{502}
2. _Observations of Intensity._
The method employed by Captain Fitz-Roy to determine the variations of the magnetic force was that of noting the time of vibration of a magnetic cylinder suspended horizontally.
The cylinder was one which had been given by M. Hansteen, in 1826, to Captain Phillip Parker King, R.N., and had been used by him during the survey of the coast of South America, which he conducted from 1826 to 1830.
The apparatus in which it was vibrated, both in Captain King"s and Captain Fitz-Roy"s voyages, was the well-known one of M. Hansteen.
By observations made with this cylinder on the 22d March 1826, and again on the 24th January 1830, in the garden of the Royal Observatory at Greenwich, it appeared that its time of performing 300 vibrations had increased from 734,45 seconds in 1826, to 775,80 seconds in 1831; or 41,35 seconds in 1,770 days. A change of such magnitude in the magnetic intensity of the instrument employed to measure the variations of the terrestrial intensity, and which ought itself, therefore, to be invariable, would, in the generality of cases, have prevented any satisfactory conclusion whatsoever being drawn from the observations. Fortunately from the nature of the duties in which Captain King was engaged, he had occasion to return frequently to the same anchorages, and by his extreme care to repeat observations on every such return, he has provided a means of computing the decrease of the intensity of the cylinder, proportioned to intervals of time, between 1826 and 1831; and of thus introducing compensations for it, which render the results on the whole nearly as satisfactory as if the cylinder had preserved an uniform magnetic condition throughout.
The voyage which Captain Fitz-Roy had to perform promised to furnish few, if any, such opportunities of examining the state of the magnetism of the cylinder, between the departure from and the return to England; and,--as it cannot but be extremely discouraging to officers to make observations which they have reason to apprehend may prove unavailing from defect in the instrument employed,--it must be regarded as exceedingly creditable to Captain Fitz-Roy and his officers, that, with the knowledge of the {503} change which the cylinder had undergone in the preceding voyage, they persevered in diligently observing, and carefully recording, its time of vibration, at most of the princ.i.p.al ports which they visited in their voyage of five years" duration. Nor was it until their return to the Cape Verd Islands, in September 1836, that they could infer, from observations repeated at the same spot as in their outward pa.s.sage in 1832, that the cylinder had not varied in any thing like the degree that it had done in the preceding voyage, and that the care and pains they had bestowed were therefore likely to be recompensed by success.
This appears a fitting opportunity to remark, how much the establishment in England of a depository for magnetic needles is needed; whence officers, and persons desirous of making such observations, might be supplied with instruments, which had been kept a sufficient time to have attained their permanent magnetic state, and had been examined from time to time to prove that they had done so. The correction for temperature should be ascertained for each needle, and given with it; as well as the time of vibration (or whatever else const.i.tuted the measure of intensity,--as, for example, the angle of deflection in Mr. Lloyd"s statical needles,)--observed at the spot which should be selected as most suitable for a point of general comparison; and the observations should be repeated at the same spot on the return of the needle. The want of such an establishment has long been greatly felt; and opportunities, where nothing was wanting but proper instruments, have been lost in consequence, where determinations of great value might have been obtained, in parts of the world of the highest magnetic interest, and where such opportunities are of rare occurrence.
The corrections necessary to render the times of vibration at the different stations strictly comparable with each other, are as follows.
1st, For the rate of the chronometer.
2d, For the temperature of the needle.
3d, For the arc of vibration.
4th, For any change in the magnetic condition of the cylinder.
In extensive voyages, the last-named correction, or that for the change in the cylinder itself, is the one which requires princ.i.p.al consideration. The corrections for temperature, and for the arc, on the first of which particularly much stress has sometimes been {504} laid, are important when extreme accuracy is sought; as for example, in comparing the force at stations which have served as the unities of different observers: their effect is, however, of little moment in observations which include great differences of the terrestrial intensity. But when the magnetic condition of the needle has varied, and interpolation becomes necessary, the instances are rare in which it can be done with entire satisfaction.
