Contributions to Terrestrial Magnetism.--No. III

| Latitude | Magnetic Force | |----------|----------------| | Fernando Noronha | 330° | 335° | 340° | 345° | 350° | | 6° | • 918 | | | | | | | • 889 | | | | | | | • 880 | | | | | | | • 862 | | | | | | 10° | • 848 | | | | | | | • 832 | | | | | | | • 83 | | | | | | 15° | • 810 | | | | | | | • 806 | | | | | | 20° | Trinidad, "Martin Vaz I" | • 813 | | | | | | • 810 | | | | | | | • 811 | | | | | | | • 815 | | | | | | | • 817 | | | | | | | • 819 | | | | | | | • 823 | | | | | | | • 826 | | | | | | | • 836 | | | | | | | • 843 | | | | | | 25° | • 819 | | | | | | | • 823 | | | | | | | • 826 | | | | | | | • 836 | | | | | | | • 843 | | | | | | 30° | • 819 | | | | | | | • 823 | | | | | | | • 826 | | | | | | | • 836 | | | | | | | • 843 | | | | | II. Contributions to Terrestrial Magnetism.—No. III. By Lieut.-Colonel Edward Sabine, R.A., F.R.S. Received December 16, 1841,—Read January 20, 1842. In the present number of these Contributions, I propose to give an account of the observations on the magnetic intensity made at sea by the officers of Her Majesty's ships Erebus and Terror, on their passage from England to Kerguelen Island, the unreduced observations, transmitted to the Admiralty by the Commanders, Captains Ross and Crozier, having been placed in my hands for that purpose. They will be divided for convenience into two sections, viz. § 5. Observations between England and the Cape of Good Hope. § 6. Observations between the Cape of Good Hope and Kerguelen Island. § 5. Observations between England and the Cape of Good Hope. The observations in the Erebus were made by the statical method devised by Mr. Fox, with one of his instruments of $7\frac{1}{2}$ inches diameter. The intensities were measured by the angles of deflection produced, in different localities, by a constant weight applied to a grooved wheel on the axle of the needle; and the ratio of the intensities is inversely as the sines of the angles of deflection, subject to a correction for differences of temperature of the needle, computed by the formula $0.0016 \cdot I' (t' - t)$, in which $t$ is the standard and $t'$ the observed temperature in degrees of Fahrenheit, $0.0016$ a coefficient determined experimentally by Mr. Fox, and $I'$ the observed intensity. At sea, where the manipulation of the weights causes an exposure of the needle, which, in bad weather particularly, is liable to occasion injury, the plan recommended by Mr. Fox, of using deflecting magnets instead of weights, was frequently resorted to. In this case the ratio of the intensity in different localities is inversely as the sines of the angles of deflection, and directly as the weights equivalent to the deflecting force of the deflector on the needle at the respective angles; or $$I' = I \cdot \frac{w'}{w} \cdot \frac{\sin v}{\sin v'},$$ where $I$, $v$, and $w$ are the intensity, angle of deflection, and equivalent weight at a base station; and $I'$, $v'$, and $w'$ corresponding values at another station. A table is usually formed for each instrument experimentally, under Mr. Fox's own direction, of the equivalent, or as they are termed by him, the coercing weights, for each deflector on each of the needles at the different angles which are likely to occur in the course of the observations. This is done by placing the deflector successively at angles from the dip*, each differing one degree from the preceding; the needle is thereby deflected to a smaller angle on the side of the dip opposite to the deflector, and is brought back to the dip by a weight applied to the grooved wheel on the axle; this weight is called the coercing weight corresponding to the angle from the dip at which the deflector was placed. For greater accuracy, the table is formed from results obtained by placing the deflector successively on either side of the needle. Owing to accidental circumstances, no table of this description was prepared for this instrument before the Expedition sailed; the pressure of other duties prevented its being done at St. Helena, the Cape of Good Hope, or at Kerguelen Island; and at Van Diemen Island the end of the axle of the needle being accidentally broken, the needle was returned to England to be repaired, and was thus separated from the instrument and from the deflectors. Under these circumstances we have no other resource for reducing the observations made with the deflectors, than to form a table from the observations of the weights and deflectors (when both methods have been employed at the same station), which shall answer the same purpose as a table of coercing weights. Fortunately the number of such stations is considerable. We may form this table in the following manner. For the primary or base station, let \( V \) be the angle of deflection with a constant weight \( W \), and \( v \) the angle of deflection produced by the deflector placed at the dip, then is \[ w = W \sin v \csc V, \] \( w \) being the weight equivalent to the deflecting force of the deflector at the angle \( v \). If several constant weights were used at the primary station, the value of \( w \) may be obtained from each separately, and an arithmetical mean taken. Then at another station, at which the angles of deflection have been observed both with the deflector and with constant weights, the equivalent weight \( w' \) to the angle \( v' \) produced by the deflector may be obtained from \[ w' = \frac{I' w \sin v'}{\sin v}, \] \( I \) being the intensity at the primary station, and \( I' \) the intensity derived by the method of constant weights at the other station. The values of \( w' \), thus computed for all the stations where the weights and deflectors were both used, being projected in a graphical representation with the corresponding values of \( v' \), the former as ordinates, the latter as abscissæ, a line drawn by the eye through the terminations of the ordinates will give the values of \( w' \) for each degree of \( v' \) produced by the deflector. In the intensity instrument of the Erebus two deflectors were used, sometimes separately and sometimes combined: they were designated N. and S, according to the pole of the needle to which they were respectively applied. They were contained in brass tubes, N. with its north pole, and S. with its south pole towards the end of the tube which screwed into the limb of the instrument; consequently "Deflector N." in * This analysis may be made when the needle is in other positions, but Mr. Fox now prefers the vertical one, or when the needle stands at 90°, the circle being perpendicular to the plane of the magnetic meridian. the Table signifies that the deflector having its north pole towards the screw was placed opposite that division of the circle which the north end of the needle had previously indicated as the dip; and the angle of deflection \( v' \) is a mean of the deflections of the needle, first on the one side and then on the other side of the deflector. In the case of this deflector we have the angle \( v \) observed in London \( 22^\circ 57' \); and the value of \( w \), derived from the angles with the four constant weights of 1, 2, 3, and 4 grains, = 2·114 grs. Regarding London as the primary station, and the intensity = 1, the values of \( w' \) at the several stations where both weights and deflectors were used are found by \[ w' = 5·422 I' \sin v'. \] The table of observations furnishes seventy-four occasions between England and the Cape of Good Hope, in which this deflector was used in comparison with the constant weights: we have consequently so many values of \( w' \) from which to form a table for each degree of deflection. The angles \( v' \) produced by this deflector increased from \( 22^\circ 57' \) in London to above \( 34^\circ \) where the intensity was weakest, and again decreased to \( 29^\circ 53' \) at the Cape; consequently the ordinates corresponding to the smaller angles are derived partly from the earlier and partly from the later observations of the series. The line drawn freely through the points forming the terminations of the ordinates shows by its continuity that the force of the deflector remained unchanged during the whole of the series; it exhibits no discordances with any of the values of \( w' \), but such as may well be attributed to the unavoidable discrepancies of single observations. By means of this graphical representation the subjoined Table has been formed of the values of \( w' \) for each degree of \( v' \), permitting the intensities \( I' \) to be computed, relative to the force unity in London, by the formula \[ I' = .1845 w' \csc v'. \] | Values of \( w' \), Deflector N. | |-------------------------------| | grs. | | 23 = 2·113 | | 24 = 2·085 | | 25 = 2·058 | | 26 = 2·031 | | 27 = 2·005 | | 28 = 1·979 | | 29 = 1·954 | | | | grs. | | 30 = 1·929 | | 31 = 1·904 | | 32 = 1·880 | | 33 = 1·857 | | 34 = 1·834 | | 35 = 1·810 | In the case of deflector S, the table of observations furnishes 109 occasions between London and the Cape of Good Hope in which the angle \( v' \) was observed in comparison with the angles produced by the constant weights; consequently we have 109 values of \( w' \) to be combined in a graphical representation. The line freely drawn through the terminations of the ordinates is continuous from August 1839 to the noon-observation of February 12, 1840, when the continuity becomes interrupted, and a second line, corresponding to a diminished force in the deflector, commences, and continues unbroken to the Cape of Good Hope. The loss of force in the de- flector, which occurred between the forenoon and afternoon observations of the 12th of February, was equivalent to nearly a degree in the angle \( v' \), and is obvious on a simple inspection of the table of observations. In this case, therefore, we require to form two tables of the values of \( w' \); the one, Table A, corresponding to the force of the deflector between August 1839 and February 12th, 1840, and the other, Table B, to the weaker force between February 12th and the Cape of Good Hope. | Values of \( w' \), Deflector S. | |---------------------------------| | (A.) August 1839 to February 12, 1840. | (B.) February 12 to March 25, 1840. | | grs. | grs. | grs. | | 30 = 2·754 | 37 = 2·410 | 35 = 2·291 | | 31 = 2·704 | 38 = 2·359 | 36 = 2·260 | | 32 = 2·655 | 39 = 2·310 | 37 = 2·235 | | 33 = 2·606 | 40 = 2·260 | 38 = 2·210 | | 34 = 2·556 | 41 = 2·210 | 39 = 2·186 | | 35 = 2·508 | 42 = 2·160 | 40 = 2·161 | | 36 = 2·459 | 43 = 2·110 | 41 = 2·135 | | | | 42 = 2·110 | For the first series we have London as the primary station, where \( I = 1, v = 30^\circ 19' \), and \( w = 2·737 \); whence \[ I' = '1845 w' \csc v'. \] the values of \( w' \) being taken from Table A. And for the second series we have the Cape as the primary station, where \( v = 35^\circ 40' \), \( w = 2·270 \), and \( I \), derived from the experiments with the constant weights = 0·715 (London = 1); consequently at other stations \[ I' = '1837 w' \csc v'. \] the values of \( w' \) being taken from Table B. The loss of force sustained by deflector S. causes a similar interruption in the continuity of the line connecting the terminations of the ordinates derived from the observations in which the deflectors N. and S. were used conjointly; we have therefore in this case also two tables of the values of \( w' \), one for the first, and the other for the second series. | Values of \( w' \), Deflectors N. and S. | |----------------------------------------| | (A.) August 1839 to February 12, 1840. | (B.) February 12 to March 25, 1840. | | grs. | grs. | grs. | | 44 = 3·784 | 53 = 3·118 | 51 = 3·037 | | 45 = 3·674 | 54 = 3·056 | 52 = 2·989 | | 46 = 3·584 | 55 = 2·995 | 53 = 2·943 | | 47 = 3·505 | 56 = 2·936 | 54 = 