval being nearly of the same duration, and the order of the experiment reversed, the pa.s.sage being in this instance from Monte Video to Rio, it having been before from Rio to Monte Video. On the supposition of an uniform, or nearly uniform rate of change in the cylinder, the errors arising therefrom during the two pa.s.sages would be of opposite kinds, and should compensate each other in a mean of the two comparisons. Calling the force at Rio unity, these comparisons give its value at Monte Video respectively as follows, namely,

September and December 1827 ... 1.197 } October and December 1828 ... 1.207 } Mean 1.202.

On the 1st of June 1830, being then on his return from the west coast of South America, and on the eve of sailing for England, Captain King again observed the time of vibration of the cylinder at Monte Video; whence, through the preceding comparison, we obtain the time of vibration at Rio, which should belong to the same date. We have thus a fourth date at Rio, which, added to those enumerated above, will include the whole of the South American stations; and we have only to distribute in each interval the loss of magnetism which the observations shew to have taken place from one date to the next, in the manner which may appear most suitable. There is no very obvious indication that the loss was other than gradual; and by considering it uniform in each separate {519} interval, the results are found extremely accordant at several other stations at which observations were repeated at distant intervals.

In the first of the subjoined tables are given the times of vibration at Rio at the four periods referred to; and the corresponding times as a dipping needle. In its three last columns are shewn,--the number of days comprised in each interval,--the increase in the time of vibration owing to the loss of magnetism,--and the resulting daily correction, on the supposition in each case of the loss having been uniform in the interval during which it occurred.

The second table contains the corrected times of horizontal vibration at each of the South American stations, at the dates respectively inserted,--the corresponding times as a dipping needle,--the times of vibration as a dipping needle at Rio de Janeiro at the same dates, derived from the observations in the first table,--and the resulting intensity at each station relatively to unity at Rio. Thus far the results are derived from Captain King's observations, unmixed with those of any other observer: but in order to bring Captain King's series into connexion with the general body of results of other observers, the values of his intensities are expressed in the final column in terms of the scale in common use, in which the force at Paris = 1,348, and at Rio de Janeiro 0,884; the latter being the mean of four independent determinations by the following observers, namely,

1817 and 1820 Freycinet 0,890 } 1827 Lutke 0,886 } 0,884 1830 Erman 0,879 } 1836 Fitz-Roy 0,878 }

Port St. Elena is not included in this table, as no dip was observed there, and the total intensity consequently cannot be computed. The three stations, Madeira, Teneriffe, and Port Praya, at which the cylinder was vibrated in the outward voyage, are also without dips observed by Captain King. The deficiency at Port Praya has been supplied from Captain Fitz-Roy's observations and my own, both having been made at the same place at which Captain King's intensity was observed,--Captain Fitz-Roy's at a later, and mine at an earlier date. At Madeira also the dip has been supplied from my observations, which were made in the British consul's garden at Funchal, where Captain King's cylinder was vibrated. I have {520} deducted, from my determination of the dip, 12' for the probable change between 1822 and 1826. At Teneriffe the dip has been frequently observed; but the values a.s.signed by different observers vary so much as to indicate a more than usual frequency of local disturbance, which might also be expected from the geological character of that island. It would be unsafe therefore to employ any dip for that station but one which was certainly obtained at the same spot at which the horizontal intensity was observed.

The dates of the observations at these three stations fall between the observations at Greenwich in March 1826, and those at Rio de Janeiro in August of the same year. Having the intensity at Greenwich = 1,372 and at Rio = 0,884, and the dip at Greenwich 69 52', and at Rio 14 00, we have the time of vibration of Captain King's cylinder as a dipping needle at Rio at the respective dates as follows, namely,

March 1826 536,2.

August 1826 537,0.

It appears therefore that but a very slight change took place in the magnetism of the cylinder during the outward voyage, and we may take 536,6 as the time of vibration at Rio, corresponding to the dates of the intermediate observations; and this is done in the table for Madeira and Port Praya. {521}

TABLE I.

