Chapter LXIII: repeats the promise of freedom to the English church (13)

§ 53. Schuster[87] has calculated a potential analogous to the
Gaussian potential, from which the regular diurnal changes of the
magnetic elements all over the earth may be derived. From the mean
summer and winter diurnal variations of the northerly and easterly
components of force during 1870 at St Petersburg, Greenwich, Lisbon
and Bombay, he found the values of 8 constants analogous to Gaussian
constants; and from considerations as to the hours of occurrence of
the maxima and minima of vertical force, he concluded that the
potential, unlike the Gaussian, must proceed in positive powers of r,
and so answer to forces external to the earth. Schuster found,
however, that the calculated amplitudes of the diurnal vertical force
inequality did not accord well with observation; and his conclusion
was that while the original cause of the diurnal variation is
external, and consists probably of electric currents in the
atmosphere, there are induced currents inside the earth, which
increase the horizontal components of the diurnal inequality while
diminishing the vertical. The problem has also been dealt with by H.
Fritsche,[88] who concludes, in opposition to Schuster, that the
forces are partly internal and partly external, the two sets being of
fairly similar magnitude. Fritsche repeats the criticism (already made
in the last edition of this encyclopaedia) that Schuster's four
stations were too few, and contrasts their number with the 27 from
which his own data were derived. On the other hand, Schuster's data
referred to one and the same year, whereas Fritsche's are from epochs
varying from 1841 to 1896, and represent in some cases a single year's
observations, in other cases means from several years. It is clearly
desirable that a fresh calculation should be made, using synchronous
data from a considerable number of well distributed stations; and it
should be done for at least two epochs, one representing large, the
other small sun-spot frequency. The year 1870 selected by Schuster
had, as it happened, a sun-spot frequency which has been exceeded
only once since 1750; so that the magnetic data which he employed were
far from representative of average conditions.

Magnetization of Vases, &c.

§ 54. It was discovered by Folgheraiter[89] that old vases from
Etruscan and other sources are magnetic, and from combined observation
and experiment he concluded that they acquired their magnetization
when cooling after being baked, and retained it unaltered. From
experiments, he derived formulae connecting the magnetization shown by
new clay vases with their orientation when cooling in a magnetic
field, and applying these formulae to the phenomena observed in the
old vases he calculated the magnetic dip at the time and place of
manufacture. His observations led him to infer that in Central Italy
inclination was actually southerly for some centuries prior to 600
B.C., when it changed sign. In 400 B.C. it was about 20°N.; since 100
B.C. the change has been relatively small. L. Mercanton[90] similarly
investigated the magnetization of baked clay vases from the lake
dwellings of Neuchatel, whose epoch is supposed to be from 600 to 800
B.C. The results he obtained were, however, closely similar to those
observed in recent vases made where the inclination was about 63°N.,
and he concluded in direct opposition to Folgheraiter that inclination
in southern Europe has not undergone any very large change during the
last 2500 years. Folgheraiter's methods have been extended to natural
rocks. Thus B. Brunhes[91] found several cases of clay metamorphosed
by adjacent lava flows and transformed into a species of natural
brick. In these cases the clay has a determinate direction of
magnetization agreeing with that of the volcanic rock, so it is
natural to assume that this direction coincided with that of the dip
when the lava flow occurred. In drawing inferences, allowance must of
course be made for any tilting of the strata since the volcanic
outburst. From one case in France in the district of St Flour, where
the volcanic action is assigned to the Miocene Age, Brunhes inferred a
southerly dip of some 75°. Until a variety of cases have been
critically dealt with, a suspension of judgment is advisable, but if
the method should establish its claims to reliability it obviously may
prove of importance to geology as well as to terrestrial magnetism.

Polar Phenomena.