2·896 | | 48 = 3·430 | 57 = 2·880 | 55 = 2·853 | | 49 = 3·366 | 58 = 2·828 | 56 = 2·813 | | 50 = 3·304 | 59 = 2·780 | 57 = 2·775 | | 51 = 3·242 | 60 = 2·738 | 58 = 2·740 | | 52 = 3·180 | | | For the first series we have London as the primary station, where $I = 1$, $\nu = 44^\circ 06'$, and $w = 3.773$; whence at other stations $$I' = 0.1845 \ w' \ \text{cosec} \ \nu',$$ $w'$ being taken from Table A; and for the second series the Cape as the primary station, where $I = 0.715$, $\nu = 51^\circ 10'$, and $w = 3.032$; whence at other stations $$I' = 0.1837 \ w' \ \text{cosec} \ \nu',$$ $w'$ being taken from Table B. Table I. contains the observations made with the weights and deflectors on shore and on board the Erebus, between London and the Cape of Good Hope. Of 647 observations comprised in this Table, I have only found it necessary to consider a single one as doubtful, namely, the second observation with the constant weight of one grain at the Cape of Good Hope; its result differs so much from that of the observation on the preceding day with the same weight, and with those of the preceding and of the same day with the weight of $1\frac{1}{2}$ grain, that I have thought it safer to omit it in taking the mean of the results at that station; but the observation itself, and its result, are both given in the Table. ### Table I. Observations of the Magnetic Intensity on Shore, and on Board Her Majesty's Ship Erebus, with Needle F, by Captain James Clark Ross. London to the Cape of Good Hope. | 1839 | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | | | | | | | | London = 1·000 = 1·372 | | Aug. 28. | Westbourn Green near London. | h m | Deflector S., Deflector N., Deflectors S. & N. weight 1 grain. weight 2 grains. weight 3 grains. weight 4 grains. S. | 30° 19' 70° | Observed on shore. | 1·000 | 1·372 | | Oct. 1. | 50° 42' 0° 35' 1 P.M. | 11 A.M. | weight 2 grains. weight 4 grains. weight 2 grains. weight 4 grains. S. | 21° 54' 60° | w. by n. | .985 | .993 | .990 | 1·358 | | | Noon. | 30° 17' 357° 26' 10° 30' A.M. | weight 2 grains. weight 4 grains. weight 2 grains. weight 4 grains. S. | 21° 41' 60° | w. by n. | 1·003 | 1·001 | 1·373 | | | 8. | 47° 47' 350° 42' 10° A.M. | weight 2 grains. S. and N. weight 2 grains. weight 2 grains. S. and N. weight 2 grains. weight 4 grains. S. and N. weight 2 grains. weight 4 grains. S. | 21° 1' 30° 43' 61° | s.w. by w. | .981 | 1·007 | 1·382 | | | 13. | 41° 63' 48° 10' 9° A.M. | weight 2 grains. S. and N. weight 2 grains. weight 2 grains. S. and N. weight 2 grains. weight 4 grains. S. and N. weight 2 grains. weight 4 grains. S. | 21° 1' 30° 43' 61° | s.w. by w. | .982 | .959 | .966 | 1·326 | | | 14. | 39° 30' 347° 51' 9° 30' A.M. | weight 2 grains. S. and N. weight 2 grains. weight 2 grains. S. and N. weight 2 grains. weight 4 grains. S. and N. weight 2 grains. weight 4 grains. S. | 21° 1' 30° 43' 61° | s.w. by w. | .993 | .978 | .977 | 1·346 | | | 10° 15' A.M. | 11° 0° A.M. | weight 2 grains. S. and N. weight 2 grains. weight 2 grains. S. and N. weight 2 grains. weight 4 grains. S. and N. weight 2 grains. weight 4 grains. S. | 21° 1' 30° 43' 61° | s.w. by w. | .959 | .948 | .949 | 1·302 | | | 21. | Funchal Roads. | 10° 0° A.M. | weight 2 grains. S. and N. weight 2 grains. weight 2 grains. S. and N. weight 2 grains. weight 4 grains. S. and N. weight 2 grains. weight 4 grains. S. | 21° 1' 30° 43' 61° | s.w. by w. | .945 | .955 | .948 | 1·297 | | | 11° 0° A.M. | Consul's House, Funchal. | 7° A.M. | weight 2 grains. S. and N. weight 2 grains. weight 2 grains. S. and N. weight 2 grains. weight 4 grains. S. and N. weight 2 grains. weight 4 grains. S. | 21° 1' 30° 43' 61° | s.w. by w. | .942 | .948 | .942 | 1·297 | | | Noon. | 32° 38' 343° 04' 0° 30' P.M. | weight 2 grains. weight 3 grains. weight 2 grains. weight 4 grains. S. | 21° 1' 30° 43' 61° | s.w. by w. | .943 | .948 | .942 | 1·297 | | | 24. | Funchal Roads. | 10° 30' A.M. | weight 1 grain. weight 2 grains. weight 3 grains. weight 1 grain. weight 2 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | 26. | 2° 0° P.M. | weight 2 grains. weight 4 grains. weight 2 grains. weight 4 grains. S. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | 3° 50' P.M. | 4° 30' P.M. | weight 1 grain. weight 2 grains. weight 3 grains. weight 1 grain. weight 2 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | Nov. 1. | 30° 47' 343° 10' 10° A.M. | to Noon. | weight 1 grain. weight 2 grains. weight 1 grain. weight 2 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | Off Santa Cruz, Teneriffe. | 10° A.M. | weight 1 grain. weight 2 grains. weight 1 grain. weight 2 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | Noon. | 6° 26' 1342° 25' 10° A.M. | weight 1 grain. weight 2 grains. weight 1 grain. weight 2 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | 10° 20' A.M. | 11° 15' A.M. | weight 2 grains. weight 3 grains. weight 2 grains. weight 3 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | 11° 50' A.M. | 1° 30' P.M. | weight 2 grains. weight 3 grains. weight 2 grains. weight 3 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | 2° 0° P.M. | 10° 30' A.M. | weight 1 grain. weight 2 grains. weight 1 grain. weight 2 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | 7. | 24° 51' 341° 18' 3° 30' P.M. | weight 1 grain. weight 2 grains. weight 1 grain. weight 2 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | | | 4° 0° P.M. | weight 1 grain. weight 2 grains. | 21° 1' 30° 43' 61° | s.w. by w. | .948 | .942 | .942 | 1·297 | ### Table I. (Continued.) | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | | Lat. | Long. E. | | | | | London = 1·000 = 1·372 | | 1839 | Nov. 9 | 22° 18' 340° 03' | 9 A.M. | S. | 33° 13' | 7° 4 | -874 | | | | | | | 25° 31' | | -874 | | | | | | | 47° 24' | | -871 | | | | | | | 25° 29' | | -871 | | | | | | weight 1 grain. | 11° 58' | | -878 | | | | | | weight 2 grains. | 25° 30' | | -878 | | | | | | S. | 33° 32' | | -885 | | | | | | | 12° 4 | | -862 | | | | | | weight 1 grain. | 25° 38' | | -862 | | | | | | weight 2 grains. | 33° 56' | | -861 | | | | | | S. | 25° 54' | | -860 | | | | | | | 47° 49' | | -859 | | | | | | S. | 34° 8 | | -859 | | | | | | | 26° 0 | | -858 | | | | | | S. | 48° 4 | | -858 | | | | | | | 12° 14' | | -855 | | | | | | weight 1 grain. | 26° 6 | | -855 | | | | | | weight 2 grains. | 41° 0 | | -855 | | | | | | weight 3 grains. | 34° 4 | | -855 | | | | | | S. | 25° 59' | | -855 | | | | | | | 48° 15' | | -855 | | | | | | S. | 12° 36' | | -855 | | | | | | | 26° 0 | | -855 | | | | | | S. | 41° 41' | | -855 | | | | | | | 33° 57' | | -855 | | | | | | S. | 34° 34' | | -855 | | | | | | | 33° 53' | | -855 | | | | | | S. | 34° 30' | | -855 | | | | | | | 34° 19' | | -855 | | | | | | S. | 33° 13' | | -855 | | | | | | | 34° 21' | | -855 | | | | | | S. | 33° 57' | | -855 | | | | | | | 34° 4 | | -855 | | | | | | S. | 26° 8 | | -855 | | | | | | | 48° 51' | | -855 | | | | | | weight 1 grain. | 13° 2 | | -855 | | | | | | weight 2 grains. | 26° 50' | | -855 | | | | | | S. | 34° 46' | | -855 | | | | | | | 27° 6 | | -855 | | | | | | S. | 49° 24' | | -855 | | | | | | | 35° 1 | | -855 | | | | | | S. | 27° 20' | | -855 | | | | | | | 49° 50' | | -855 | | | | | | S. | 13° 45' | | -855 | | | | | | | 28° 24' | | -855 | | | | | | S. | 35° 59' | | -855 | | | | | | | 28° 38' | | -855 | | | | | | S. | 51° 6 | | -855 | | | | | | | 14° 3 | | -855 | | | | | | weight 1 grain. | 29° 14' | | -855 | | | | | | weight 2 grains. | 36° 13' | | -855 | | | | | | S. | 29° 1 | | -855 | | | | | | | 51° 51' | | -855 | | | | | | S. | 37° 8 | | -855 | | | | | | | 29° 23' | | -855 | | | | | | S. | 52° 29' | | -855 | *Observed on shore.* | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | | Lat. | Long. E. | | | | | London = 1·000. = 1·372. | | Nov. 30. | 2° 6' 33" 25' 10" A.M. | S. | 37° 17' 8° 1' | s.s.w. ½ w. | -729 | -711 | -730 | | | | N. | 30° 3' | | -734 | -735 | -734 | | | | S. and N. | 52° 34' | | -725 | -726 | -727 | | | | weight 1 grain. | 14° 30' | | -726 | -726 | -726 | | | | weight 2 grains. | 30° 24' | | -717 | -718 | -719 | | Dec. 2. St. Paul's Rocks. | 8° A.M. | S. | 37° 8' 90° | Observed on shore. | -717 | -718 | -719 | | | 0° 56' 33" 40" | N. | 29° 34' | | -706 | -707 | -708 | | | | S. and N. | 52° 44' | | -697 | -698 | -699 | | | | weight 1 grain. | 14° 39' | | -699 | -700 | -701 | | | | weight 2 grains. | 30° 50' | | -690 | -691 | -692 | | | | S. | 37° 41' 84° | s.s.w. ½ w. | -683 | -684 | -685 | | | | N. | 30° 11' | | -679 | -680 | -681 | | | | S. and N. | 53° 15' | | -674 | -675 | -676 | | | | weight 1 grain. | 37° 46' | | -674 | -675 | -676 | | | | weight 2 grains. | 30° 29' | | -674 | -675 | -676 | | | | S. and N. | 53° 49' | | -674 | -675 | -676 | | | | weight 1 grain. | 38° 10' | | -674 | -675 | -676 | | | | weight 2 grains. | 30° 29' | | -674 | -675 | -676 | | | | S. and N. | 54° 2' | | -674 | -675 | -676 | | | | weight 1 grain. | 15° 27' | | -674 | -675 | -676 | | | | weight 2 grains. | 32° 6' | | -674 | -675 | -676 | | | | S. | 38° 47' | | -674 | -675 | -676 | | | | S. and N. | 54° 48' | | -674 | -675 | -676 | | | | N. | 30° 51' | | -674 | -675 | -676 | | | | S. and N. | 39° 7' | | -674 | -675 | -676 | | | | weight 1 grain. | 55° 1' | | -674 | -675 | -676 | | | | weight 2 grains. | 31° 24' | | -674 | -675 | -676 | | | | S. and N. | 39° 35' | | -674 | -675 | -676 | | | | weight 1 grain. | 16° 25' | | -674 | -675 | -676 | | | | weight 2 grains. | 32° 26' | | -674 | -675 | -676 | | | | weight 3 grains. | 56° 7' | | -674 | -675 | -676 | | | | S. | 39° 50' | | -674 | -675 | -676 | | | | S. and N. | 55° 50' | | -674 | -675 | -676 | | | | N. | 32° 47' | | -674 | -675 | -676 | | | | weight 1 grain. | 16° 37' | | -674 | -675 | -676 | | | | weight 2 grains. | 34° 42' | | -674 | -675 | -676 | | | | S. | 40° 13' | | -674 | -675 | -676 | | | | S. and N. | 57° 2' | | -674 | -675 | -676 | | | | N. | 33° 7' | | -674 | -675 | -676 | | | | weight 1 grain. | 16° 41' | | -674 | -675 | -676 | | | | weight 2 grains. | 35° 53' | | -674 | -675 | -676 | | | | S. | 41° 0' | | -674 | -675 | -676 | | | | S. and N. | 57° 18' | | -674 | -675 | -676 | | | | N. | 33° 29' | | -674 | -675 | -676 | | | | weight 1 grain. | 17° 7' | | -674 | -675 | -676 | | | | weight 2 grains. | 36° 8' | | -674 | -675 | -676 | | | | S. | 41° 24' | | -674 | -675 | -676 | | | | S. and N. | 57° 23' | | -674 | -675 | -676 | | | | N. | 33° 40' | | -674 | -675 | -676 | | | | weight 1 grain. | 17° 56' | | -674 | -675 | -676 | | | | weight 2 grains. | 36° 56' | | -674 | -675 | -676 | | | | S. | 18° 0' | | -674 | -675 | -676 | | | | S. and N. | 37° 57' | | -674 | -675 | -676 | | | | weight 1 grain. | 41° 51' | | -674 | -675 | -676 | | | | weight 2 grains. | 41° 42' | | -674 | -675 | -676 | | | | S. | 18° 1' | | -674 | -675 | -676 | | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|------------|------------|-----------| | | | | | | | | London = 1·000. = 1·372. | | Dec. 14. | -15° 4' 33" 06' 10" A.M. | S. | 41° 58' 80" | s.s.E. | .596 | .610 | .600 | .599 | .822 | | | | S. and N. | 58° 17' | | | | .591 | .598 | | | | | N. | 34° 13' | | | | .594 | .602 | | | | | weight 1 grain. | 18° 7' 81" | | | | .597 | .596 | .818 | | | | weight 2 grains. | 38° 27' | | | | .589 | .597 | | | | | S. | 42° 3' 79" | | | | .591 | .596 | | | | | S. and N. | 58° 40' | | | | .598 | .597 | | | | | N. | 34° 21' | | | | .591 | .593 | .814 | | | | weight 1 grain. | 18° 10' 79" | | | | .587 | .593 | | | | | weight 2 grains. | 38° 30' | | | | .593 | .593 | | | | | S. | 42° 12' 78" | | | | .593 | .592 | | | | | N. | 34° 23' | | | | .598 | .592 | | | | | S. and N. | 59° 1' 78" | | | | .591 | .592 | | | | | weight 1 grain. | 18° 7' 78" | | | | .587 | .595 | | | | | weight 1 grain. | 18° 14' | | | | .594 | .603 | | | | | weight 2 grains. | 38° 47' | | | | .598 | .598 | | | | | weight 2 grains. | 38° 48' | | | | .598 | .598 | | | | | S. | 42° 5' 80" | | | | .593 | .592 | | | | | N. | 34° 34' | | | | .598 | .592 | | | | | S. and N. | 59° 1' 80" | | | | .587 | .595 | | | | | weight 1 grain. | 18° 14' 80" | | | | .594 | .603 | | | | | weight 2 grains. | 38° 42' | | | | .598 | .598 | | | | | S. | 42° 3' 78" | | | | .593 | .592 | | | | | N. | 34° 5' | | | | .598 | .592 | | | | | S. and N. | 59° 4' | | | | .597 | .596 | | | | | weight 1 grain. | 18° 6' 79" | | | | .590 | .598 | | | | | weight 1 grain. | 18° 9' | | | | .598 | .598 | | | | | weight 2 grains. | 38° 28' | | | | .598 | .598 | | | | | weight 2 grains. | 38° 27' | | | | .598 | .598 | | | | | S. | 42° 8' 76" | | | | .592 | .597 | | | | | N. | 34° 13' | | | | .598 | .597 | | | | | S. and N. | 58° 55' | | | | .597 | .596 | | | | | weight 1 grain. | 41° 52' 79" | | | | .590 | .598 | | | | | weight 2 grains. | 37° 37' | | | | .598 | .598 | | | | | S. | 41° 58' 76" | | | | .592 | .597 | | | | | S. | 41° 16' 77" | | | | .598 | .596 | | | | | N. | 34° 8' 77" | | | | .597 | .596 | | | | | S. and N. | 59° 5' 77" | | | | .599 | .599 | | | | | weight 1 grain. | 17° 52' 78" | | | | .600 | .599 | | | | | weight 2 grains. | 37° 34' | | | | .600 | .599 | | | | | S. | 41° 48' 76" | | | | .600 | .599 | | | | | N. | 34° 4' | | | | .600 | .599 | | | | | S. and N. | 58° 33' | | | | .600 | .599 | | | | | weight 1 grain. | 41° 56' 76" | | | | .600 | .599 | | | | | weight 2 grains. | 37° 16' 78" | | | | .600 | .599 | | | | | S. | 41° 50' 77" | | | | .600 | .599 | | | | | N. | 33° 53' | | | | .600 | .599 | | | | | S. and N. | 58° 53' | | | | .600 | .599 | | | | | weight 1 grain. | 41° 10' 77" | | | | .600 | .599 | | | | | weight 2 grains. | 33° 43' | | | | .600 | .599 | | | | | S. | 58° 9' | | | | .600 | .599 | | | | | weight 1 grain. | 17° 44' 78" | | | | .600 | .599 | | | | | weight 1 grain. | 17° 44' | | | | .600 | .599 | | | | | weight 2 grains. | 36° 56' | | | | .600 | .599 | | | | | weight 2 grains. | 36° 56' | | | | .600 | .599 | | MDCCXLII. | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|------------|-----------| | | | | | | | | London = 1·000. = 1·372. | | Dec. 22. | -26° 49' 33" 30' | h m | S. | 41° 30' | 77° | N. by E. | -608 | | | | 2 to 3 P.M. | N. | 33° 23' | | | -620 | | | | | S. and N. weight 1 grain. | 58° 16' | 77° | N. by E. | -611 | | | | | weight 2 grains. | 36° 48' | | | -613 | | | | | N. | 33° 17' | 78° | N. ½ W. | -604 | | | | | N. | 33° 57' | 78° | S.E. | -620 | | | | | S. | 41° 46' | 78° | | -606 | | | | Noon. | S. and N. weight 1 grain. | 58° 55' | 78° | S.E. | -602 | | | | | weight 2 grains. | 17° 45' | 77° | | -599 | | | | | S. | 41° 35' | 76° | E. | -602 | | | | | S. | 41° 31' | 77° | | -613 | | | | | N. | 34° 0' | 77° | S.E. by E. | -605 | | | | | S. and N. weight 1 grain. | 58° 37' | 76° | | -608 | | | | | weight 2 grains. | 17° 40' | | | -605 | | | | | weight 2 grains. | 36° 56' | | | -618 | | | | | S. | 41° 13' | 76° | S. by E. | -622 | | | | | N. | 33° 23' | 76° | | -615 | | | | | S. and N. | 58° 1' | 76° | | -620 | | | | | S. | 41° 22' | 76° | N.N.E. ½ E. | -615 | | | | | N. | 33° 47' | | | -611 | | | | | S. and N. weight 1 grain. | 58° 11' | 76° | | -611 | | | | | weight 2 grains. | 17° 22' | | | -617 | | | | | weight 2 grains. | 35° 47' | | | -845 | | | | | S. | 41° 23' | 76° | N.N.E. | -635 | | | | | N. | 33° 47' | | | -611 | | | | | S. and N. weight 1 grain. | 57° 46' | 78° | | -610 | | | | | weight 2 grains. | 17° 40' | | | -619 | | | | | weight 2 grains. | 37° 0' | | | -613 | | | | | S. | 41° 41' | 77° | N. by E. | -605 | | | | | N. | 33° 55' | 77° | | -618 | | | | | S. and N. | 58° 8' | 77° | | -603 | | | | | weight 2 grains. | 37° 46' | 76° | | -607 | | | | | weight 1 grain. | 17° 51' | 76° | N.E. by N. | -613 | | | | | S. | 41° 55' | 78° | | -607 | | | | | N. | 34° 3' | 78° | S.E. by E. | -607 | | | | | S. and N. weight 1 grain. | 58° 32' | 78° | | -607 | | | | | weight 2 grains. | 17° 51' | 79° | | -607 | | | | | weight 1 grain. | 38° 12' | 79° | S.E. | -599 | | | | | weight 2 grains. | 17° 53' | 76° | S.E. by s. | -604 | | | | | S. | 41° 37' | 79° | | -607 | | | | | N. | 34° 0' | 79° | | -607 | | | | | S. and N. weight 2 grains. | 58° 44' | 79° | S.E. by E. | -607 | | | | | weight 2 grains. | 37° 46' | 79° | | -607 | | | | | weight 2 grains. | 37° 45' | 79° | | -607 | | | | | weight 1 grain. | 17° 46' | 79° | | -607 | | | | | weight 2 grains. | 17° 49' | 79° | | -607 | | | | | S. | 41° 24' | 79° | | -607 | | | | | N. | 33° 56' | 79° | | -607 | | | | | S. and N. weight 1 grain. | 58° 50' | 79° | | -607 | | | | | weight 1 grain. | 17° 55' | 77° | S.E. | -607 | | | | | weight 1 grain. | 17° 50' | 77° | | -607 | | | | | weight 2 grains. | 37° 16' | 77° | | -607 | | | | | weight 2 grains. | 37° 18' | | | -607 | | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | | Lat. | Long. E. | | | | | London = 1·000. | 1·372. | | Dec. 31. | -27° 43' 338" 34° 10' A.M. | S. | 41° 27' 78° | S.E. by E. ½ E. | {610} | {600} | {602} | {607} | {833} | | | | N. | 34° 12' 78° | | {600} | {602} | {607} | {833} | | | -27° 44' 338" 40° 2' P.M. | S. and N. | 58° 50' 78° | | {600} | {602} | {607} | {833} | | | | weight 2 grains. | 36° 47' 76° | | {600} | {602} | {607} | {833} | | | | weight 1 grain. | 17° 48' 76° | | {600} | {602} | {607} | {833} | | 1840. | Jan. 1. | 10° 30' A.M. | S. | 41° 39' 77° | E. by s. | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 5' | | {604} | {603} | {599} | {603} | {828} | | | | 11° 30' A.M. | S. and N. | 58° 56' | | {604} | {603} | {599} | {603} | {828} | | | | Noon. | weight 1 grain. | 17° 52' | | {604} | {603} | {599} | {603} | {828} | | | | 9° 30' P.M. | weight 2 grains. | 37° 20' | | {604} | {603} | {599} | {603} | {828} | | | | 10° 00' P.M. | S. | 41° 49' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 33° 59' | | {604} | {603} | {599} | {603} | {828} | | | | 10° 30' P.M. | S. and N. | 59° 0' | | {604} | {603} | {599} | {603} | {828} | | | | 11° 0' P.M. | weight 1 grain. | 17° 56' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 ½ grain. | 27° 7' | | {604} | {603} | {599} | {603} | {828} | | | | weight 2 grains. | 37° 36' | | {604} | {603} | {599} | {603} | {828} | | | | 9° 30' A.M. | S. | 41° 31' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 33° 58' | | {604} | {603} | {599} | {603} | {828} | | | | 10° 40' A.M. | S. and N. | 58° 45' | | {604} | {603} | {599} | {603} | {828} | | | | S. | 41° 32' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 5' | | {604} | {603} | {599} | {603} | {828} | | | | 10° 30' A.M. | S. and N. | 58° 59' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 2' | | {604} | {603} | {599} | {603} | {828} | | | | weight 2 grains. | 38° 7' | | {604} | {603} | {599} | {603} | {828} | | | | 11° 30' A.M. | S. | 41° 37' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 23' | | {604} | {603} | {599} | {603} | {828} | | | | S. and N. | 59° 1' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 7' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 ½ grain. | 26° 40' | | {604} | {603} | {599} | {603} | {828} | | | | weight 2 grains. | 38° 20' | | {604} | {603} | {599} | {603} | {828} | | | | S. | 41° 52' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 23' | | {604} | {603} | {599} | {603} | {828} | | | | S. and N. | 59° 13' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 9' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 14' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 12' | | {604} | {603} | {599} | {603} | {828} | | | | S. | 42° 2' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 34' | | {604} | {603} | {599} | {603} | {828} | | | | S. and N. | 59° 23' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 7' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 20' | | {604} | {603} | {599} | {603} | {828} | | | | S. | 42° 3' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 30' | | {604} | {603} | {599} | {603} | {828} | | | | S. and N. | 59° 33' | | {604} | {603} | {599} | {603} | {828} | | | | S. | 42° 10' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 34' | | {604} | {603} | {599} | {603} | {828} | | | | S. and N. | 59° 39' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 6' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 26' | | {604} | {603} | {599} | {603} | {828} | | | | S. | 42° 13' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 36' | | {604} | {603} | {599} | {603} | {828} | | | | S. and N. | 59° 48' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 9' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 7' | | {604} | {603} | {599} | {603} | {828} | | | | S. | 42° 0' | | {604} | {603} | {599} | {603} | {828} | | | | N. | 34° 7' | | {604} | {603} | {599} | {603} | {828} | | | | S. and N. | 59° 9' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 3' | | {604} | {603} | {599} | {603} | {828} | | | | weight 1 grain. | 18° 6' | | {604} | {603} | {599} | {603} | {828} | D 2 | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|------------|------------|-----------| | | | | | | | | London = 1'000. = 1'372. | | 1840 | Jan. 11 | 17° 39' 346" | 10° 30 A.M. | S. | 41 54 | N.E. by E. | .598 | | | | | | | | | -605 | | | | | | | | | .595 | | | | | | | | | .597 | | | | | | | | | .820 | | | | | | | | | .594 | | | | | | | | | .595 | | | | | | | | | .598 | | | | | | | | | .588 | | | | | | | | | .595 | | | | | | | | | .817 | | | | | | | | | .597 | | | | | | | | | .592 | | | | | | | | | .597 | | | | | | | | | .593 | | | | | | | | | .594 | | | | | | | | | .816 | | | | | | | | | .595 | | | | | | | | | .594 | | | | | | | | | .592 | | | | | | | | | .821 | | | | | | | | | .596 | | | | | | | | | .598 | | | | | | | | | .597 | | | | | | | | | .606 | | | | | | | | | .589 | | | | | | | | | .593 | | | | | | | | | .592 | | | | | | | | | .813 | | | | | | | | | .594 | | | | | | | | | .598 | | | | | | | | | .597 | | | | | | | | | .600 | | | | | | | | | .596 | | | | | | | | | .599 | | | | | | | | | .600 | | | | | | | | | .585 | | | | | | | | | .587 | | | | | | | | | .593 | | | | | | | | | .587 | | | | | | | | | .590 | | | | | | | | | .590 | | | | | | | | | .597 | | | | | | | | | .599 | | | | | | | | | .594 | | | | | | | | | .815 | | | | | | | | | .599 | | | | | | | | | .600 | | | | | | | | | .588 | | | | | | | | | .582 | | | | | | | | | .589 | | | | | | | | | .810 | | | | | | | | | .594 | | | | | | | | | .594 | | | | | | | | | .594 | | | | | | | | | .594 | | | | | | | | | .594 | | | | | | | | | .816 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .816 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .816 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .816 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .591 | | | | | | | | | .816 | | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|------------|------------|-----------| | | | | | | | | London = 1·000. = 1·372. | | 1840 | | | | | | | | | Jan. 21 | -14° 51' 35" 31' | h. m. | S. | 42° 16' | W. | .589 | | | | | S. | 42° 7' | N.W. | .593 | | | | | S. | 41° 44' | N. | .601 | | | | | S. | 42° 6' | N.E. | .593 | | | | | S. | 42° 15' | E. | .589 | | | | | weight 1 grain. | 17° 52' | | .598 | | | | | S. | 41° 51' | S.W. | .599 | | | | | S. | 17° 41' | | .604 | | | | | S. | 41° 52' | | .598 | | | | | weight 1 grain. | 17° 45' | | .601 | | | | | S. | 41° 46' | N.E. | .601 | | | | | S. | 17° 50' | | .599 | | | | | S. | 41° 47' | | .601 | | | | | weight 1 grain. | 33° 54' | | .607 | | | | | S. | 58° 36' | | .605 | | | | | weight 1 grain. | 17° 54' | | .597 | | | | | S. | 17° 56' | | .596 | | | | | S. | 41° 47' | s. by w. | .601 | | | | | S. | 33° 59' | | .605 | | | | | S. | 58° 44' | | .603 | | | | | weight 1 grain. | 17° 57' | | .596 | | | | | S. | 41° 51' | s. by w. | .599 | | | | | S. | 17° 59' | | .595 | | | | | S. | 