The time of vibration of this cylinder at Plymouth, with corrections applied for the chronometer"s rate, the temperature, and the arc, was, in December 1831, 770,8 seconds, with a dip of 69 27",6, and in October 1836, 777,3 seconds with a dip of 69 17",5. The difference in its magnetic state, at the two periods, is shown by the squares of the times of vibration multiplied by the cosine of the dip observed at the respective periods. This makes known what the time of vibration of the same cylinder would have been, had it been free to move in the direction of the dipping-needle, instead of being suspended horizontally; and, consequently, if it had been acted upon by the total magnetic intensity, instead of by the horizontal component only. It is here a.s.sumed that the total terrestrial intensity is constant at the same place. This is doubtless not strictly true; but the amount of the change must be too small to require consideration in the period occupied by Captain Fitz-Roy"s observations.
The horizontal component must, however, necessarily vary with the changes in the dip: and it is, therefore, from the time of vibration in the direction of the dipping-needle, and not from the time of horizontal vibration, that the change, or otherwise, in the magnetism of the needle is to be inferred. We find, then, the equivalent time of vibration of this cylinder in the direction of the dipping-needle to have been 456,4 seconds in December 1831, and 462,2 seconds in October 1836; showing a difference of 5,8 seconds in fifty-eight months. If we compare this amount with the far greater loss of intensity sustained by this cylinder in the preceding voyage, it seems a probable supposition that, at the commencement of Captain Fitz-Roy"s voyage, the cylinder had nearly attained its permanent magnetic state; and that its further loss of magnetism, occasioning an increase of 5,8 seconds in the time of vibration, took place in the early part of the voyage: supposing the loss to have been progressive, and not sudden, as from accidents, of which the observations give no indications.
{505} Consistently with this supposition, the loss has been distributed through the first half, or twenty-nine months, of this voyage, in the proportion of three-tenths of a second per month in the first ten months, commencing December 1st 1831; two-tenths per month in the next nine months; and one-tenth per month in the remaining nine months. In the last twenty-nine months of the voyage, the intensity of the cylinder is supposed to have been uniform, and the same which it was found to possess on the return to England in 1836.
It is satisfactory that, with this compensation, the observations at Port Praya, in January 1832, and in September 1836, a.s.sign almost identically the same relative magnetic intensity to that station.
The correction for temperature for this cylinder not having been previously examined, I received it from Captain Fitz-Roy for that purpose, and made with it the following observations. The cylinder, in its own apparatus, was placed in a large earthen jar, glazed at the top, and standing in a larger earthen vessel, into which warm water could be poured at pleasure, and the cylinder was then vibrated alternately in heated air and in air of the natural temperature. These experiments were made at Tortington, in Suss.e.x.
{506}
+---------+-------+----------+----------+------------+Thermom.TimeDay.Hour.(Reaumur.)of Vibration+---------+-------+----------+----------+------------+h. m.s{June 1211 50 A.M.12,00774,91{--0 12 P.M.12,00775,37{--0 34 P.M.12,00775,71{--1 58 P.M.31,65777,31Exp. 1 {--2 21 P.M.32,20777,09{--2 42 P.M.31,30776,86{--4 00 P.M.15,45774,00{--4 22 P.M.15,80774,25{--4 44 P.M.15,70774,23{June 1310 49 A.M.14,90773,77{--11 16 A.M.15,10773,37Exp. 2 {--0 20 P.M.34,80775,94{--0 42 P.M.36,55775,89{--1 30 P.M.17,70774,06{--1 52 P.M.15,90774,11+---------+-------+----------+----------+------------+
+---------+------------------------------------------------------ -++---------+------------------------------------------------------ -+{} h. m. s{} Mean, 0 12 P.M. 12,0 775,33 Natural Temperature.{}{}Exp. 1 {} Mean, 2 20 P.M. 31,72 777,09 Heated Air.{}{}{} Mean, 4 22 P.M. 15,65 774,16 Natural Temperature.{}{} Mean, 11 02 A.M. 15,0 773,57 Natural Temperature.{}Exp. 2 {} Mean, 0 31 P.M. 35,7 775,92 Heated Air.{}{} Mean, 1 41 P.M. 16,8 774,08 Natural Temperature.{}+---------+--------------------------------------------------------+
In the first Experiment, T=777,09; T" 774.74; t=103,4 Faht.; and t"=63,1 Faht.