§ 55. Magnetic phenomena in the polar regions have received
considerable attention of late years, and the observed results are of
so exceptional a character as to merit separate consideration. One
feature, the large amplitude of the regular diurnal inequality, is
already illustrated by the data for Jan Mayen and South Victoria Land
in Tables VIII. to XI. In the case, however, of declination allowance
must be made for the small size of H. If a force F perpendicular to
the magnetic meridian causes a change [Delta]D in D then [Delta]D =
F/H. Thus at the "Discovery's" winter quarters in South Victoria Land,
where the value of H is only about 0.36 of that at Kew, a change of
45´ in D would be produced by a force which at Kew would produce a
change of only 16´. Another feature, which, however, may not be
equally general, is illustrated by the data for Fort Rae and South
Victoria Land in Table XVII. It will be noticed that it is the 24-hour
term in the Fourier analysis of the regular diurnal inequality which
is specially enhanced. The station in South Victoria Land--the winter
quarters of the "Discovery" in 1902-1904--was at 77° 51´ S. lat.; thus
the sun did not set from November to February (midsummer), nor rise
from May to July (midwinter). It might not thus have been surprising
if there had been an outstandingly large seasonal variation in the
type of the diurnal inequality. As a matter of fact, however, the type
of the inequality showed exceptionally small variation with the
season, and the amplitude remained large throughout the whole year.
Thus, forming diurnal inequalities for the three midsummer months and
for the three midwinter months, we obtain the following amplitudes for
the range of the several elements[92]:--

D. H. V. I.

Midsummer 64´.1 57[gamma] 58[gamma] 2´.87
Midwinter 26´.8 25[gamma] 18[gamma] 1´.23

The most outstanding phenomenon in high latitudes is the frequency and
large size of the disturbances. At Kew, as we saw in § 25, the
absolute range in D exceeds 20´ on only 12% of the total number of
days. But at the "Discovery's" winter quarters, about sun-spot
minimum, the range exceeded 1° on 70%, 2° on 37%, and 3° on fully 15%
of the total number of days. One day in 25 had a range exceeding 4°.
During the three midsummer months, only one day out of 111 had a range
under 1°, and even at midwinter only one day in eight had a range as
small as 30´. The H range at the "Discovery's" station exceeded
100[gamma] on 40% of the days, and the V range exceeded 100[gamma] on
32% of the days.

The special tendency to disturbance seen in equinoctial months in
temperate latitudes did not appear in the "Discovery's" records in the
Antarctic. D ranges exceeding 3° occurred on 11% of equinoctial days,
but on 40% of midsummer days. The preponderance of large movements at
midsummer was equally apparent in the other elements. Thus the
percentage of days having a V range over 200[gamma] was 21 at
midsummer, as against 3 in the four equinoctial months.

At the "Discovery's" station small oscillations of a few minutes'
duration were hardly ever absent, but the character of the larger
disturbances showed a marked variation throughout the 24 hours. 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 3½ 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°½ below the horizon, the horizontal projection of its line of action
being inclined about 41°½ 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
200[gamma]. A somewhat similar type of disturbance was observed by Kr.
Birkeland[93] 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,[92] 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.

Magnetic Poles.

In 1831 Sir James Ross[94] 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 "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_.

FOOTNOTES:

[A] For explanation of these numbers, see end of article.

[1] E. Walker, _Terrestrial and Cosmical Magnetism_ (Cambridge and
London, 1856).

[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.

[6] _M.Z._ 1906, 23, p. 145.

[7] (3) p. 62.

[8] _K. 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. 1, &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 Heft II. des
Archivs des Erdmagnetismus)_ (Potsdam, 1906).

[21] _M.Z._ 1888, 5, p. 225.

[22] _M.Z._ 1904, 21, p. 129.

[23] _P.T._ 202 A, p. 335.

[23a] _Comb. Phil. Soc. Trans._ 20, p. 165.

[24] _P.T._ 208 A, p. 205.

[25] _P.T._ 203 A, p. 151.

[26] _P.T._ 171. p. 541; _P.R.S._ 63, p. 64.

[27] _R.A.S. Notices_ 60, p. 142.

[28] _Rendiconti del R. Ist. Lomb._ 1902, Series II. vol. 35.

[29] _R._ 1889, vol. 12, no. 8.

[30] _B.A. Report_, 1898, p. 80.

[31] _P.R.S._ (A) 79, p. 151.

[32] _P.T._ 204 A, p. 373.

[33] _Ann. du Bureau Central Météorologique, année 1897_, 1 Mem. p.
B65.

[34] _P.T._ 161, p. 307.

[35] _M.Z._ 1895, 12, p. 321.

[35a] _P.T._ 1851, p. 123; and 1852, p. 103, see also (4) § 38.

[36] _P.T._ 159, p. 363.

[37] (1) p. 92.

[38] _R.A.S. Notices_ 65, p. 666.

[39] _R.A.S. Notices_, 65, pp. 2 and 538.

