Page:EB1911 - Volume 17.djvu/399

Those of a very rapid oscillatory character were especially numerous in the morning between 4 and 9 a.m. In the late afternoon and evening disturbances of a more regular type became prominent, especially in the winter months. In particular there were numerous occurrences of a remarkably regular type of disturbance, half the total number of cases taking place between 7 and 9 p.m. This “special type of disturbance” was divisible into two phases, each lasting on the average about 20 minutes. During the first phase all the elements diminished in value, during the second phase they increased. In the case of D and H the rise and fall were about equal, but the rise in V was about 31/2 times the preceding fall. The disturbing force—on the north pole—to which the first phase might be attributed was inclined on the average about 5°1/2 below the horizon, the horizontal projection of its line of action being inclined about 41°1/2 to the north of east. The amplitude and duration of the disturbances of the “special type” varied a good deal; in several cases the disturbing force considerably exceeded 200γ. A somewhat similar type of disturbance was observed by Kr. Birkeland^[1] at Arctic stations also in 1902–1903, and was called by him the “polar elementary” storm. Birkeland’s record of disturbances extends only from October 1902 to March 1903, so it is uncertain whether “polar elementary” storms occur during the Arctic summer. Their usual time of occurrence seems to be the evening. During their occurrence Birkeland found that there was often a great difference in amplitude and character between the disturbances observed at places so comparatively near together as Iceland, Nova Zembla and Spitzbergen. This led him to assign the cause to electric currents in the Arctic, at heights not exceeding a few hundred kilometres, and he inferred from the way in which the phenomena developed that the seat of the disturbances often moved westward, as if related in some way to the sun’s position. Contemporaneously with the “elementary polar” storms in the Arctic Birkeland found smaller but distinct movements at stations all over Europe; these could generally be traced as far as Bombay and Batavia, and sometimes as far as Christchurch, New Zealand. Chree,^[2] on the other hand, working up the 1902–1904 Antarctic records, discovered that during the larger disturbances of the “special type” corresponding but much smaller movements were visible at Christchurch, Mauritius, Kolaba, and even at Kew. He also found that in the great majority of cases the Antarctic curves were specially disturbed during the times of Birkeland’s “elementary polar” storms, the disturbances in the Arctic and Antarctic being of the same order of magnitude, though apparently of considerably different type.

Examining the more prominent of the sudden commencements of magnetic disturbances in 1902–1903 visible simultaneously in the curves from Kew, Kolaba, Mauritius and Christchurch, Chree found that these were all represented in the Antarctic curves by movements of a considerably larger size and of an oscillatory character. In a number of cases Birkeland observed small simultaneous movements in the curves of his co-operating stations, which appeared to be at least sometimes decidedly larger in the equatorial than the northern temperate stations. These he described as “equatorial” perturbations, ascribing them to electric currents in or near the plane of the earth’s magnetic equator, at heights of the order of the earth’s radius. It was found, however, by Chree that in many, if not all, of these cases there were synchronous movements in the Antarctic, similar in type to those which occurred simultaneously with the sudden commencements of magnetic storms, and that these Antarctic movements were considerably larger than those described by Birkeland at the equatorial stations. This result tends of course to suggest a somewhat different explanation from Birkeland’s. But until our knowledge of facts has received considerable additions all explanations must be of a somewhat hypothetical character.

In 1831 Sir James Ross^[3] observed a dip of 89° 59′ at 70° 5′ N., 96° 46′ W., and this has been accepted as practically the position of the north magnetic pole at the time. The position of the south magnetic pole in 1840 as deduced from the Antarctic observations made by the “Erebus” and Magnetic Poles. “Terror” expedition is shown in Sabine’s chart as about 73° 30′ S., 147° 30′ E. In the more recent chart in J. C. Adams’s Collected Papers, vol. 2, the position is shown as about 73° 40′ S., 147° 7′ E. Of late years positions have been obtained for the south magnetic pole by the “Southern Cross” expedition of 1898–1900 (A), by the “Discovery” in 1902–1904 (B), and by Sir E. Shackleton’s expedition 1908–1909 (C). These are as follow:

(A)	72° 40′ S.,	152° 30′ E.
(B)	72° 51′ S.,	156° 25′ E.
(C)	72° 25′ S.,	155° 16′ E.