In the second Experiment, T=775,92; T" 773,83; t=112,3 Faht.; and t"=67,8 Faht.
The first Experiment gives (T-T")/T(t-t") = 2.35/(777,0940,3)=.000075.
The second Experiment gives (T-T")/T(t-t") = 2,09/(775,9244,5)=.000061.
{507}
Whence the formula T"=T [1 0.000068 (60-t)], in which T is the time of vibration at any station, t, the temperature of the cylinder in degrees of Fahrenheit, and T" is the equivalent time at a standard temperature of 60.
The thermometer was noted at the beginning and ending of every set of vibrations, and was always placed in the box with the cylinder.
At all Captain Fitz-Roy"s stations the apparatus was placed for observation on a stand, which raised it from two to three feet above the ground, thereby rendering the cylinder somewhat less liable to be disturbed by local influences: it was not furnished with a means of examining the strict horizontality of the cylinder, that improvement having been introduced into M. Hansteen"s apparatus at a later period. On this point Captain Fitz-Roy remarks: "A small leaden tripod was used as a stand, whose upper surface was adjusted by a small spirit-level--or roughly by the trough of an artificial horizon, filled with mercury. Upon the leaden stand the box containing the needle was adjusted by its foot-screws, so that the suspending fibre of silk hung centrally in the wooden tube, the needle"s centre being over that of the graduated circle, and the needle itself near, but not touching, the bottom of the box. The needle was not always strictly parallel to the bottom of the box, nor strictly horizontal, because I would not move the bra.s.s stirrup in which it was suspended, but its deviation from strict horizontality _never_ exceeded two degrees, and was seldom nearly so much."
The time of completing every tenth vibration was recorded. The time of performing 300 vibrations is deduced from a mean generally of seven partial results: _i.e._ from the 0th to the 300th; 10th to the 310th; and so on, to the 60th and 360th vibrations; the commencing vibration being always at an arc of 20. In a very few instances the number of vibrations observed, after the commencing arc of 20, was less than 360; in such cases the first vibration after the arc had become 20 has still been taken as the commencing one, though previous ones may have been recorded; it being kept strictly in view, to obtain the relative time of vibration in arcs as nearly the same as possible, and not exceeding 20 as the initial. The arc was reduced to 10 generally about the 100th vibration; and one thousandth of the time of vibration has been taken throughout the series as the correction to infinitely small arcs. {508}
The object of noting the time of every tenth vibration is to check errors in the counting, which will sometimes occur in the course of the 360, particularly with the very short and quick-moving needles of M. Hansteen"s very portable apparatus, and at stations of low dip, where the horizontal force is greatest, and the needle consequently moves most quickly. Several such mistakes evidently occurred. When the time of completing every supposed tenth vibration is observed with tolerable exactness, and the duration of each pair of vibrations decidedly exceeds any irregularity of probable occurrence, apart from miscounting the number of vibrations, such mistakes can be discovered with ease, and rectified with certainty. This has been done in every case where no doubt could exist of a mistake of the kind having occurred; such as when all the intervals are of nearly equal duration, with one or two exceptions, which differ as much as three or four seconds from the general body. There are two stations, however, Callao and Keeling Islands, where the rectification is not so clear, or the true result so obvious. At Callao there are three series of horizontal intensities, each of forty observed intervals, which should be of ten vibrations each. Several of these intervals are between 17,5 and 18,5 seconds, and several others between 20,5 and 21,5 seconds. These can hardly represent an equal number of vibrations, because the difference between them is greater than can easily be supposed due to any uncertainty in seizing the particular beat of the chronometer at which the vibration was completed; it is, moreover, about the time that would be occupied by two vibrations more or less. The question then arises, do the longer intervals represent 12, and the shorter 10 vibrations, or do the longer represent 10, and the shorter 8? In the former supposition the intensity at Callao would be about 1.01 (Paris = 1.348): in the latter about 0.75. The difference shews how great an error would be risked by either a.s.sumption. If we take a mean of all the intervals as they stand, the amount of error risked would be certainly lessened; but we should a.s.suredly not have the true time of three hundred vibrations, except on one supposition: namely, that the irregularities in question are not errors in estimating the number of vibrations, but that each interval really represented an equal number, and that some unusual and accidental cause occasioned the needle to differ so greatly in successive intervals. But this {509} supposition would imply a disturbing cause vitiating the series as a measure of the magnetic intensity at the station. I have not ventured, therefore, to draw any conclusion from these observations, farther than to notice, as above, the limits within which, in either of the two first suppositions, the intensity would fall.