41° 54' | | .598 | | | | | weight 1 grain. | 17° 55' | | .597 | | | | | S. | 42° 1' | E.N.E. | .595 | | | | | weight 1 grain. | 17° 48' | | .601 | | | | | S. | 41° 40' | s.s.w. | .604 | | | | | weight 1 grain. | 17° 40' | | .605 | | | | | S. | 41° 46' | s. ½ w. | .601 | | | | | weight 1 grain. | 17° 48' | | .601 | | | | | S. | 41° 26' | s. by w. | .601 | | | | | S. | 58° 18' | | .610 | | | | | S. | 33° 45' | | .611 | | | | | weight 1 grain. | 17° 43' | | .611 | | | | | S. | 26° 36' | | .611 | | | | | weight 1 ½ grain. | 37° 13' | | .611 | | | | | S. | 42° 35' | | .611 | | | | | weight 2 grains. | 59° 56' | | .611 | | | | | S. | 34° 53' | | .611 | | | | | weight 1 grain. | 18° 13' | | .611 | | | | | S. | 27° 17' | | .611 | | | | | weight 1 ½ grain. | 39° 52' | | .611 | | | | | S. | 42° 23' | | .611 | | | | | weight 2 grains. | 42° 26' | | .611 | | | | | S. | 41° 47' | | .611 | | | | | weight 1 grain. | 41° 52' | | .611 | | | | | S. | 42° 20' | | .611 | | | | | weight 1 ½ grain. | 42° 0' | | .611 | | | | | S. | 42° 19' | | .611 | | | | | weight 2 grains. | 42° 11' | | .611 | | | | | S. | 42° 19' | | .611 | | | | | weight 1 grain. | 41° 49' | | .611 | | | | | S. | 41° 51' | s.s.w. ½ w. | .601 | | | | | S. | 41° 51' | s.w. ½ s. | .599 | | | | | weight 1 grain. | 17° 32' | | .609 | | | | | S. | 41° 46' | s.s.w. ½ w. | .601 | | | | | S. | 41° 43' | | .603 | ### Table I. (Continued.) | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | | | | | | | | London = 1·000. = 1·372. | | 1840 | Feb. 12 | -20° 41' 352" E | h m | S. | 41° 56' | s.s.w. | .597 | | | | | | S and N. | 58° 41' | | .604 | | | | | | N. | 34° 29' | | .594 | | | | Noon | weight 1 grain. | 17° 28' | 76 | s.s.w. ½ W. | .612 | | | | 1 12 P.M. | | 17° 36' | | | .607 | | | | 1 30 P.M. | | 41° 57' | | | .597 | | | | 6 P.M. | | 41° 2' | 74 | s.w. ½ W. | .597 | | | | 6 30 P.M. | | 41° 4' | 72 | s.w. by s. | .596 | | | | 13. | | 40° 57' | 72 | s. by E. | .599 | | | | -21° 13' 51" | | 40° 57' | 75 | | .599 | | | | 10 A.M. | | S. | 57° 34' | | .599 | | | | 10 30 | | S. and N. | 34° 1' | | .605 | | | | 10 45 | | N. | 41° 0' | | .598 | | | | 11 0 | | S. | 18° 1' | | .594 | | | | 11 30 | | weight 1 grain. | 18° 0' | | .594 | | | | -22° 12' 35" | | weight 1 grain. | 41° 4' | | .596 | | | | 23 Noon | | weight 1 grain. | 41° 11' | | .594 | | | | 1 0 P.M. | | S. | 41° 5' | | .595 | | | | 22° 19' 35" | | S. | 40° 56' | | .599 | | | | 22° 23' 35" | | S. | 40° 41' | s.s.w. | .604 | | | | 14. | | S. | 40° 58' | w. | .599 | | | | -23° 33' 35" | | S. | 41° 0' | | .595 | | | | 10 A.M. | | S. | 57° 45' | | .597 | | | | 8 30 | | S. and N. | 34° 12' | | .599 | | | | 9 30 | | N. | 17° 56' | | .596 | | | | 10 0 | | weight 1 grain. | 40° 58' | | .599 | | | | 10 40 | | S. | 41° 5' | | .595 | | | | -23° 42' 35" | | weight 1 grain. | 17° 51' | | .596 | | | | 28 Noon | | weight 1 grain. | 17° 51' | | .599 | | | | 1 30 P.M. | | S. | 41° 12' | | .594 | | | | 23° 47' 35" | | S. | 41° 4' | | .596 | | | | 28° 30' | | weight 1 grain. | 17° 45' | | .595 | | | | 23° 51' 35" | | weight 1 grain. | 17° 45' | | .602 | | | | 16° 530 | | S. | 40° 59' | | .602 | | | | 9 A.M. | | S. | 41° 7' | | .595 | | | | 10 | | weight 1 grain. | 17° 50' | | .600 | | | | 10 30 | | weight 1 grain. | 17° 51' | | .599 | | | | 11 00 | | S. | 41° 4' | | .594 | | | | 11 30 | | S. | 41° 10' | | .596 | | | | -24° 36' 34" | | S. | 41° 2' | | .595 | | | | 30 Noon | | weight 1 grain. | 17° 45' | | .597 | | | | 24° 39' 34" | | weight 1 grain. | 17° 45' | | .602 | | | | 17° 40 | | weight 1 grain. | 17° 45' | | .602 | | | | 24° 42' 34" | | weight 1 grain. | 17° 56' | | .598 | | | | 10 | | S. | 41° 4' | | .594 | | | | 7 0 | | S. | 41° 12' | | .594 | | | | -25° 0' 34" | | weight 1 grain. | 17° 56' | | .596 | | | | Midnight | | weight 1 grain. | 17° 51' | | .596 | | | | 0 40 | | weight 1 grain. | 17° 51' | | .599 | | | | 16. | | weight 1 grain. | 17° 51' | | .601 | | | | -25° 15' 34" | | weight 1 grain. | 17° 51' | | .599 | | | | 59° A.M. | | S. | 40° 49' | | .601 | | | | 7 | | S. | 40° 57' | | .599 | | | | 11 | | S. | 58° 13' | | .591 | | | | 11 30 | | S and N. | 34° 9' | | .602 | | | | Noon | | N. | 17° 47' | | .602 | | | | 0 30 P.M. | | weight 1 grain. | 27° 18' | | .602 | | | | 1 0 | | weight 1½ grain. | 38° 11' | | .601 | | | | 1 30 | | weight 2 grains. | 40° 13' | | .605 | | | | -25° 38' 34" | | weight 1 grain. | 17° 36' | | .607 | | | | 41° | | S. | 40° 50' | | .601 | | | | 6 0 | | S. | 41° 10' | | .594 | | | | 6 30 | | S. | 40° 24' | | .609 | | | | 17. | | S. | 40° 20' | | .611 | | | | -26° 8' 34" | | S. | 57° 11' | | .605 | | | | 0347 03° 10" | | S. | 33° 19' | | .621 | | | | 10 A.M. | | S. | 17° 7' | | .624 | | | | 10 30 | | S. | | | | | | | 10 40 | | S. | | | | | | | 11 20 | | N. | | | | | | | -27° 6' 34" | | weight 1 grain. | | | | | 1840 | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|------------|-----------| | | Lat. | Long. E. | | | | | London = 1·000. = 1·372. | | Feb. 19. | -27° 55' 346" 41' 10" A.M. | 10 10 30 | S. weight 1 grain. | 40 28 78 | S.E. | -610 | -612 | | | | | | | | | -607 | -834 | | | | | | | | | -601 | -607 | | | | | | | | | -607 | -607 | | | | | | | | | -612 | -607 | | | | | | | | | -599 | -612 | | | | | | | | | -618 | -609 | | | | | | | | | -611 | -836 | | | | | | | | | -611 | -611 | | | | | | | | | -606 | -606 | | | | | | | | | -618 | -618 | | | | | | | | | -629 | -629 | | | | | | | | | -622 | -852 | | | | | | | | | -618 | -618 | | | | | | | | | -621 | -621 | | | | | | | | | -605 | -605 | | | | | | | | | -632 | -632 | | | | | | | | | -619 | -849 | | | | | | | | | -610 | -610 | | | | | | | | | -619 | -619 | | | | | | | | | -627 | -627 | | | | | | | | | -636 | -636 | | | | | | | | | -635 | -872 | | | | | | | | | -635 | -635 | | | | | | | | | -638 | -872 | | | | | | | | | -634 | -634 | | | | | | | | | -627 | -627 | | | | | | | | | -628 | -628 | | | | | | | | | -652 | -652 | | | | | | | | | -640 | -878 | | | | | | | | | -652 | -652 | | | | | | | | | -630 | -630 | | | | | | | | | -647 | -647 | | | | | | | | | -653 | -653 | | | | | | | | | -658 | -658 | | | | | | | | | -636 | -636 | | | | | | | | | -653 | -888 | | | | | | | | | -647 | -647 | | | | | | | | | -658 | -658 | | | | | | | | | -644 | -644 | | | | | | | | | -645 | -645 | | | | | | | | | -638 | -638 | | | | | | | | | -876 | -876 | | | | | | | | | -627 | -627 | | | | | | | | | -645 | -645 | | | | | | | | | -645 | -886 | | | | | | | | | -645 | -645 | | | | | | | | | -666 | -915 | | | | | | | | | -663 | -663 | | | | | | | | | -657 | -657 | | | | | | | | | -665 | -913 | | | | | | | | | -674 | -674 | | | | | | | | | -667 | -667 | | | | | | | | | -672 | -672 | | | | | | | | | -671 | -671 | | | | | | | | | -674 | -922 | | | | | | | | | -667 | -667 | | | | | | | | | -686 | -686 | | | | | | | | | -668 | -668 | | | | | | | | | -678 | -678 | | | | | | | | | -671 | -925 | | | | | | | | | -671 | -925 | ## Table I. (Continued.) | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|------------|-----------| | | | | | | | | London = 1'000. = 1'372. | | March 4. | -33° 8' 10" 11' | h m | S. | 37° 16' 6" | 66 | S.E. | .677 | | | | 10 0 A.M. | N. and S. | 53 20 | .670 | | | | 10 30 | N. | 31 1' 5" | .679 | | | | 11 0 | Noon. | weight 1 grain. | 15 28 | .932 | | | | 6 30 A.M. | S. | 37 11 | .681 | | | | 9 30 | S. | 37 11' 7" | .687 | | | | 11 0 | weight 1 grain. | 15 30' 2" | .678 | | | | 11 30 | weight 1 grain. | 15 32' 2" | .682 | | | | 6 32 57' 14 00' | S. | 36 39' 5" | .683 | | | | 7 32 33' 15 24' | S. | 36 25' 4" | .685 | | | | 8 32 17' 17 0' | Noon. | S. | 36 13 | .691 | | | | 9 32 31' 17 41' | Noon. | S. | 36 33' 2" | .694 | | | | 10 32 45' 16 37' | 0 30 P.M. | S. | 36 29' 5" | .697 | | | | 11 33' 3 16 46' | 10 0 A.M. | S. | 36 2' 4" | .697 | | | | 12 33' 17' 16 51' | 10 0 A.M. | S. | 35 54' 2" | .697 | | | | 10 30 | S. | 35 47' 2" | .697 | | | | 3 30 | S. | 35 55' 5" | .697 | | | | 3 50 | S. | 50 56' 6" | .697 | | | | 4 10 | weight 1 grain. | 29 19' 5" | .697 | | | | 4 30 | weight 1 grain. | 14 36' 7" | .697 | | | | 10 A.M. | S. | 14 42 | .697 | | | | 10 30 | S. | 35 20 | .697 | | | | 11 0 | S. | 50 57' 2" | .697 | | | | 11 30 | weight 1 grain. | 29 28 | .697 | | | | weight 1½ grain. | 14 42' 2" | .697 | | | | Noon. | S. | 22 0' 2" | .697 | | | | 0 30 P.M. | S. | 35 20' 2" | .697 | | | | 1 30 | S. | 35 21 | .697 | | | | 1 50 | S. | 50 55' 2" | .697 | | | | 2 10 | S. | 29 26 | .697 | | | | 2 30 | weight 1 grain. | 14 40' 3" | .697 | | | | weight 1 grain. | 35 38' 9" | .697 | | | | 10 A.M. | S. | 35 34' 5" | .697 | | | | 10 A.M. | S. | 51 9' 7" | .697 | | | | 10 20 | S. | 29 37' 7" | .697 | | | | 10 40 | S. | 14 39' 7" | .697 | | | | 0 30 P.M. | weight 1 grain. | 14 34' 2" | .697 | | | | 1 0 | weight 1 grain. | 35 46' 2" | .697 | | | | S. | 36 1' 5" | .697 | | | | S. | 35 59' 5" | .697 | | | | S. | 35 53' 2" | .697 | | | | S. | 35 39' 4" | .697 | | | | S. | 35 17 | .697 | | | | S. | 35 47' 6" | .697 | | | | S. | 35 35 | .697 | | | | S. | 35 25' 8" | .697 | | | | S. | 35 1 | .697 | | | | Noon. | S. | 34 56' 2" | .697 | | | | S. | 35 45 | .697 | | | | S. | 35 24 | .697 | | | | S. | 35 31' 8" | .697 | | | | S. | 35 35 | .697 | | | | S. | 35 24' 8" | .697 | | | | 4 P.M. | S. | 35 26' 3" | .697 | | | | 6 P.M. | S. | 35 39' 2" | .697 | | | | S. | 35 39' 7" | .697 | | | | On shore. | .697 | Observations in the Terror.—The observations in the Terror were made with a Fox's needle of four inches diameter; one of equal size with that in the Erebus, which was not ready when the Expedition sailed, having been sent out subsequently, and received by Captain Crozier, at Van Diemen Island, in August 1840. An instrument of only two inches radius, for the purpose of determining both the dip and intensity at sea, might previously have been regarded by many persons as scarcely more than a philosophical toy; and as little likely to yield results having the precision which is now required in such determinations. It has, however, in Captain Crozier's hands, fully justified the expectations which Mr. Fox, from his own experiments with it, had ventured to entertain. On land, the instruments of the Erebus and Terror are, for the most part, as far as they have yet reached us, nearly identical in their results. Confining our attention to the intensity as the subject immediately before us, the intensities at James Town in St. Helena, and at Longwood in the same island, measured by the instruments of the two ships, the days and spots of observation being the same, are by the Erebus in the proportion of '586 at Longwood to '611 at James Town, and by the Terror as '587 to '611. The agreement is in this case the more valuable, because we are justified by it in ascribing the difference thus found between places so geographically near to each other, to a really existing difference (viz. to station error), rather than to accident or to observation error, as might have been done, had only a single instrument been used. A similar accord in the determinations of the two instruments is shown by the results at the Cape of Good Hope and Kerguelen Island, which, though more properly belonging to the next section, may be instanced here in evidence of the confirmation which the two instruments * Omitted in the mean. mutually afford each other; the Erebus making the intensity on shore at Christmas Harbour, in Kerguelen Island, to be as $1.068$ to $0.715$ at Simon's Bay, and the Terror as $1.0675$ to $0.715$. At sea, where in consequence of the motion of the ship, the inferior size of the four-inch instrument cannot be compensated by additional time given to the reading, or by other arrangements conducing to minute accuracy, the probable error of a single result appears, as might be expected, to be somewhat greater with the four inch than with the $7\frac{1}{2}$-inch needle; but, with