------------------+------------------------+---------+-----+--------- Time of Vibration. -----------+------------+ Rio de Janeiro. Horizontal. As a Dipping Interval. Loss. Per Diem.

Needle. ------------------+-----------+------------+---------+-----+--------- s. s. Days. s. August 29, 1826 545,2 537,0 } 382 6,5 .017 September 15, 1827 551,7 543,5 } December 21, 1828 561,1 552,7 462 9,2 .020 June 1, 1830 563,8 555,4 527 2,7 .005 ------------------+-----------+------------+---------+-----+---------

TABLE II.

---------------+-------------+------------------------+------------------ Time of Vibration. Intensity.

------------------------+-----------+------ At the Station. At Rio. --------+-------+-------+ As a As a Dipping Dipping Rio= Station. Date. Horizon. Needle. Needle. Rio=1,000. 0,884.

---------------+-------------+--------+-------+-------+-----------+------ s. s. s. Madeira 1826 May 31 627,79 430,1 536,6 1,556 1,377 Port Praya { 1826 June 22 }557,08 465,4 536,6 1,330 1,177 { and 24 } Sta Catharina 1827 Nov. 3 553,58 532,7 544,5 1,045 0,920 Gorriti { 1826 Oct. 29 }549,44 497,0 538,1 1,172} -- { and Nov. 6 } }1,175 1,041 -- 1829 Jan. 10 562,78 509,0 552,8 1,179} Monte Video 1827 Dec. 18 553,87 496,7 545,4 1,206} -- 1828 Oct. 8 560,95 503,0 551,3 1,201}1,203 1,065 -- 1830 June 1 564,89 506,6 555,4 1,202} Sea Bear Bay 1829 Mar. 20 576,37 446,0 553,1 1,538 1,361 St. Martin { 1827 Jan. 15 }584,29 414,9 539,4 1,691} Cove { and 22 } }1,692 1,498 -- -- Mar. 27 585,08 415,4 540,6 1,694} Port Famine 1828 Jan. 28 589,36 417,5 546,2 1,712 -- -- May 8 596,54 422,6 548,2 1,683} -- { -- June 18 }595,81 422,1 549,3 1,694}1,700 1,505 { and July 30 } } -- 1830 April 26 598,97 424,3 555,1 1,712} Chiloe { 1829 Sept. 1 }565,23 453,7 554,2 1,402 1,321 { and Dec. 15 } Juan Fernandez 1830 Feb. 19 551,83 464,7 554,8 1,425 1,262 Talcahuano 1829 Dec. 18 555,59 466,5 554,6 1,413}1,412 1,250 -- 1830 May 12 557,18 467,4 555,3 1,412} Valparaiso 1829 Aug. 4 548,59 479,5 553,9 1,334}1,329 1,176 -- { 1830 Jan. 11 }551,60 482,1 554,6 1,324} { and Feb. 1 } ---------------+-------------+--------+-------+-------+-----------+------

{522}

4.--The following Table exhibits the MAGNETIC OBSERVATIONS of Captains King and Fitz-Roy collected in one view.

---------------+---------+----------+----------+--------------+---------- Intensity, Paris= Stations. Lat.i.tude. Longitude. Variation. Dip. 1,348.