A nearly similar reasoning applies to the observations at Keeling Islands; of three series, one is decidedly so irregular, that no inference could be drawn from it; in the two other series the irregularities are neither so frequent, nor so large: my general impression (in the uncertainty created by the irregularity of the first series), is, that the majority of the intervals are of twelve vibrations, and not of ten: if of twelve, the intensity would be about 1,21; if of ten, about 0,85.
The inconvenience of the rapid motion of the needle, occasioned, at one part of the voyage, the practice to be discontinued of observing every tenth vibration, and every twentieth was subst.i.tuted. This no doubt relieved the perplexity in which the observer occasionally found himself, in having to observe, and record, and be prepared again to observe, at every twenty seconds or less, and so far the change enabled him to observe better. But still, the disadvantage remains, in so quick moving a needle, that if a mistake of two vibrations is made, the difference of time occasioned is not of so marked and decided a character as to be at all times at once distinguished. It is of much more importance that there should be no miscount of the vibrations, than that the times should be recorded correctly to the fraction of a second. It is only the earlier and later times that are finally influential; but every undetected error in the number of vibrations falls with its whole weight upon the result.
The occasional discrepancies in the results of the same, or of different, observers, or on the same, or on different, days, which are seen in the subjoined table, are not, I believe, traceable to the source I have been discussing, nor apparently to any other than an actual difference in the time of the cylinder performing its vibration. A mean has been taken as the result at each station, except at St. Helena, where the discrepancy on the 11th and 13th of July was so considerable, that it has been thought more satisfactory to collect the observations of each day into separate results.
The subjoined table comprises the result of each observation, {510} and the general results deduced for each station. The column ent.i.tled "Time" is that of 300 vibrations; and the "Corrected Time" is the mean of these, corrected for the rate of the chronometer and the arc, and reduced to an average temperature of 60. The dips are those observed by Captain Fitz-Roy; except at Port Famine, where, as Captain Fitz-Roy did not observe, it has been supplied from Captain King"s observations; and at Coquimbo, where, for the purpose of computing the intensity, it has been supplied by estimation from the other geographic positions on this coast, at which Captain Fitz-Roy observed the dip. In the column showing the time of vibration as a dipping-needle at Plymouth corresponding to the periods of observation at the several stations, the compensations have been introduced for the variation in the intensity of the cylinder, agreeably to what has been said above on that subject. The two final columns exhibit the values of the total magnetic intensity at the different stations derived from these observations. In the first of the two columns, the values are given relatively to the force at Plymouth, considered as unity; and in the second column, relatively to the force at Plymouth, expressed by 1.375; for the purpose of exhibiting Captain Fitz-Roy"s results in direct comparison with the determinations of continental observers, who have taken Paris as their basis, giving the force at Paris the arbitrary expression of 1.3482.