this reservation, the observations made at sea with the two instruments, when the ships were in company, are highly confirmatory each of the other. The table which has been received from the Terror contains almost daily observations from the 1st of January 1840 until her arrival at the Cape of Good Hope in the following March. The intensities were observed both by deflectors and by constant weights, which latter, in the four-inch instrument, were $.5$ and $.3$ of a grain. St. Helena is the only land station observed at in the passage; the intensities observed at sea have therefore been computed relatively to the observations at James Town St. Helena, taken as a base station; and the value of the intensity at James Town has been taken as determined by the Erebus, viz. as $0.611$, to unity in London, or as $0.838$ to $1.372$ in London. The values of $w'$ for the Terror's deflectors N. and S, required, instead of a table of coercing weights, in computing the intensities when the deflectors were used, have been derived by the method already explained. Tables of these values for each degree of deflection are subjoined: neither on a careful examination of the observations, nor in the process of forming these tables, does there appear any reason to suppose that the deflectors or the needle sustained any change in magnetic condition during the period embraced by the observations under notice. | Values of $w'$, Terror's Deflectors. | |-------------------------------------| | N. | S. | | gr. | gr. | | $38 = .837$ | $33 = .732$ | | $39 = .828$ | $34 = .726$ | | $40 = .819$ | $35 = .721$ | | $41 = .811$ | $36 = .716$ | | $42 = .803$ | $37 = .711$ | | $43 = .797$ | $38 = .707$ | | $44 = .7925$ | $39 = .703$ | | $45 = .791$ | $40 = .700$ | | $46 = .7895$ | | | $47 = .7885$ | | With the values of $w'$ taken from this Table, the intensities at sea are obtained relatively to $0.611$ at St. Helena (London = 1), by the formula $$I' = .5398 \cdot w' \csc v'.$$ ### Table II Observations of the Magnetic Intensity on Shore, and on Board Her Majesty's Ship Terror, with a four-inch Fox's Needle. | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|---------------------|-------------|-------------|-----------| | | | | | | | | London = 1·000. = 1·372. | | 1840 | | | | | | | | | Jan. | | | | | | | | | 1. | -28° 17' | 339" 59' | h m | N. | 46° 27' | 77° | E.S.E. | '587 | | | | | | S. | 38° 54' | 77° | | '604 | | | | | | weight ·5 grain. | 26° 50' | 77° | | '597 | | | | | | weight ·3 grain. | 16° 20' | 77° | | '575 | | | | | | N. | 46° 02' | 76° | E. I ½ N. | '593 | | | | | | S. | 39° 57' | 76° | | '588 | | | | | | weight ·5 grain. | 27° 31' | 76° | | '587 | | | | | | weight ·3 grain. | 16° 08' | 76° | | '583 | | | | | | N. | 45° 35' | 76° | N.E. ½ E. | '582 | | | | | | S. | 38° 57' | 76° | | '596 | | | | | | weight ·5 grain. | 27° 18' | 76° | | '603 | | | | | | weight ·3 grain. | 15° 33' | 76° | | '587 | | | | | | N. | 45° 40' | 76° | N.E. by E.| '595 | | | | | | S. | 39° 01' | 76° | | '602 | | | | | | weight ·5 grain. | 27° 20' | 76° | | '587 | | | | | | weight ·3 grain. | 15° 41' | 76° | | '595 | | | | | | N. | 45° 03' | 76° | N.E. | '602 | | | | | | S. | 39° 03' | 76° | | '601 | | | | | | weight ·5 grain. | 26° 55' | 76° | | '595 | | | | | | weight ·3 grain. | 15° 33' | 76° | | '603 | | | | | | N. | 45° 25' | 76° | N.E. by N.| '598 | | | | | | S. | 39° 07' | 76° | | '600 | | | | | | weight ·5 grain. | 26° 18' | 76° | | '607 | | | | | | weight ·3 grain. | 15° 28' | 76° | | '607 | | | | | | N. | 45° 27' | 76° | | '598 | | | | | | S. | 39° 26' | 76° | | '596 | | | | | | weight ·5 grain. | 26° 27' | 76° | | '605 | | | | | | weight ·3 grain. | 16° 27' | 76° | | '593 | | | | | | N. | 45° 34' | 73° | N.E. ½ N.| '571 | | | | | | S. | 40° 10' | 73° | | '596 | | | | | | weight ·5 grain. | 26° 57' | 73° | | '585 | | | | | | weight ·3 grain. | 15° 34' | 73° | | '594 | | | | | | N. | 46° 06' | 76° | E.N.E. | '594 | | | | | | S. | 41° 05' | 76° | | '602 | | | | | | weight ·5 grain. | 26° 32' | 76° | | '591 | | | | | | weight ·3 grain. | 16° 00' | 76° | | '574 | | | | | | N. | 46° 21' | 74° | E. I ½ N.| '589 | | | | | | S. | 40° 23' | 74° | | '603 | | | | | | weight ·5 grain. | 27° 48' | 74° | | '586 | | | | | | weight ·3 grain. | 16° 07' | 74° | | '586 | | | | | | N. | 46° 04' | 76° | N.N.F. | '588 | | | | | | S. | 40° 02' | 76° | | '583 | | | | | | weight ·5 grain. | 26° 39' | 76° | | '591 | | | | | | weight ·3 grain. | 15° 40' | 76° | | '587 | | | | | | N. | 45° 51' | 78° | N.E. | '593 | | | | | | S. | 39° 48' | 78° | | '594 | | | | | | S. | 39° 21' | 78° | | '590 | | | | | | weight ·5 grain. | 26° 51' | 78° | | '597 | | | | | | weight ·3 grain. | 15° 47' | 78° | | '594 | | | | | | N. | 46° 05' | 76° | N.E.by E.½ E.| '591 | | | | | | S. | 40° 25' | 76° | | '583 | | | | | | weight ·5 grain. | 26° 53' | 76° | | '596 | | | | | | weight ·3 grain. | 15° 38' | 76° | | '600 | | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | | | | | | | | London = 1·000. = 1·372. | | 1840 | Jan. 13. | 16° 43' 34" | 02 h 10 m A.M. | N. 45° 56' 74" | E. 1/2 N. | .593 | | | | | | | | | .595 | | | | | | | | | .594 | | | | | | | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 45° 39' 74" | | | .591 | | | | | | S. 39° 32' 74" | | | .591 | | | | | | S. 39° 45' 74" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 46° 50' 76" | E.N.E. | | .591 | | | | | | S. 39° 17' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 46° 18' 76" | N.E. by E. | | .591 | | | | | | S. 39° 04' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 45° 26' 76" | S.W. by S. | | .591 | | | | | | S. 38° 12' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 45° 26' 76" | E. by N. | | .591 | | | | | | S. 39° 43' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 46° 10' 76" | | | .591 | | | | | | S. 39° 24' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 45° 19' 76" | | | .591 | | | | | | S. 39° 01' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 45° 39' 76" | S.W. 1/2 W. | | .591 | | | | | | S. 38° 49' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 46° 09' 76" | | | .591 | | | | | | S. 39° 04' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 46° 06' 76" | | | .591 | | | | | | S. 39° 14' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 45° 23' 76" | | | .591 | | | | | | S. 38° 59' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 45° 32' 76" | | | .591 | | | | | | S. 38° 37' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | | | | | N. 45° 21' 76" | | | .591 | | | | | | S. 38° 19' 76" | | | .591 | | | | | | weight ·5 grain. | | | .591 | | | | | | weight ·3 grain. | | | .591 | | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | | | | | | | | London = 1·000. = 1·372. | | 1840 | Jan. 25. | -14° 55' 35" | 9 15 A.M. | N. | 45° 36' | 77° | s.s.w. | .596 | .608 | .604 | .829 | | | | | | S. | 38° 40' | 77° | | .600 | .611 | .601 | .830 | | | | | | weight ·5 grain. | 26° 39' | 77° | | .610 | .605 | .830 | | | | | | weight ·3 grain. | 15° 21' | 77° | | .604 | | | | | | | | | N. | 45° 08' | 77° | | .597 | .611 | .602 | .825 | | | | | | S. | 38° 32' | 77° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 26° 25' | 77° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 31' | 77° | | .612 | .607 | .834 | | | | | | N. | 45° 31' | 77° | E.N.E. | .597 | .602 | .604 | .829 | | | | | | S. | 38° 27' | 77° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 26° 33' | 77° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 45' | 77° | | .612 | .607 | .834 | | | | | | N. | 45° 0 | 77° | s. ½ W. | .597 | .602 | .604 | .829 | | | | | | S. | 38° 41' | 77° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 25° 40' | 77° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 18' | 77° | | .612 | .607 | .834 | | | | | | N. | 45° 17' | 76° | s.s.w. | .597 | .602 | .604 | .829 | | | | | | S. | 38° 30' | 76° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 26° 29' | 76° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 30' | 76° | | .612 | .607 | .834 | | | | | | N. | 45° 29' | 76° | E.N.E. | .597 | .602 | .604 | .829 | | | | | | S. | 39° 02' | 76° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 25° 56' | 76° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 18' | 76° | | .612 | .607 | .834 | | | | | | N. | 45° 58' | 78° | N.E. by E. | .597 | .602 | .604 | .829 | | | | | | S. | 39° 14' | 78° | | .603 | .601 | .838 | | | | | | weight ·3 grain. | 15° 36' | 78° | | .608 | .621 | .611 | .838 | | | | | | N. | 45° 39' | 78° | s. ½ W. | .597 | .602 | .604 | .829 | | | | | | S. | 38° 28' | 78° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 26° 19' | 78° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 58' | 78° | | .612 | .607 | .834 | | | | | | N. | 45° 47' | 77° | s.s.w. | .597 | .602 | .604 | .829 | | | | | | S. | 38° 37' | 77° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 26° 12' | 77° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 10' | 77° | | .612 | .607 | .834 | | | | | | N. | 45° 03' | 78° | s. by w. | .597 | .602 | .604 | .829 | | | | | | S. | 39° 17' | 78° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 27° 02' | 78° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 41' | 78° | | .612 | .607 | .834 | | | | | | N. | 44° 21' | 81° | | .597 | .602 | .604 | .829 | | | | | | S. | 39° 09' | 81° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 26° 09' | 81° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 21' | 81° | | .612 | .607 | .834 | | | | | | N. | 46° 07' | 70° | | .597 | .602 | .604 | .829 | | | | | | S. | 39° 09' | 70° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 27° 10' | 70° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 16° 07' | 70° | | .612 | .607 | .834 | | | | | | N. | 44° 42' | 77° | s.w. by s. | .597 | .602 | .604 | .829 | | | | | | S. | 39° 09' | 77° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 26° 46' | 77° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 15° 55' | 77° | | .612 | .607 | .834 | | | | | | N. | 45° 12' | 76° | s.w. | .597 | .602 | .604 | .829 | | | | | | S. | 39° 04' | 76° | | .603 | .601 | .838 | | | | | | weight ·5 grain. | 26° 20' | 77° | | .608 | .621 | .611 | .838 | | | | | | weight ·3 grain. | 16° 02' | 77° | | .612 | .607 | .834 | | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | 1840 | Feb. 13 | 21° 55' 35" | h m | N | 44° 52' 5" | 76 | S.S.E. | London = 1·000. = 1·372. | | | | 9 10 A.M. | | N | 45° 04' 3" | 76 | | .603 | .828 | | | | 9 24 A.M. | | N | 45° 03' 6" | 76 | | .602 | .826 | | | | 9 55 A.M. | | N | 45° 03' 2" | 76 | | s. by w. | | | | | 10 10 A.M. | | N | 45° 01' 2" | 76 | | s. ½ w. | | | | | 10 24 A.M. | | N | 45° 01' 3" | 76 | | | | | | | 10 36 A.M. | | N | 44° 58' 5" | 76 | | | | | | | 10 55 A.M. | | N | 45° 06' 2" | 76 | | | | | | | 11 05 A.M. | | N | 45° 07' 5" | 76 | | | | | | | 1 20 P.M. | | N | 45° 03' 8" | 76 | | | | | | | 1 35 P.M. | | N | 45° 03' 6" | 76 | | | | | | | 1 55 P.M. | | N | 45° 10 | 76 | | | | | | | 2 10 P.M. | | N | 45° 00' 2" | 74 | | | | | | | 5 55 P.M. | | N | 45° 00' 6" | 74 | | | | | | | 6 10 P.M. | | N | 26 36 | 77 | W. | | | | | | 9 00 A.M. | weight ·5 grain.| S | 16 05 | 77 | | | | | | | | weight ·3 grain.