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Published online by Cambridge University Press: 17 January 2013
In 1845, a paper by me on the balance magnetometer was read to the Royal Society of Edinburgh (see Trans. vol. xvi. p. 67), which contained an examination of some of the difficulties to be considered and overcome in relation to that instrument. No similar examination has yet appeared, as far as I am aware, of the bifilar magnetometer. When it is considered that the value of the results obtained from this instrument in so many observatories is dependent on an exact knowledge and elimination or correction of its errors; and, as will appear hereafter, that the temperature coefficients employed in the discussion of the Colonial observations are in some cases so erroneous that the unconnected observations would have been nearer the truth than after correction, the following communication may not appear unnecessary.
page 469 note * See Introduction to Makerstoun Observations, Trans. Roy. Soc. Ed., vol. xvii. part 1. p. 27
page 469 note † Introduction to Mak. Obs., 1843, p. lxv.; 1844, p. xxviii.; 1845, p. xxvii.
page 469 note ‡ I have only lately learned that Mr Brooke has employed the process by weights for his self-registering instruments (Phil. Trans., p. 85, 1850).
page 475 note * I first proposed to Professor James D. Forbes (in a letter dated 21st November 1842) to determine the temperature coefficient from the usual hourly observations. (See Trans. Roy. Soc. Ed., Vol. XVI. p. 74.)
P.S.—Since this paper was written, I have received the third volume of the Toronto Observations (London 1357), where I am glad to perceive that General Sabine has followed the principle described in the text, for the determination of the temperature coefficient. The mode he has adopted is however far from accurate, including as it does irregular annual secular and instrumental changes, with too few results to render plus and minus errors equally probable; as a consequence, the coefficient obtained by him is still erroneous (1·63 sc. div., whereas I find 1·36 sc. div.). General Sabine notices my result for the Makerstoun bifilar (Adjustments, &c, part v., Toronto Observations, vol. iii.), but he omits to remark that I had pointed out, twelve years before, the necessity of applying the process to all bifilars, and that I had applied it to the very magnet (the Toronto one) under his discussion. See Trans. Roy. Soc. Ed., Vol. XVI. p. 77 (20), p. 102 (11), and curves in No. 5, Plate IV.
page 475 note † See the Introduction to the Makerstoun Observations for 1844 and 1845, where the process is described.
page 476 note * Magnetic Observations, Cape of Good Hope, vol. i., printed under the superintendence of Lieut.-Colonel E. Sabine.
page 478 note * Printed under the superintendence of Lieut.-Colonel E. Sabine.
page 480 note * At least I have found this to be the case with magnetic rocks in South India.
page 480 note † Adjustments, &c., “St Helena Observation,” vol. i., p. 34.
page 480 note ‡ Ibid. p. 35. This Table has some inaccuracies. The coefficient varies, and the temperature zero is not always 60°, as stated.
page 480 note § Hobarton Observations, vols. i., ii., and iii., printed under the superintendence of Lieut.-Colonel E. Sabine.
page 482 note * Observations in manuscript.
page 484 note * Observations in manuscript and printed, but not published.
page 485 note * Toronto Observations, 1842–43, vols. i. and ii., printed under the superintendence of Lieutenant-Colonel E. Sabine.
page 485 note † The value employed by me in 1845, to correct the Toronto bifilar observations for the annual law, was obtained from a few groups of days (the sums of differences of temperature being 117°), and was qʹ = 2·55 sc. div., almost identical with that here obtained from the whole year.
page 487 note * This includes the six months of 1848, and the two months of 1843.
page 487 note † Six months of 1848 only, no differences having been rejected in November and December 1843.