References.—Ternant, Le Télégraphe, Paris, 1881, p. 11; Linguet, “Mém. manuscrit ... signaux par la lumière,” Paris, 1782; all about the “Mercure de France,” in “Bulletin du Bibliophile” No. 7 of July 15, 1902; “Biog. Dict.,” Alex Chalmers, 1815, Vol. XX. p. 290; “Nouv. Biog. Gén.” (Hœfer), Paris, 1860, Vol. XXXI. p. 279; “Biog. Univ.” (Michaud), Vol. XXIV. p. 565.

A.D. 1782–1791.—Cassini (Jean Jacques Dominique, Comte de), son of Cassini de Thury, eminent astronomer, makes the very important announcement that, besides the secular variation of the declination, the magnetic needle is subject to an annual periodical fluctuation depending on the position of the sun in reference to the equinoctial and solstitial points.

Cassini’s discovery is contained in a Memoir consisting of two parts, the first part being a letter addressed to L’Abbé Rosier and published by him in the Journal de Physique, while the second part, composed at request of the Académie des Sciences, is that which specially treats of the annual variation in declination.

Besides the last named, we have thus far learned of the secular variation discovered by Gellibrand (Hellibrand) in 1635, as well as of the diurnal and horary variations, first accurately observed by George Graham during the year 1722, and we have likewise been informed of the earliest observations of the dip or inclination, made independently by both Georg Hartmann (A.D. 1543–1544) and by Robert Norman (A.D. 1576), as well as of the determination of the intensity of the inclination by J. C. Borda (at A.D. 1776). For accounts of the secular and annual, as well as of the diurnal and horary variations of the dip, the reader should consult the First Section of Humboldt’s “Cosmos” treating of telluric phenomena and some of the very numerous references therein given.

Speaking of the influence of the sun’s position upon the manifestation of the magnetic force of the earth, Humboldt remarks that the most distinct intimation of this relation was afforded by the discovery of horary variations, although it had been obscurely perceived by Kepler, who surmised that all the axes of the planets were magnetically directed toward one portion of the universe. He says that the sun may be a magnetic body, and that on that account the force which impels the planets may be centred in the sun (Kepler, in “Stella Martis,” pp. 32–34—compare with it his treatise, “Mysterium Cosmogr.,” cap. 20, p. 71). He further observes that the horary variations of the declination, which, although dependent upon true time are apparently governed by the sun as long as it remains above the horizon, diminish in angular value with the magnetic latitude of place. Near the equator, for instance, in the island of Rawak, they scarcely amount to three or four minutes, whilst the variations are from thirteen to fourteen minutes in the middle of Europe. As in the whole northern hemisphere the north point of the needle moves from east to west on an average from 8½ in the morning until 1½ at midday, in the southern hemisphere the same north point moves from west to east (Arago, Annuaire, 1836, p. 284, and 1840, pp. 330–358). Attention has been drawn, with much justice, to the fact that there must be a region of the earth, between the terrestrial and the magnetic equator, where no horary deviations in the declination are to be observed. This fourth curve (in contradistinction to the isodynamic, isoclinic and isogonic lines, or those respectively of equal force, equal inclination and equal declination), which might be called the curve of no motion, or rather the line of no variation of horary declination, has not yet been discovered. No point has hitherto been found at which the needle does not exhibit a horary motion, and, since the erection of magnetic stations, the important and very unexpected fact has been evolved that there are places in the southern magnetic hemisphere at which the horary variations of the dipping needle alternately participate in the phenomena (types) of the hemispheres.

Humboldt also alludes, in the article on “Magnetic Variation,” to his recognition of the “four motions of the needle, constituting, as it were, four periods of magnetic ebbing and flowing, analogous to the barometrical periods,” which will be found recorded in Hansteen’s “Magnetismus der Erde,” 1819, s. 459, and he likewise refers to the long-disregarded nocturnal alterations of variation, for which he calls attention to Faraday “On the Night Episode,” ss. 3012–3024. (See also, Poggendorff’s Annalen der Physik, Bd. XV. s. 330, and Bd. XIX. s. 373.)

The Phil. Trans. for 1738, p. 395, contain the description of a new compass for ascertaining the variation “with greater ease and exactness than any ever yet contrived for that purpose.” This was devised by Capt. Christopher Middleton, whose many interesting observations are to be found in the same volume of the Phil. Trans., p. 310, as well as in the volumes for 1726, p. 73; 1731–1732, 1733–1734, p. 127; 1742, p. 157, and in John Martyn’s abridgment, Vol. VIII. part i. p. 374. Reference should also be made to the volumes for 1754 (p. 875) and 1757 (p. 329), giving the reports of W. Mountaine and J. Dodson upon the magnetic chart and tables of 50,000 observations, likewise to the volume for 1766 containing the report of W. Mountaine on Robert Douglass’ observation, as well as for the record of investigations of the variation made by David Ross on board the ship “Montagu” during the years 1760–1762.

References.—Sabine, “On the Annual and Diurnal Variations” in Vol. II of “Observations made ... at Toronto,” pp. xvii-xx, also his Memoir “On the Annual Variation of the Magnetic Needle at Different Periods of the Day,” in Phil. Trans. for 1851, Part II. p. 635, as well as the Introduction to his “Observations ... at Hobart Town,” Vol. I. pp. xxxiv-xxxvi, and his Report to the British Association at Liverpool, 1854, p. 11—Phil. Trans. for 1857, Art. 1, pp. 6, 7—relative to the lunar diurnal magnetic variation. See likewise C. Wolf, “Histoire de l’observatoire depuis sa fondation à 1793”; Houzeau et Lancaster, “Bibl. Gen.,” Vol. II. p. 102; “Mém. de Paris,” Vol. II. p. 74, and Vol. VII. pp. 503, 530; Walker, “Ter. and Cos. Magn.,” Chap. III; Mme. J. Le Breton, “Histoire et Applic.,” etc., Paris, 1884, p. 17; Robison, “Mech. Phil.,” Vol. IV. p. 356; Thos. Young, “Nat. Phil.,” 1845, p. 583.

Cassini Family

This celebrated family, to which allusion was made under A.D. 1700, deserves here additional notice.