| S | 39 02 | 77 | | | | | | | | | N | 45 50 | 77 | | | | | | | | | N | 45 40 | 77 | | | | | | | | | N | 45 46 | 77 | | | | | | | | | N | 45 47 | 77 | | | | | | | | | N | 45 52 | 77 | | | | | | | | | N | 45 47 | 77 | | | | | | | | | N | 45 40 | 77 | | | | | | | | | N | 45 49 | 77 | | | | | | | | | N | 27 08 | 77 | | | | | | | | | N | 16 14 | 77 | | | | | | | | | S | 39 43' 5" | 77 | | | | | | | | | N | 45 24 | 77 | | | | | | | | | N | 45 26 | 77 | | | | | | | | | N | 45 22 | 78 | | | | | | | | | N | 45 29 | 78 | | | | | | | | | N | 45 21 | 80 | | | | | | | | | N | 45 32' 5" | 80 | | | | | | | | | N | 45 38 | 76 | | | | | | | | | N | 45 41 | 76 | | | | | | | | | N | 45 28 | 75 | | | | | | | | | N | 45 35 | 75 | | | | | | | | | N | 44 41 | 80 | | | | | | | | | N | 44 41 | 80 | | | | | | | | | S | 38 48 | 80 | | | | | | | | | N | 26 26 | 80 | | | | | | | | | N | 16 12' 5" | 80 | | | | | | | | | N | 45 05 | 77 | | | | | | | | | N | 45 01 | 77 | | | | | | | | | S | 38 45 | 77 | | | | | | | | | N | 26 53 | 77 | | | | | | | | | N | 15 43' 5" | 77 | | | | | | | | | N | 44 44' 7" | 78 | | | | | | | | | S | 38 48' 0" | 78 | | | | | | | | | N | 27 13' 7" | 78 | | | | | | | | | N | 15 13' 1" | 78 | | | | | | | | | N | 44 34' 7" | 78 | | | | | | | | | S | 38 01' 9" | 78 | | | | | | | | | N | 26 28' 7" | 78 | | | | | | | | | N | 15 46' 9" | 78 | | | | | | | | | N | 44 11' 2" | 77 | | | | | | | | | S | 38 18 | 77 | | | | | | | | | N | 26 02 | 77 | | | | | | | | | N | 15 08 | 77 | | | | | Year | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|------------|-----------| | | | | | | | | London = 1·000. | 1·372 | | Feb. | | | | | | | | | | 21. | -29°56'35"42" | 8 45 A.M. | N. | 44°24'79" | S.E. | -610 | -611 | -838 | | | | | S. | 38°28'79" | | -611 | -838 | | | 22. | -31°00'35"40" | 9 20 A.M. | N. | 43°18'57" | S.E. ½ E. | -626 | -625 | -856 | | | | | S. | 37°39'57" | | -625 | -856 | | | 23. | -31°40'35"30" | 9 10 A.M. | N. | 43°29'57" | S.E. by E. | -632 | -626 | -857 | | | | | S. | 37°21'17" | | -630 | -857 | | | 24. | -31°13'35"26" | 9 15 A.M. | N. | 43°34'79" | E. ½ S. | -621 | -628 | -862 | | | | | S. | 37°03'79" | | -635 | -862 | | | 25. | -30°10'35"53" | 9 00 A.M. | N. | 43°14'75" | E.N.E. | -627 | -627 | -858 | | | | | S. | 37°33'75" | | -625 | -858 | | | | | | weight ·5 grain.| 25°31'75" | | -622 | -858 | | | | | | weight ·3 grain.| 15°04'75" | | -622 | -858 | | | 25. | -30°12'35"54" | 10 00 A.M. | N. | 43°04'75" | S.S.W. | -629 | -638 | -876 | | | | | S. | 36°23'75" | | -647 | -876 | | | 26. | -31°07'35"28" | 9 20 A.M. | N. | 42°21'71" | S. by W. | -641 | -641 | -876 | | | | | S. | 36°18'71" | | -649 | -876 | | | | | | weight ·5 grain.| 25°21'71" | | -649 | -876 | | | | | | weight ·3 grain.| 14°32'57" | | -649 | -876 | | | 27. | -31°15'35"48" | 9 10 A.M. | N. | 42°45'27" | S.E. | -634 | -634 | -876 | | | | | S. | 37°01'71" | | -636 | -876 | | | | | | weight ·5 grain.| 24°23'71" | | -636 | -876 | | | | | | weight ·3 grain.| 14°45'71" | | -636 | -876 | | | 28. | -31°57'20"02" | 9 00 A.M. | N. | 42°15'67" | S.E. ½ S. | -643 | -647 | -888 | | | | | S. | 36°11'57" | | -651 | -888 | | | 29. | -32°35'42"00" | 9 40 A.M. | N. | 41°29'47" | S.E. | -656 | -650 | -892 | | | | | S. | 36°33'47" | | -656 | -892 | | | March | 1. | 33°02'54"00"| 9 00 A.M. | N. | 41°06'71" | S.E. | -664 | -670 | -919 | | | | | S. | 34°59'71" | | -664 | -919 | | | 2. | -33°20'73"20"| 9 10 A.M. | N. | 41°01'57" | S.E. | -666 | -668 | -917 | | | | | S. | 35°19'71" | | -666 | -917 | | | 3. | -33°10'93"00"| 9 30 A.M. | N. | 40°54'57" | N.E. | -668 | -674 | -923 | | | | | S. | 35°09'67" | | -668 | -923 | | | | | | weight ·5 grain.| 34°41'87" | | -668 | -923 | | | | | | weight ·3 grain.| 23°39'37" | | -668 | -923 | | | 4. | -33°03'95"05"| 9 05 A.M. | N. | 40°16'16" | S.E. ½ S. | -681 | -685 | -940 | | | | | S. | 34°24'26" | | -690 | -940 | | | 5. | -33°08'11"43"| 9 15 A.M. | N. | 39°45'68" | S.E. by S. | -693 | -698 | -957 | | | | | S. | 33°53'88" | | -702 | -957 | | | 6. | -32°56'13"48"| 9 15 A.M. | N. | 39°35'70" | S.E. by E. | -696 | -699 | -959 | | | | | S. | 33°51'17" | | -703 | -959 | | | 7. | -32°14'15"20"| 9 30 A.M. | N. | 39°10'57" | E.S.E. | -705 | -707 | -970 | | | | | S. | 32°35'70" | | -709 | -970 | | | 8. | -32°16'16"52"| 9 20 A.M. | N. | 39°17'28" | S.E. ½ S. | -703 | -711 | -975 | | | | | S. | 33°12'68" | | -719 | -975 | | | 9. | -32°31'17"45"| 9 45 A.M. | N. | 39°49'61" | N.W. | -691 | -698 | -957 | | | | | S. | 33°43'21" | | -706 | -957 | | | 10. | -32°44'16"27"| 9 00 A.M. | N. | 38°32'96" | S.E. | -720 | -724 | -994 | | | | | S. | 32°51'63" | | -728 | -994 | | | 11. | -33°01'16"41"| 9 20 A.M. | N. | 39°03'69" | S.E. ½ E. | -708 | -712 | -976 | | | | | S. | 33°21'69" | | -716 | -976 | | | 12. | -33°13'16"46"| 3 45 P.M. | N. | 38°23'69" | S.S.E. | -724 | -732 | 1·004 | | | | | S. | 32°18'69" | | -740 | -732 | | | 13. | -33°52'18"04"| 9 0 A.M. | N. | 38°35'65" | s. by W. | -719 | -722 | -991 | | | | | S. | 33°00'65" | | -725 | -991 | | | 14. | -34°16'17"34"| 9 30 A.M. | N. | 38°31'81" | E. by N. | -721 | -712 | -976 | | | | | S. | 33°50'71" | | -703 | -976 | | Minimum Intensity.—In the passage from England to the Cape of Good Hope the Expedition traversed three times that large space of the Atlantic in which the magnetic intensity is less than in any other part of the surface of the globe; first in a southerly course, in and about the meridian of $330^\circ$ E.; a second time in beating up to St. Helena, in and about the meridian of $345^\circ$ E.; and a third time in the course from St. Helena to the Cape of Good Hope, in and about the meridian of $350^\circ$ E. Before we examine more particularly the results of the observations made during these traverses, it will be proper to clear them from the effects of the ship’s iron, as far as the data furnished will enable us to do so. It is obvious, on a simple inspection of the results in the tables, that, in the southern hemisphere, when the ship’s head was on the points from S.E. to S.W., the intensity observed was generally slightly in excess, and on the contrary, when on the points from N.E. to N.W., slightly in defect; and that such was the case in both ships. At St. Helena and at the Cape of Good Hope, an endeavour was made to ascertain more precisely the effect of the ship’s iron in modifying the results, by placing the ship’s head successively on the principal points of the compass, and observing the intensity in each position. At St. Helena, the experiment failed, owing, apparently, to the disturbing influence of the island itself, which, even at the distance at which the vessels were anchored, was found to be sufficient to mask the local attraction of the ship, and to produce anomalies which were not experienced at sea. At the Cape, the geological character of the land interposed no such difficulty. The following Table shows the differences found at Simon’s Bay between the intensity observed on each of the sixteen principal points of the compass, and the arithmetical mean of the whole. Each difference has the sign prefixed which would be required for a correction to the arithmetical mean. Allowing for discrepancies incidental to single observations, the general aspect of the differences is sufficiently systematic to justify us in regarding them as principally occasioned by the influence of the ship’s iron. | Ship's head | Corrections | Mean of the two ships | Corrections | Mean of the two ships | |-------------|-------------|-----------------------|-------------|----------------------| | | Erebus | Terror | | | | E. | +·002 | +·001 | W. | −·000 | | E.S.E. | +·006 | +·004 | W.N.W. | 000 | | S.E. | −·002 | −·004 | N.W. | +·005 | | S.S.E. | +·001 | −·004 | N.N.W. | −·004 | | S. | −·007 | −·015 | N. | +·002 | | S.S.W. | −·015 | −·015 | N.N.E. | +·003 | | S.W. | −·012 | −·007 | N.E. | +·007 | | W.S.W. | +·005 | −·004 | E.N.E. | +·001 | | Ship's head | Corrections | Mean of the two ships | |-------------|-------------|-----------------------| | | Erebus | Terror | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This experiment was repeated at Kerguelen Island, but with the ship's head placed only on the eight principal points: the season and weather were unfavourable, and the errors of observation were consequently greater than at the Cape; but the general indication is the same: the results are as follows: | Ship's head | Differences from the mean | Mean of the two ships | |-------------|--------------------------|----------------------| | | Erebus | Terror | | | E. | + ·008 | - ·003 | + ·002 | | S.E. | - ·007 | 000 | - ·003 | | S. | - ·016 | - ·027 | - ·021 | | S.W. | - ·014 | + ·001 | - ·006 | | W. | + ·006 | + ·007 | + ·006 | | N.W. | 000 | + ·008 | + ·004 | | N. | + ·007 | + ·005 | + ·006 | | N.E. | + ·014 | + ·008 | + ·011 | The experiments at the Cape and Kerguelen Island agree in indicating the points of greatest disturbance to be from S.E. (through south) to S.W., causing an augmentation,—and from N.E. (through north) to N.W., causing a diminution,—in the regular magnetic intensity; the augmentation on the southerly points being rather greater than the diminution on the northerly, compensated by there being a greater number of the remaining points in defect than in excess; the latter points being affected in a minor degree. Considering that the differences shown in the two last Tables necessarily combine the errors of observation with the influence of local attraction, we may regard the effect of the ship's iron on the intensity needle as probably amounting, in extreme cases, to $\frac{1}{100}$th of the earth's magnetic force; but on much the greater number of points as probably far less than that amount. In Table III., which contains the results of the almost daily observations made in the Erebus between December 7, 1839, and February 29, 1840, in the space of the Atlantic comprised between the two portions of the isodynamic curve of 0·9, I have employed the following scale of correction*: \[ \begin{align*} \text{Ship's head} & \quad \text{Correction} \\ \text{S.W. by S. to S.E. by S.} & \quad - ·008 \\ \text{S.W. and S.E.} & \quad - ·007 \\ \text{W.S.W. and E.S.E.} & \quad - ·002 \\ \text{W. by S. and E. by S.} & \quad 000 \\ \text{Ship's head} & \quad \text{Correction} \\ \text{W. and E.} & \quad + ·001 \\ \text{W. by N. and E. by N.} & \quad + ·002 \\ \text{W.N.W. and E.N.E.} & \quad + ·003 \\ \text{N.W. and N.E.} & \quad + ·004 \\ \text{N.W. by N. to N.E.by N.} & \quad + ·005 \end{align*} \] * After the communication of this paper to the Royal Society, I received from Captain Crozier the results of a third experiment of the same kind, made in the Terror on the 20th of October 1840 in the river Derwent in Van Diemen Island. I subjoin these results in further evidence of the general correctness of the deductions which have been drawn in regard to the influence of the ship's iron on the intensity observations made at sea. | Ship's head | Correction | Ship's head | Correction | |-------------|-----------|-------------|-----------| | E. | + ·001 | W. | + ·003 | | E.S.E. | - ·001 | W.N.W. | 000 | | S.E. | - ·005 | N.W. | + ·007 | | S.S.E. | - ·008 | N.N.W. | + ·005 | | S. | - ·008 | N. | + ·004 | | S.S.W. | - ·007 | N.N.E. | + ·005 | | S.W. | - ·005 | N.E. | + ·008 | | W.S.W. | + ·001 | E.N.E. | + ·002 | MDCCCXLII. ### Table III. Abstract of the Intensities observed in Her Majesty's Ship Erebus, in the space of the Atlantic comprised within the isodynamic curve of 0°9. | Position | Intensity | Ship's head | Correction for ship's head | Corrected intensity. London = 1°372. | |----------|-----------|-------------|---------------------------|-------------------------------------| | Lat. | Long. E. | | | | | -4° 49' 32" 43' | .921 | s.s.w. | -.008 | .913 | | -6° 24' 32" 24' | .897 | s. | -.008 | .889 | | -7° 50' 32" 28' | .888 | s. | -.008 | .880 | | -9° 21' 32" 58' | .870 | s.s.e. | -.008 | .862 | | -11° 03' 32" 22' | .856 | s.s.e. | -.008 | .848 | | -12° 32' 32" 57' | .840 | s.e. by s. | -.008 | .832 | | -14° 03' 28" 82' | .822 | s.s.e. | -.008 | .814 | | -15° 04' 33" 06' | .822 | s.s.e. | -.008 | .814 | | -16° 52' 33" 27' | .818 | s.by e. | -.008 | .810 | | -19° 01' 33" 45' | .814 | s. ½ E. | -.008 | .806 | | -21° 31' 33" 47' | .818 | s. | -.008 | .810 | | -21° 47' 33" 50' | .819 | s. | -.008 | .811 | | -23° 14' 33" 55' | .823 | s. ½ W. | -.008 | .815 | | -24° 22' 33" 51' | .829 | s.e. by s. | -.008 | .821 | | -25° 40' 33" 46' | .828 | s.e. by e. | -.005 | .823 | | -26° 50' 33" 30' | .839 | s.e. by e. | -.004 | .835 | | -26° 12' 33" 22' | .833 | s.e. by e. | -.007 | .826 | | -27° 04' 33" 13' | .836 | s.e. by e. | -.005 | .831 | | -27° 43' 33" 06' | .845 | s.e. by e. | -.002 | .843 | | -26° 53' 33" 26' | .840 | s.s.e. | -.002 | .838 | | -26° 01' 33" 20' | .833 | n.by e. | +.005 | .838 | | -25° 21' 33" 28' | .826 | n.e. by n. | +.005 | .819 | | -26° 12' 33" 12' | .829 | s.e. by e. | -.005 | .824 | | -27° 04' 33" 25' | .832 | s.e. | -.007 | .825 | | -27° 43' 33" 37' | .833 | s.e. | -.002 | .831 | | -28° 19' 34" 10' | .828 | e.by s. | .000 | .828 | | -28° 01' 34" 45' | .828 | e.n.e. | +.003 | .831 | | -27° 26' 34" 29' | .830 | n.e. by e. | +.003 | .833 | | -26° 47' 34" 57' | .824 | n.e. by n. | +.005 | .829 | | -25° 34' 34" 58' | .823 | n.n.e. | +.005 | .828 | | -24° 10' 34" 04' | .812 | n.e. | +.005 | .817 | | -22° 40' 34" 42' | .811 | n.e. ½ E. | +.004 | .815 | | -21° 30' 34" 17' | .810 | n.e. | +.003 | .813 | | -20° 20' 34" 12' | .810 | n.e. | +.003 | .813 | | -18° 53' 34" 46' | .817 | n.e. by e. | +.003 | .820 | | -17° 30' 34" 15' | .820 | n.e. | +.003 | .823 | | -17° 11' 34" 40' | .817 | n.e. | +.003 | .820 | | -16° 30' 34" 17' | .816 | n.e. | +.004 | .820 | | -15° 15' 34" 01' | .821 | n.e. by e. ½ E. | +.003 | .824 | | -15° 20' 34" 07' | .813 | n.e. | +.003 | .812 | | -15° 49' 34" 09' | .815 | s.s.w. | +.003 | .812 | | Position | Intensity | Ship's head | Correction for ship's head | Corrected intensity. London = 1°372. | |----------|-----------|-------------|---------------------------|-------------------------------------| | Lat. | Long. E. | | | | | -15° 30' 34" 51' | .810 | e. | +.001 | .811 | | -14° 33' 34" 37' | .816 | n.e. by e. | -.001 | .815 | | -13° 37' 35" 33' | .822 | s.w. by s. | -.005 | .817 | | -14° 19' 35" 33' | .813 | e. by n. | +.002 | .815 | | -14° 51' 35" 31' | .815 | e. w. | +.003 | .818 | | -14° 08' 35" 31' | .825 | n.w. | +.002 | .823 | | -13° 43' 35" 01' | .823 | s.by w. ½ W. | -.008 | .815 | | -14° 26' 35" 57' | .825 | s. | -.008 | .817 | | -15° 04' 35" 54' | .825 | s.by w. | -.008 | .811 | | -15° 23' 35" 06' | .819 | s.by w. | -.008 | .812 | | -15° 17' 35" 35' | .820 | s.e.n.e. | +.003 | .823 | | -15° 19' 35" 13' | .820 | s.s.w. | -.008 | .821 | | -15° 07' 35" 44' | .829 | s.by w. | -.008 | .817 | | -15° 05' 35" 08' | .825 | s.by w. | -.008 | .816 | | -17° 26' 35" 28' | .823 | s.w. | -.007 | .816 | | -18° 53' 35" 45' | .827 | s.s.w. | -.008 | .819 | | -20° 51' 35" 55' | .823 | s.by e. | -.008 | .815 | | -22° 02' 35" 27' | .821 | s.by w. | -.008 | .813 | | -22° 21' 35" 22' | .818 | s.by w. | -.008 | .810 | | -23° 35' 35" 05' | .821 | s.s.w. | -.002 | .819 | | -23° 47' 35" 29' | .818 | w. | +.001 | .819 | | -24° 34' 34" 39' | .818 | w. | +.001 | .819 | | -24° 46' 34" 12' | .820 | w. | -.001 | .819 | | -25° 19' 34" 54' | .820 | s.w. | -.007 | .813 | | -25° 31' 34" 45' | .829 | s.w. | -.007 | .822 | | -26° 08' 34" 03' | .826 | w.e. | -.002 | .824 | | -26° 03' 34" 32' | .844 | s.s.w. | -.008 | .836 | | -27° 03' 34" 46' | .844 | s.s.w. | -.007 | .827 | | -27° 56' 34" 44' | .836 | s.e. | -.007 | .829 | | -29° 02' 34" 46' | .836 | s.e. | -.007 | .829 | | -30° 14' 35" 37' | .852 | s.e. | -.007 | .845 | | -31° 08' 35" 56' | .849 | s.e. ½ E. | -.006 | .843 | | -31° 46' 35" 38' | .872 | s.e. by e. | -.006 | .866 | | -31° 13' 35" 38' | .872 | e.by s. | .000 | .872 | | -30° 20' 35" 50' | .878 | e. | -.005 | .873 | | -31° 13' 35" 39' | .882 | s.e. | -.007 | .875 | | -32° 01' 2' 17' | .886 | s.w. | -.007 | .879 | | -32° 54' 4' 53' | .914 | s.e. | -.007 | .907 | When these results are transferred to the map of the portion of the Atlantic to which they refer, and are examined in detail, their systematic character becomes much more obvious than in the Table, where, in consequence of the successive alternations of increasing and decreasing latitude, their consistency is not so easily followed by the eye. On attentive examination of the map it is not difficult to trace within small limits the course of an ideal line, which should connect the points in the several meridians, where the intensity was weakest at the epoch of Captain Ross's voyage. The determination of the position of this line is easier, and in some respects more sure, than that of an isodynamic line, because it is independent of the permanency of the magnetism of the needle employed, for more than the few days occupied in the immediate research; and it is also independent of the correctness of an assumed intensity at a base station. It is therefore to be expected that the position of this line will become in future years the subject of frequent examination, serving to mark, from time to time, the progress of the secular change in its position. This may be done with the more interest and advantage, because there is reason to believe that its position is changing rapidly in the space referred to, particularly in the eastern meridians; and that the southern magnetic hemisphere, in so far as its boundary may be indicated by this line, is in that quarter of the globe gaining rapidly upon the northern. In the first of the present series of "Contributions"*, the line of least intensity was drawn from observations corresponding nearly to the epoch of 1825, and this line of 1825 is lightly retraced in the present map for the purpose of comparison. It will be seen, that whilst its general direction is consistent with the observations of Captain Ross in 1840, its earlier position is everywhere three or four degrees south of that which would be now inferred. It is readily admitted that many of the observations from which the line of 1825 was drawn are inferior in precision to those of Captain Ross; and I rejoice in the late improvement in this class of observations, for which we are mainly indebted to the method and instrument devised by Mr. Fox, and to the zeal and unwearied patience of our naval officers. To an observer, however, who is proceeding in a nearly north and south direction, very little uncertainty attends the determination of the time and place at which he finds the weakest intensity; and if we compare the observations of Dunlop, Erman, and Sullivan, with those of Ross and Crozier, we invariably find that the earlier observer makes the place of the minimum a little more southerly than later determinations. A glance at the map suffices to show where determinations are now most wanted, and to point out the track where additional observations would be most valuable: it would be nearly that of a vessel making the eastern passage to the Cape of Good Hope. * Philosophical Transactions, 1840, Plate V. § 6. Observations of Intensity between the Cape of Good Hope and Kerguelen Island. On the 6th of April 1840 the Expedition quitted Simon’s Bay, and on the same night the Erebus and Terror parted company, and made their passage to Kerguelen Island on separate courses. Although the weather was very unfavourable, the practice of daily observation with the magnetical instruments was continued with very few exceptions, by the Erebus during the whole passage, and by the Terror from Simon’s Bay as far as the meridian of the Crozet Islands, which was the first rendezvous. The observations of intensity on board the Erebus were chiefly made with deflector S, the other deflector N having been used only five times during the whole passage, whilst the number of observations with S amounts to thirty-six. For the values of $w'$, in the case of deflector S, we have the comparisons with the constant weights at the Cape and Kerguelen Island, and three good intermediate comparisons at sea, viz. on the 11th, 12th, and 18th of April; a fourth attempt on the 1st of May failed from some accidental error in the observation with the constant weight. Pursuing the plan of graphical representation already described, we find that the line connecting the terminations of the ordinates at the Cape and at Kerguelen Island passes either through or extremely near the terminations of the other three ordinates, indicating the unchanged magnetism of the deflector; and we obtain the following Table of the values of $w'$ for the degrees of deflection in the Table: | Values of $w'$, deflector S, Erebus; Cape of Good Hope to Kerguelen Island. | |-----------------------------| | grs. | | 25 = 2·628 | | 26 = 2·594 | | 27 = 2·560 | | 28 = 2·527 | | 29 = 2·494 | | 30 = 2·460 | | 31 = 2·426 | | 32 = 2·392 | | 33 = 2·358 | | 34 = 2·324 | | 35 = 2·291 | | 36 = 2·260 | Regarding the Cape as the primary station, and its intensity = 0·715 (London = 1), the intensity at the other stations is given by the formula $$I' = 0·1837 w' \cosec v'.$$ The observations with deflector N between the Cape and Kerguelen Island being few, and the two intermediate comparisons at sea with the constant weights exhibiting considerable discordances, either from the unfavourable circumstances of weather, or possibly in consequence of an actual small change of force in the deflector, I have not attempted to deduce results from the observations either with deflector N, or with N and S combined. I have also omitted in the mean deductions the results of the observations with the constant weight of one grain on the 1st of May and 29th of June, these observations being obviously affected with some accidental error. For the Terror’s deflectors we have only the comparisons with the constant weights at the Cape and Kerguelen Island from which to derive the values of $w'$ for the intermediate degrees of $v'$. Connecting the values of $w'$ obtained by those comparisons for deflector N, with those in the former table for the same deflector, and presuming that the values corresponding to the intermediate degrees change in a nearly uniform progression, we derive the following Table for the degrees of $v'$ observed between the Cape and Kerguelen Island: | Values of $w'$, deflector N, Terror; Cape of Good Hope to Kerguelen Island. | |-----------------------------| | 26 = .917 | 33 = .873 | | 27 = .911 | 34 = .866 | | 28 = .906 | 35 = .859 | | 29 = .900 | 36 = .852 | | 30 = .894 | 37 = .845 | | 31 = .887 | 38 = .837 | | 32 = .880 | 39 = .828 | The Cape being the primary station, and its intensity = 0.715, we obtain the intensities at the other stations by the formula $$I' = .529 \cdot w' \csc v'.$$ In the case of deflector S, the values of $w'$ which result from the comparisons with the constant weights at the Cape and Kerguelen Island are so nearly the same (.733 at the Cape, and .735 at Kerguelen Island), that we may take the arithmetical mean .734 for all the intermediate stations without sensible inconvenience; whence the formula for the calculation of the intensity becomes $$I' = .388 \csc v'.