I have taken the ratio of the force at Plymouth to that at Paris to be as 1.375 to 1.348, which I believe will prove a very near approximation; it is that which results from Captain Fitz-Roy"s observations at Plymouth, in October 1836 (page 17), and mine, at Tortington, in Suss.e.x, in June 1837 (page 10): the dip at Tortington, at the period in question being 68 57", and the intensity, compared with Paris, through the medium of London, 1.368. {511}
+--------------------+-------+------+--------+------+---------+---------+Chron."sCorrectedStation.Date.Time.Rate.Therm.Observer.Time+--------------------+-------+------+--------+------+---------+---------+1831s.s.s.PlymouthDec. 1770,79-3,050Kg.770,61832{Jan. 27600,17-6,483F. }600,1Port Praya {-- 27604,58-6,476K. }{-- 28598,33-6,475F. }{-- 31602,19-6,485K. }{Feb. 1602,50-6,485F. }Rio de JaneiroMay 1582,78-4,382S.581,3Blanco Bay {Sept.21592,330,068F. }591,9{-- 21593,340,068F. }1833{Mar. 12603,22+1,550F. }Falkland Islands, {-- 14604,56+1,563F. }603,0Magellan Cove {-- 15603,12+1,655F. }{-- 15603,40+1,664F. }Monte Video {Nov. 25583,93+7,190F. }581,9{-- 25583,61+7,190F. }Port Desire {Dec. 29597,68+3,962F. }597,1{-- 29597,98+3,962F. }1834Mar. 26603,10+5,060K. }Falkland Islands {-- 26598,72+5,060K. }595,9Port Louis {-- 26596,36+5,060K. }{-- 26590,29+5,060K. }{-- 26594,20+5,061K. }+--------------------+-------+------+--------+------+---------+---------+
+---------------------+--------+--------------------+------------+Time of Vibrationas a Dipping-Needle.Intensity---------+----------+-----+------+ObservedAt eachAtStation.DipStation.Plymouth.1.2.+---------------------+--------+---------+----------+-----+------+"s.s.Plymouth69 27,6456,4456,41,0001,375{46 20,2498,8457,00,8391,154Port Praya {{{{Rio de Janeiro13 37,8573,1457,90,6380,878Blanco Bay {41 54510,7459,40,8101,113{{Falkland Islands, {53 30,4465,0460,60,9811,349Magellan Cove {{Monte Video {34 51,2527,1461,80,7671,055{Port Desire {52 43,5464,7461,90,9881,359{Falkland Islands {53 19,6460,5462,11,0071,385Port Louis {{{+---------------------+--------+---------+----------+-----+------+
{512}
+--------------------+-------+------+--------+------+---------+---------+Chron."sCorrectedStation.Date.Time.Rate.Therm.Observer.Time+--------------------+-------+------+--------+------+---------+---------+1831s.s.s.{May 10601,67+8,053K. }R. Santa Cruz {-- 10601,72+8,054St. }601,1{-- 10601,03+8,050St. }Port Famine {June 4613,25+6,070S. }612,6{-- 4614,03+6,070S. }Chiloe {Dec. 10586,31+9,262K. }586,0{-- 10587,14+9,262K. }1835Port Low {Jan. 15587,14+8,856K. }588,7{-- 15591,16+8,856K. }Valdivia {Feb. 19587,67+8,060K. }588,7{-- 19591,01+8,061K. }{Apr. 13584,63+10,565K. }Concepcion {-- 13585,30+8,665K. }583,1{-- 14584,94+8,664K. }{-- 14581,03+8,664K. }Coquimbo {June 4566,81+7,664K. }565,8{-- 4566,37+7,664K. }Galapagos Islands,{Oct. 16529,66-0,488St. }527,9Charles Island {-- 16529,21-0,488St. }{Nov. 16580,33+6,091K. }Otaheite, {-- 16580,66+6,093K. }578,4or Tahiti} {-- 23578,44+6,085K. }{-- 23581,00+6,085K. }+--------------------+-------+------+--------+------+---------+---------+
+---------------------+--------+--------------------+------------+Time of Vibrationas a Dipping-Needle.Intensity---------+----------+-----+------+ObservedAt eachAtStation.DipStation.Plymouth.1.2.+---------------------+--------+---------+----------+-----+------+"s.s.{R. Santa Cruz {55 15,7454,0462,21,0371,425{Port Famine {59 52,6434,0462,21,1361,560{Chiloe {48 58,9474,7462,20,9481,304{Port Low {51 20,1465,3462,20,9641,260{Valdivia {46 46,5487,2462,20,9001,238{{Concepcion {43 15,4497,7462,20,8631,186{{Coquimbo {34 20514,2462,20,8081,111{Galapagos Islands, {9 28,6524,2462,20,7771,069Charles Island {{Otaheite, {30 13,5537,6462,20,7391,017or Tahiti} {{+---------------------+--------+---------+----------+-----+------+