---------------+---------+----------+----------+--------------+---------- ' ' ' ' Madeira 32 38 N. 16 56 W. -- K. 1,377 Terceira 38 39 N. 27 13 W. 24 18 W. F. 68 06 N. F. 1,457 { -- K. 1,177 Port Praya 14 54 N. 23 30 W. 16 30 W. {F. 46 20,2 N. F. 1,154 {F. 45 46,5 N. F. 1,157 Ascension 7 56 S. 14 24 W. 13 30 W. F. 1 39,2 N. F. 0,873 St. Helena 15 55 S. 5 43 W. 18 00 W. F. 18 01,2 S. {F. 0,822 {F. 0,850 Falkland 51 32 S. 58 07 W. 19 00 E. {F. 53 30,4 S. F. 1,349 Islands {F. 53 19,6 S. F. 1,385 Pernambuco 8 04 S. 34 51 W. 5 54 W. F. 13 12,9 N. F. 0,914 Bahia 12 59 S. 38 30 W. 4 18 W. {F. 4 55,8 N. }F. 0,871 {F. 5 53,5 N. } Rio de Janeiro 22 55 S. 43 09 W. 2 00 E. {K. 14 00,1 S. }F. 0,878 {F. 13 37,4 S. } Sta Catharina 27 26 S. 48 33 W. -- K. 22 12,4 S. K. 0,920 Gorriti 34 57 S. 54 57 W. -- K. 35 05,9 S. K. 1,041 Monte Video 34 53 S. 56 13 W. 12 00 E. {K. 36 28,4 S. K. 1,065 {F. 34 51,2 S. F. 1,055 Blanco Bay 38 57 S. 61 59 W. 15 00 E. F. 41 54,0 S. F. 1,113 Port Desire 47 45 S. 65 55 W. 20 12 E. F. 52 43,5 S. F. 1,359 Sea Bear Bay 47 51 S. 65 48 W. -- K. 53 13,5 S. K. 1,361 R. Santa Cruz 50 07 S. 68 24 W. 20 54 E. F. 55 15,7 S. F. 1,425 St. Martin Cove 55 51 S. 67 34 W. -- K. 59 43,8 S. K. 1,498 Port Famine 53 38 S. 70 58 W. 23 00 E. K. 59 52,6 S. {K. 1,505 {F. 1,560 Port San Andres 46 35 S. 75 35 W. 20 48 E. F. 54 13,6 S. -- Port Low 43 48 S. 74 02 W. 19 48 E. F. 51 20,1 S. F. 1,326 Chiloe 41 51 S. 73 56 W. 18 00 E. {K. 49 52,6 S. K. 1,321 {F. 48 58,9 S. F. 1,304 Valdivia 39 53 S. 73 29 W. 17 30 E. F. 46 46,5 S. F. 1,238 Concepcion 36 42 S. 73 10 W. 16 48 E. {K. 45 10,0 S. K. 1,250 {F. 43 15,4 S. F. 1,186 Valparaiso 33 02 S. 71 41 W. 15 18 E. {K. 40 10,7 S. }K. 1,176 {F. 38 03,1 S. } Coquimbo 29 59 S. 71 26 W. 14 24 E. -- F. 1,111 Callao 12 04 S. 74 14 W. 10 00 E. F. 7 02,8 S. Galapagos 0 50 S. 89 37 W. 9 30 E. F. 8 41,2 N. Islands Ditto 0 15 S. 90 31 W. 9 30 E. F. 9 28,6 N. F. 1,069 Juan Fernandez 33 38 S. 78 53 W. K. 44 49,8 S. K. 1,262 Otaheite 17 29 S. 149 30 W. 7 54 E. F. 30 13,5 S. F. 1,017 Cape of Good 34 11 S. 18 26 E. 28 30 W. F. 52 35,0 S. F. 1,014 Hope Mauritius 20 09 S. 57 31 E. 11 18 W. F. 54 00,8 S. F. 1,192 Keeling Islands 12 05 S. 96 55 E. 1 12 W. F. 38 33,1 S. New Zealand 35 16 S. 174 00 E. 14 00 E. F. 59 32,0 S. F. 1,591 King George 35 02 S. 117 56 E. 5 36 W. F. 64 41,3 S. F. 1,709 Sound Hobart Town 42 53 S. 147 24 E. 11 06 E. F. 70 34,9 S. F. 1,817 Sydney 33 51 S. 151 17 E. 10 24 E. F. 62 49,4 S. F. 1,685 ---------------+---------+----------+----------+--------------+----------

{523}

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. Lutke 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.