$$ As we have only the comparisons with the constant weights at the Cape and Kerguelen Island from which to derive the values of $w'$ for the Terror's deflectors for all the intermediate degrees of $v'$, we might be disposed to fear that the data were scarcely sufficient for that purpose; but when we examine the intensities deduced from the observations with the two deflectors (both having been used at all the intermediate stations except one), we perceive that their accordance is in general remarkably good, which would scarcely be the case unless the elements of calculation were tolerably correct. So close an agreement in the partial observations, in a passage made in tempestuous weather, is certainly very creditable both to the instrument and to the observers. Those who interest themselves in examining the progress which magnetic maps of the portion of the globe occupied by sea are making towards accuracy, will compare the intensities between the Cape of Good Hope and Kerguelen Island, contained in the subjoined Tables, with the isodynamic lines drawn from Mr. Dunlop's observations in the first Number of these Contributions*. The prolongation of those lines into the more southerly latitudes traversed by the Erebus and Terror would suit extremely well with the intensities which are here given. * Philosophical Transactions, 1840, Plate V. ### Table IV. Observations of the Magnetic Intensity on Shore and on Board Her Majesty's Ship Erebus, with Needle F 1, between the Capé of Good Hope and Kerguelen Island. | 1840 | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|-------------|-----------| | | Lat. | Long. E. | | | | | London = 1·000. = 1·372. | | April | | | | | | | | | 3. | -34 11' | 18 26' | h m | S. | 35 35 | 72° | .717 | | | -35 14' | 18 27' | 11 40 A.M. | S. | 35 23 | 71·5 | .723 | | | -35 48' | 18 47' | 10 A.M. | S. | 35 11·1 | 70 | .728 | | | -36 4 | 19 10' | 10 45 | S. | 35 12 | 70 | .728 | | | -36 11' | 20 42' | 9 40 A.M. | S. | 35 0·7 | 66·5 | .720 | | | | | 10 15 A.M. | S. | 34 36·2 | 66·5 | .733 | | | | | 10 40 A.M. | S. and N. | 49 24·4 | 66·5 | .745 | | | | | 11 0 A.M. | N. | 28 54·5 | 66·5 | .742 | | | | | 11 30 A.M. | weight 1 grain. | 14 16·2 | 71 | .742 | | | | | Noon. | weight 1½ grain. | 23 10·2 | 71 | .731 | | | | | 0 30 P.M. | weight 2 grains. | 29 40·0 | 71 | .750 | | | | | 11 A.M. | S. | 33 48·8 | 72·5 | .769 | | | | | 11 40 | weight 1 grain. | 13 54·2 | 72·5 | .784 | | | | | 2 15 P.M. | S. | 33 53·1 | 72 | .763 | | | | | 10 40 A.M. | S. | 34 0·7 | w.s.w. | .766 | | | | | 10 40 A.M. | S. | 34 25·5 | s.s.e. | .763 | | | | | 11 30 A.M. | S. | 33 5·5 | s.e. by s. | .750 | | | | | 9 30 A.M. | S. | 33 29·1 | s.e. by s. | .792 | | | | | 10 10 A.M. | S. and N. | 47 59·5 | 56 | .780 | | | | | 10 50 A.M. | N. | 27 32·5 | 56 | .799 | | | | | 9 20 A.M. | S. | 32 52·7 | 61·5 | .800 | | | | | 10 30 A.M. | S. and N. | 47 9 | 60 | .810 | | | | | 10 40 A.M. | N. | 26 48 | 60 | .832 | | | | | 11 0 A.M. | weight 1 grain. | 12 44·3 | 59 | .830 | | | | | 11 20 A.M. | weight 1½ grain. | 18? 6·5* | 59 | .841 | | | | | 11 45 A.M. | weight 2 grains. | 25 53·3 | 59 | .849 | | | | | 11 30 A.M. | S. | 31 58·6 | s.s.e. | .830 | | | | | 11 30 A.M. | S. | 31 11 | 48·5 | .859 | | | | | 11 30 A.M. | S. | 30 56·2 | 45 | .868 | | | | | 11 30 A.M. | S. | 31 15·6 | 44 | .856 | | | | | 11 30 A.M. | S. | 30 3·4 | | .802 | | | | | 11 30 A.M. | S. | 29 23 | 45 | .827 | | | | | 11 30 A.M. | S. | 29 10·7 | 44 | .837 | | | | | 11 30 A.M. | S. | 29 6·2 | 44 | .841 | | | | | 11 30 A.M. | S. | 28 36·7 | 43 | .862 | | | | | 11 30 A.M. | S. | 28 20·5 | s.w. by w. | .874 | | | | | 11 30 A.M. | S. | 28 25 | s. by e. | .870 | | | | | 11 30 A.M. | S. and N. | 42 36·2 | | .831 | | | | | 11 30 A.M. | N. | 21 49 | | .839† | | | | | Noon. | weight 1 grain. | 11 13·3 | | .939† | | | | | 10 15 A.M. | S. | 28 23 | s.e. | .972 | | | | | 10 30 A.M. | S. | 27 33·5 | 40 | 1·009 | | | | | 10 50 A.M. | S. and N. | 42 0·5 | | | | | | | 11 10 A.M. | N. | 21 36 | | | | | | | 9 40 A.M. | S. | 26 22·5 | 43 | | | | | | Noon. | S. | 26 7·5 | | | | | | | 9 30 A.M. | S. | 25 54·7 | 39 | | | | | | 11 30 A.M. | S. | 25 49·5 | s.w. by s. | | | | | | 0 30 P.M. | S. | 25 45·5 | s.w. by w. | | * 18° is probably an error of transcription, and should be 19°; the result of 18° 06′5 would be .885; that of 19° 06′5 is .841, as entered in the Table. † Omitted in the mean. ### Table IV. (Continued.) | 1840 | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|------------|-----------| | | Lat. Long. E. | | | | | | London = 1·000. = 1·372. | | May 18 | Christmas Harbour. | h A.M. | S. | 26° 19' 7" 39' | N.W. | 1·069 | 1·467 | | June 26 | —48° 41' 68" 54' | Noon. | S. | 26° 21' 3" 34' 5" | Observed on shore. | 1·068 | 1·465 | | | 4 P.M. | S. and N. | 40° 42' 7" | 1·010* | 1·068 | | | 10 A.M. to 2 P.M. | weight 1 grain. | 20° 6' 3" | 1·063 | 1·465 | | | | weight 1½ grain. | 10° 24' 1" | 1·073 | 1·465 | | | | weight 2 grains. | 14° 48' 6" | 1·062 | 1·465 | | July 7 | Moored in Christmas Harbour. | S. | 26° 28' 1" | N.E. | 1·068 | 1·465 | | | | S. | 26° 21' 8" | E. | 1·083 | 1·476 | | | | S. | 26° 5' 2" | S.E. | 1·092 | 1·476 | | | | S. | 25° 54' 2" | S. | 1·090 | 1·476 | | | | S. | 25° 56' 4" | S.W. | 1·090 | 1·476 | | | | S. | 26° 19' 8" 41" | N. | 1·069 | 1·476 | | | | S. | 26° 11' 1" | N.W. | 1·076 | 1·476 | | | | S. | 26° 19' 9" | W. | 1·070 | 1·476 | ### Table V. Observations of the Magnetic Intensity on Shore and on Board Her Majesty's Ship Terror, with a four-inch Fox's Needle, between the Cape of Good Hope and Kerguelen Island. | 1840 | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|--------------------|-------------|------------|-----------| | | Lat. Long. E. | | | | | | London = 1·000. = 1·372. | | March 18 | Simon's Bay. | h m A.M. | N. | 38° 44' 1" | N.W. | 7·02 | 9·77 | | | —34° 11' 18" 26" | 8 A.M. | N. | 38° 35' 8" | W.N.W. | 7·05 | 9·77 | | | | N. | 38° 39 | W. | 7·04 | 9·77 | | | | N. | 38° 04' 4" | W.S.W. | 7·18 | 9·77 | | | | N. | 38° 43' 7" | N. | 7·02 | 9·77 | | | | N. | 38° 32' 6" | N.N.W. | 7·06 | 9·77 | | | | N. | 38° 46' 3" | N.N.E. | 7·02 | 9·77 | | | | N. | 38° 23' 5" | N.E. | 7·10 | 9·77 | | | | N. | 38° 13' 7" | E.N.E. | 7·13 | 9·77 | | | | N. | 38° 22' 9" | E. | 7·10 | 9·77 | | | | N. | 38° 26' 0" | E.S.E. | 7·09 | 9·77 | | | | N. | 38° 02' 1" | S.E. | 7·18 | 9·77 | | | | N. | 38° 04' 3" | S.S.E. | 7·18 | 9·77 | | | | N. | 37° 40' 8" | S. | 7·29 | 9·77 | | | | N. | 37° 43' 2" | S.S.W. | 7·28 | 9·77 | | | | N. | 37° 58' 5" | S.W. | 7·20 | 9·77 | | | | N. | 38° 07' 1" | Observed on shore. | 7·15 | 9·81 | | | | N. | 38° 16' 2" | 7·15 | 9·81 | | | | S. | 32° 59' 3" | 7·15 | 9·81 | | | | S. | 32° 43' 8" | 7·15 | 9·81 | | | | weight ½ grain. | 21° 46' 1" | 7·15 | 9·81 | | | | weight ½ grain. | 21° 47' 9" | 7·15 | 9·81 | | | | weight ¾ grain. | 12° 38' 2" | 7·15 | 9·81 | | | | weight ¾ grain. | 12° 56' 5" | 7·15 | 9·81 | * Omitted in the mean. | 1840 | Position | Time of day | Method employed | Deflection observed | Thermometer | Ship's head | Intensity | |------|----------|-------------|-----------------|---------------------|-------------|------------|-----------| | | Lat. | Long. E. | | | | | London = 1·000. | = 1·372. | | April 8. | -36° 16' 20" 04' | 3 50 P.M. | N. | 37° 31' 7" 70' | s. by E. | .730 | .731 | 1·003 | | | | | S. | 32° 00' 2" 70' | | .733 | | | | | | | N. | 37° 53' 1" 66' | | .722 | .724 | .994 | | | | | S. | 32° 21' 1" 66' | W. 1/2 N. | .726 | | | | | | | N. | 38° 6' 66' | W.S.W. | .717 | .722 | .991 | | | | | S. | 32° 15' 6" 66' | | .728 | | | | | | | N. | 37° 56' 7" 72' | S.W. | .720 | .721 | .990 | | | | | S. | 32° 31' 6' | | .722 | | | | | | | N. | 37° 20' 2" 70' | | .735 | .734 | 1·007 | | | | | S. | 31° 59' 9" 70' | | .733 | | | | | | | N. | 36° 47' 8" 62' | | .748 | .750 | 1·028 | | | | | S. | 31° 07' 4" 62' | S. | .752 | | | | | | | N. | 36° 49' 4" 68' | | .747 | .760 | 1·044 | | | | | S. | 30° 09' 7" 68' | | .773 | | | | | | | N. | 35° 37' 7" 56' | S.E. by E. | .776 | .775 | 1·063 | | | | | S. | 30° 11' 56' | | .773 | | | | | | | N. | 35° 21' 7" 61' | | .782 | .788 | 1·081 | | | | | S. | 29° 15' 3" 61' | | .795 | | | | | | | N. | 34° 28' 5" 60' | S. | .807 | .805 | 1·104 | | | | | S. | 28° 56' 4" 60' | | .803 | | | | | | | N. | 33° 48' 0" 59' | S.S.E. | .828 | | | | | | | N. | 33° 56' 0" 48' | S.E. by E. | .821 | .822 | 1·127 | | | | | S. | 28° 13' 1" 48' | | .822 | | | | | | | N. | 32° 12' 1" 44' | | .873 | .866 | 1·189 | | | | | S. | 26° 52' 8" 44' | | .859 | | | | | | | N. | 31° 28' 7" 45' | S.E. 1/2 E. | .896 | | | | | | | S. | 25° 24' 7" 45' | | .905 | .901 | 1·236 | | | | | N. | 30° 37' 7" 44' | | .924 | | | | | | | S. | 24° 52' 1" 44' | | .924 | .924 | 1·268 | | July 3. | Christmas Harbour, Kerguelen Island. | 10 A.M. | N. | 27° 03' 1" 36' | | .1·0675 | | | | | | | N. | 26° 41' 36' | | .1·0675 | | | | | | | A.M. | 21° 31' 3" 36' | | .1·0675 | | | | | | | P.M. | 21° 14' 0" 36' | | .1·070 | | | | | | | A.M. | 14° 18' 5" 36' | | .1·065 | | | | | | | P.M. | 14° 13' 7" 36' | | .1·079 | | | | | | | A.M. | 8° 29' 7" 36' | | .1·076 | | | | | | | P.M. | 8° 29' 5" 36' | | .1·084 | | | | | | | A.M. | 26° 37' 1" 38' | | .1·087 | | | | | | | N. | 26° 41' 4" 38' | N.E. | .1·084 | | | | | | | N. | 26° 26' 2" 38' | E. | .1·083 | | | | | | | N. | 26° 29' 8" 38' | S.E. | .1·084 | | | | | | | N. | 25° 54' 7" 38' | S. | .1·111 | | | | | | | N. | 26° 32' 1" 38' | S.W. | .1·083 | | | | | | | N. | 26° 40' 3" 38' | W. | .1·077 | | | | | | | N. | 26° 40' 8" 38' | N.W. | .1·076 | | | Observed on shore. Weight 5 grain. Weight 3 grain. ### Table VI Abstract of the Intensities observed in Her Majesty's Ships Erebus and Terror between the Cape of Good Hope and Kerguelen Island. | Position | In which ship observed. | Intensity. | Ship's head when at sea. | Correction for ship's head. | Corrected intensity. London = 1°372. | |----------|-------------------------|------------|--------------------------|-----------------------------|----------------------------------| | -34° 11' | Erebus and Terror. | 0·981 | Observed on shore. | | 0·981 | | -37° 44' | Terror. | 0·990 | S.W. | -0·007 | 0·983 | | -35° 14' | Erebus. | 0·992 | S. by E. | -0·008 | 0·984 | | -36° 04' | Erebus. | 0·988 | W. by S. | +0·000 | 0·988 | | -37° 16' | Terror. | 0·991 | W.S.W. | -0·002 | 0·989 | | -36° 16' | Terror. | 1·003 | S. by E. | -0·008 | 0·995 | | -36° 52' | Terror. | 0·994 | W. ½ N. | +0·002 | 0·996 | | -36° 11' | Erebus. | 1·005 | S.E. by S. | -0·008 | 0·997 | | -35° 48' | Erebus. | 0·999 | E.S.E. | -0·001 | 0·998 | | -38° 47' | Terror. | 1·007 | W. by S. | -0·008 | 0·999 | | -36° 35' | Erebus. | 1·018 | S. | -0·008 | 1·010 | | -38° 58' | Terror. | 1·028 | -0·008 | 1·020 | | -40° 05' | Erebus. | 1·029 | S.S.E. | -0·008 | 1·021 | | -40° 45' | Terror. | 1·044 | S. | -0·008 | 1·036 | | -38° 13' | Erebus. | 1·047 | W.S.W. | -0·002 | 1·045 | | -42° 40' | Terror. | 1·063 | S.E. by E. | -0·005 | 1·058 | | -41° 24' | Erebus. | 1·071 | S.E. by S. | -0·008 | 1·063 | | -42° 56' | Terror. | 1·081 | S. | -0·008 | 1·073 | | -40° 29' | Erebus. | 1·087 | S.E. by S. | -0·008 | 1·079 | | -41° 58' | Erebus. | 1·096 | S.S.E. | -0·008 | 1·088 | | -44° 28' | Terror. | 1·104 | S. | -0·008 | 1·096 | | -46° 41' | Terror. | 1·127 | S.E. by E. | -0·005 | 1·122 | | -46° 0 | Terror. | 1·136 | S.S.E. | -0·008 | 1·128 | | -44° 19' | Erebus. | 1·139 | S.E. by E. | -0·008 | 1·131 | | -43° 07' | Erebus. | 1·142 | S.E. by E. | -0·008 | 1·134 | | -47° 00' | Erebus. | 1·175 | S.E. by E. | -0·005 | 1·170 | | -45° 44' | Erebus. | 1·179 | S.S.E. | -0·008 | 1·171 | | -46° 45' | Terror. | 1·189 | S.E. ¼ E. | -0·006 | 1·183 | | -47° 00' | Erebus. | 1·191 | S.E. by E. | -0·005 | 1·186 | | -47° 0 | Terror. | 1·236 | S.E. ¼ E. | -0·006 | 1·230 | | -46° 46' | Erebus. | 1·237 | S.E. by E. | -0·005 | 1·232 | | -47° 50' | Terror. | 1·268 | S.E. ½ S. | -0·007 | 1·261 | | -47° 01' | Erebus. | 1·275 | S.E. ¼ E. | -0·006 | 1·269 | | -46° 41' | Erebus. | 1·285 | S.E. by S. | -0·008 | 1·277 | | -46° 28' | Erebus. | 1·290 | W.S.W. | -0·002 | 1·288 | | -46° 29' | Erebus. | 1·319 | S.W. by W. | -0·005 | 1·314 | | -46° 25' | Erebus. | 1·331 | S. by E. ¼ E. | -0·008 | 1·323 | | -46° 57' | Erebus. | 1·333 | S.E. | -0·007 | 1·326 | | -46° 18' | Erebus. | 1·336 | S.S.W. | -0·008 | 1·328 | | -47° 19' | Erebus. | 1·384 | S.E. | -0·007 | 1·377 | | -47° 41' | Erebus. | 1·466 | S.E. | -0·007 | 1·459 | | -48° 41' | Erebus and Terror. | 1·465 | Observed on shore. | | 1·465 | | -48° 41' | Erebus. | 1·467 | N.W. | +0·004 | 1·471 | | -48° 39' | Erebus. | 1·488 | N. | 0·000 | 1·488 | | -48° 36' | Erebus. | 1·497 | S.W. by W. ½ W. | -0·008 | 1·489 | | -48° 36' | Erebus. | 1·485 | N.N.W. | +0·005 | 1·490 | | -48° 30' | Erebus. | 1·502 | S.W. by W. ½ W. | -0·005 | 1·497 | MDCCCXLII.