Unless the diurnal inequality vanishes in its neighbourhood, a somewhat improbable contingency considering the large range at the “Discovery’s” winter quarters, the position of the south magnetic pole has probably a diurnal oscillation, with an average amplitude of several miles, and there is not unlikely a larger annual oscillation. Thus even apart from secular change, no single spot of the earth’s surface can probably claim to be a magnetic pole in the sense popularly ascribed to the term. If the diurnal motion were absolutely regular, and carried the point where the needle is vertical round a closed curve, the centroid of that curve—though a spot where the needle is never absolutely vertical—would seem to have the best claim to the title. It should also be remembered that when the dip is nearly 90° there are special observational difficulties. There are thus various reasons for allowing a considerable uncertainty in positions assigned to the magnetic poles. Conclusions as to change of position of the south magnetic pole during the last ten years based on the more recent results (A), (B) and (C) would, for instance, possess a very doubtful value. The difference, however, between these recent positions and that deduced from the observations of 1840–1841 is more substantial, and there is at least a moderate probability that a considerable movement towards the north-east has taken place during the last seventy years.

See publications of individual magnetic observatories, more especially the Russian (Annales de l’Observatoire Physique Central), the French (Annales du Bureau Central Météorologique de France), and those of Kew, Greenwich, Falmouth, Stonyhurst, Potsdam, Wilhelmshaven, de Bilt, Uccle, O’Gyalla, Prague, Pola, Coimbra, San Fernando, Capo di Monte, Tiflis, Kolaba, Zi-ka-wei, Hong-Kong, Manila, Batavia, Mauritius, Agincourt (Toronto), the observatories of the U.S. Coast and Geodetic Survey, Rio de Janeiro, Melbourne.

In the references below the following abbreviations are used: B.A. = British Association Reports; Batavia = Observations made at the Royal . . . Observatory at Batavia; M.Z. = Meteorologische Zeitschrift, edited by J. Hann and G. Hellman; P.R.S. = Proceedings of the Royal Society of London; P.T. = Philosophical Transactions; R. = Repertorium für Meteorologie, St Petersburg; T.M. = Terrestrial Magnetism, edited by L. A. Bauer; R.A.S. Notices = Monthly Notices of the Royal Astronomical Society. Treatises are referred to by the numbers attached to them; e.g. (1) p. 100 means p. 100 of Walker’s Terrestrial Magnetism.