{513}
+--------------------+-------+------+--------+------+---------+---------+Chron."sCorrectedStation.Date.Time.Rate.Therm.Observer.Time+--------------------+-------+------+--------+------+---------+---------+1835s.s.s.New Zealand {Dec. 22604,76+7,880K. }603,6{-- 22605,37+7,880K. }1836{Jan. 14620,63-5,478S. }Sydney {-- 14619,83-5,482S. }617,9{-- 22618,14-5,886K. }{-- 22619,21-5,887K. }Hobart Town {Feb. 6698,63+2,365S. }697,4{-- 6698,13+2,365S. }King George Sound {Mar. 7634,17-5,672S. }634,1{-- 10636,71+2,480S. }{May 3647,95-9,287K. }Mauritius {-- 3650,79-9,294K. }647,7{-- 3650,37-9,294K. }{June 1691,13-7,268S. }Cape of Good Hope {-- 1691,42-7,269S. }690,6{-- 10691,62+0,461F. }{-- 10692,00+0,463F. }{July 11612,90-8,078S. }St. Helena {-- 11615,30-8,079St. }613,2{-- 11615,40-8,078S. }{-- 13604,69-8,076S. }St. Helena {-- 13605,23-8,077S. }602,9{-- 13602,34-8,072S. }{July 21581,58-9,475S. }Ascension {-- 21581,31-9,475S. }580,2{-- 21581,09-9,475S. }{Aug. 3583,78-9,475S. }Bahia {-- 3583,26-9,475S. }582,3{-- 4583,13-9,479S. }{-- 4583,89-9,479S. }+--------------------+-------+------+--------+------+---------+---------+
+---------------------+--------+--------------------+------------+Time of Vibrationas a Dipping-Needle.Intensity---------+----------+-----+------+ObservedAt eachAtStation.DipStation.Plymouth.1.2.+---------------------+--------+---------+----------+-----+------+"s.s.New Zealand {59 32,0429,8462,21,1571,591{{Sydney {62 49,4417,6462,21,2251,685{{Hobart Town {70 34,9402,1462,21,3211,817{King George Sound {64 41,3414,6462,21,2431,709{{Mauritius {54 00,8496,5462,20,8671,192{{Cape of Good Hope {52 35,0538,4462,20,7371,014{{{St. Helena {18 01,2598,0462,20,5980,822{{St. Helena {18 01,2587,9462,20,6180,850{{Ascension {1 39,2580,1462,20,6350,873{{Bahia {5 53,5580,7462,20,6330,871{{+---------------------+--------+---------+----------+-----+------+
{514}
+--------------------+-------+------+--------+------+---------+---------+Chron."sCorrectedStation.Date.Time.Rate.Therm.Observer.Time+--------------------+-------+------+--------+------+---------+---------+s.s.s.{Aug. 13576,12-9.280S. }Pernambuco {-- 13576,00-9.281S. }574.5{-- 17575,96-9.285S. }{-- 17575,71-9.287S. }{Sept. 1603,13-9.891S. }Port Praya {-- 1602,84-9.892S. }603,4{-- 1607,63-9.891S. }{-- 1607,49-9.889S. }Terceira {Sept.21736,73-9.277S. }735,3{-- 21737,01-9.277S. }{Oct. 15778,10+5.063Ss. }Plymouth {-- 15778,26+5.064Ss. }777,3{-- 15778,65+5.064Ss. }+--------------------+-------+------+--------+------+---------+---------+
+---------------------+--------+--------------------+------------+Time of Vibrationas a Dipping-Needle.Intensity---------+----------+-----+------+ObservedAt eachAtStation.DipStation.Plymouth.1.2.+---------------------+--------+---------+----------+-----+------+"s.s.{Pernambuco {13 12,9566,8462,20,6650,914{{{Port Praya {45 46,5503,9462,20,8411,157{{Terceira {68 06449,1462,21,0591,457{{Plymouth {69 17,5462,2462,21,0001,375{+---------------------+--------+---------+----------+-----+------+
Observers:--F. Captain Fitz-Roy. Ss. Mr. J.L. Stokes. Kg. Capt. King.
S. Lieut. Sulivan. St. Mr. P.B. Stewart. K. Mr. P.G. King.
{515}
3. _Captain King"s Observations of Dip and Intensity._
Captain King, having hitherto made known his observations with the same cylinder in the years 1826 to 1830 only by communicating them to M.