¹ E. Walker, Terrestrial and Cosmical Magnetism (Cambridge and London, 1866). ^1a H. Lloyd, A Treatise on Magnetism General and Terrestrial (London, 1874). ² E. Mascart, Traité de magnétisme terrestre (Paris, 1900). ³ L. A. Bauer, United States Magnetic Declination Tables and Isogonic Charts, and Principal Facts relating to the Earth’s Magnetism (Washington, 1902). ⁴ Balfour Stewart, “Terrestrial Magnetism” (under “Meteorology”), Ency. brit. 9th ed. ⁵ C. Chree, “Magnetism, Terrestrial,” Ency. brit. 10th ed. ⁶M.Z. 1906, 23, p. 145. ⁷(3) p. 62. ⁸K. Akad. van Wetenschappen (Amsterdam, 1895; Batavia, 1899, &c.). ⁹ Atlas des Erdmagnetismus (Riga, 1903). ¹⁰ (1) p. 16, &c. ¹¹ Kolaba (Colaba) Magnetical and Meteorological Observations, 1896, Appendix Table II. ¹² (1) p. 21. ¹³ Report for 1906, App. 4, see also (3) p. 102. ¹⁴(1) p. 166. ¹⁵ Ergebnisse der mag. Beobachtungen in Potsdam, 1901, p. xxxvi. ¹⁶ U.S. Coast and Geodetic Survey Report for 1895, App. I, &c. ¹⁷ T.M. 1, pp. 62, 89, and 2, p. 68. ¹⁸ (3) p. 45. ¹⁹ Die Elemente des Erdmagnetismus, pp. 104-108. ²⁰ Zur täglichen Variation der mag. Deklination (aus Hefl II. des Archivs des Erdmagnetismus) (Potsdam, 1906). ²¹ M.Z. 1888, 5, p. 225. ²²M.Z. 1904, 21, p. 129. ²³P.T. 202 A, p. 335. ^23aComb. Phil. Soc. Trans. 20, p. 165. ²⁴P.T. 208 A, p. 205. ²⁵P.T. 203 A, p. 151. ²⁶P.T. 171. p. 541; P.R.S. 63, p. 64. ²⁷R.A.S. Notices 60, p. 142. ²⁸Rendiconti del R. Ist. Lomb. 1902, Series II. vol. 35. ²⁹R. 1889, vol. 12, no. 8. ³⁰B.A. Report, 1898, p. 80. ³¹P.R.S. (A) 79, p. 151. ³²P.T. 204 A, p. 373. ³³Ann. du Bureau Central Météorologique, année 1897, 1 Mem. p. B65. ³⁴P.T. 161, p. 307. ³⁵M.Z. 1895, 12, p. 321. ^35aP.T. 1851, p. 123; and 1852, p. 103, see also (4) § 38. ³⁶P.T. 159, p. 363. ³⁷(1) p. 92. ³⁸R.A.S. Notices 65, p. 666. ³⁹R.A.S. Notices, 65, pp. 2 and 538. ⁴⁰K. Akad. van Wetenschappen (Amsterdam, 1906) p. 266. ⁴¹R.A.S. Notices 65, p. 520. ⁴²B.A. Reports, 1880, p. 201 and 1881, p. 463. ⁴³Anhang Ergebnisse der mag. Beob. in Potsdam, 1896. ⁴⁴M.Z. 1899, 16, p. 385. ⁴⁵P.T. 166, p. 387. ⁴⁶Trans. Can. Inst. 1898–1899, p. 345, and Proc. Roy. Ast. Soc. of Canada, 1902–1903, p. 74, 1904, p. xiv., &c. ⁴⁷R.A.S. Notices 65, p. 186. ⁴⁸T.M. 10, p. 1. ⁴⁹Expédition norvégienne de 1899–1900 (Christiania, 1901). ⁵⁰Thèses présentées à la Faculté des Sciences (Paris, 1903). ⁵¹Nat. Tijdschrift voor Nederlandsch-Indië, 1902, p. 71. ⁵²Wied. Ann. 1882, p. 336. ⁵³Sitz. der k. preuss. Akad. der Wiss., 24th June 1897, &c. ⁵⁴T.M. 12, p. 1. ⁵⁵P.T. 143, p. 549; St Helena Observations, vol. ii., p. cxlvi., &c., (1) § 62. ⁵⁶Trans. R.S.E. 24, p. 669. ⁵⁷P.T. 178 A, p. 1. ⁵⁸Batavia, vol. 16, &c. ⁵⁹Batavia, Appendix to vol. 26. ⁶⁰R. vol. 17, no. 1. ⁶¹T.M. 3, p. 1, &c. ⁶²P.T. 181 A, p. 53 and 188 A. ⁶³Ann. du Bureau Central Mét. vol. i. for years 1884 and 1887 to 1895. ⁶⁴Ann. dell’ Uff. Centrale Met. e Geod. vol. 14, pt. i. p. 57. ⁶⁵A Magnetic Survey of the Netherlands for the Epoch 1st Jan. 1891 (Rotterdam, 1895). ⁶⁶Kg. Svenska Vet. Akad. Handlingar, 1895, vol. 27, no. 7. ⁶⁷Denkschriften der math. naturwiss. Classe der k. Akad. des Wiss. (Wien), vols. 62 and 67. ⁶⁸Journal of the College of Science, Tōkyō, 1904, vol. 14. ⁶⁹Ann. de l’observatoire . . . de Toulouse, 1907, vol. 7. ⁷⁰Ann. du Bureau Central Mét. 1897, I. p. B36. ⁷¹T.M. 7, p. 74. ⁷²Bull. Imp. Univ. Odessa 85, p. 1, and T.M. 7, p. 67. ⁷³P.T. 187 A, p. 345. ⁷⁴P.R.S. 76 A, p. 181. ⁷⁵Bull. Soc. Imp. des Naturalistes de Moskau, 1893, no. 4, p. 381, and T.M. 1, p. 50. ⁷⁶Forsch. zur deut. Landes- u. Volkskunde, 1898, Bd. xi, 1, and T.M. 3, p. 77. ⁷⁷P.R.S. 76 A, p. 507. ⁷⁸Adams, Scientific Papers, II. p. 446. ⁷⁹B.A. Report for 1898, p. 109. ⁸⁰Abhand. der bayer, Akad. der Wiss., 1895, vol. 19. ⁸¹Sitz. k. Akad. der Wiss. (Berlin), 1897, no. xviii., also T.M. 3, p. 191. ⁸²T.M. 2, p. 11. ⁸³Die Elemente des Erdmagnetismus (St Petersburg,

---

1. The Norwegian Aurora Polaris Expedition 1902–1903, vol. i.
2. National Antarctic Expedition 1901–1904, “Magnetic Observations.”
3. (1) p. 163.