[40] _K. Akad. van Wetenschappen_ (Amsterdam, 1906) p. 266.

[41] _R.A.S. Notices_ 65, p. 520.

[42] _B.A. Reports_, 1880, p. 201 and 1881, p. 463.

[43] _Anhang Ergebnisse der mag. Beob. in Potsdam_, 1896.

[44] _M.Z._ 1899, 16, p. 385.

[45] _P.T._ 166, p. 387.

[46] _Trans. Can. Inst._ 1898-1899, p. 345, and Proc. Roy. Ast. Soc.
of Canada, 1902-1903, p. 74, 1904, p. xiv., &c.

[47] _R.A.S. Notices_ 65, p. 186.

[48] _T.M._ 10, p. 1.

[49] _Expédition norvégienne de 1899-1900_ (Christiania, 1901).

[50] _Thèses présentées à la Faculté des Sciences_ (Paris, 1903).

[51] _Nat. Tijdschrift voor Nederlandsch-Indië_, 1902, p. 71.

[52] _Wied. Ann._ 1882, p. 336.

[53] _Sitz. der k. preuss. Akad. der Wiss._, 24th June 1897, &c.

[54] _T.M._ 12, p. 1.

[55] _P.T._ 143, p. 549; _St Helena Observations_, vol. ii., p.
cxlvi., &c., (1) § 62.

[56] _Trans. R.S.E._ 24, p. 669.

[57] _P.T._ 178 A, p. 1.

[58] _Batavia_, vol. 16, &c.

[59] _Batavia_, Appendix to vol. 26.

[60] _R._ vol. 17, no. 1.

[61] _T.M._ 3, p. 1, &c.

[62] _P.T._ 181 A, p. 53 and 188 A.

[63] _Ann. du Bureau Central Mét._ vol. i. for years 1884 and 1887 to
1895.

[64] _Ann. dell' Uff. Centrale Met. e Geod._ vol. 14, pt. i. p. 57.

[65] _A Magnetic Survey of the Netherlands for the Epoch 1st Jan.
1891_ (Rotterdam, 1895).

[66] _Kg. Svenska Vet. Akad. Handlingar_, 1895, vol. 27, no. 7.

[67] _Denkschriften der math. naturwiss. Classe der k. Akad. des
Wiss._ (Wien), vols. 62 and 67.

[68] _Journal of the College of Science, Tokyo_, 1904, vol. 14.

[69] _Ann. de l'observatoire ... de Toulouse_, 1907, vol. 7.

[70] _Ann. du Bureau Central Mét._ 1897, I. p. B36.

[71] _T.M._ 7, p. 74.

[72] _Bull. Imp. Univ. Odessa_ 85, p. 1, and _T.M._ 7, p. 67.

[73] _P.T._ 187 A, p. 345.

[74] _P.R.S._ 76 A, p. 181.

[75] _Bull. Soc. Imp. des Naturalistes de Moskau_, 1893, no. 4, p.
381, and _T.M._ 1, p. 50.

[76] _Forsch. zur deut. Landes- u. Volkskunde_, 1898, Bd. xi, 1, and
_T.M._ 3, p. 77.

[77] _P.R.S._ 76 A, p. 507.

[78] Adams, _Scientific Papers_, II. p. 446.

[79] _B.A. Report_ for 1898, p. 109.

[80] _Abhand. der bayer, Akad. der Wiss._, 1895, vol. 19.

[81] _Sitz. k. Akad. der Wiss_. (Berlin), 1897, no. xviii., also
_T.M._ 3, p. 191.

[82] _T.M._ 2, p. 11.

[83] _Die Elemente des Erdmagnetismus_ (St Petersburg, 1899), p.
103.

[84] _T.M._ 9, p. 113.

[85] _T.M._ 1, p. 77, and Nature, 57, pp. 160 and 180.

[86] _M.Z._ 15, p. 175.

[86a] _Sitz, der k. k. Akad. der Wiss. Wien, math. nat. Classe_,
1898, Bd. cvii., Abth. ii.

[87] _P.T._ (A) 180, p. 467.

[88] _Die Tägliche Periode der erdmagnetischen Elemente_ (St
Petersburg, 1902).

[89] _R. Accad. Lincei Atti_, viii. 1899, pp. 69, 121, 176, 269 and
previous volumes, see also _Séances de la Soc. Franc. de Physique_,
1899, p. 118.

[90] _Bull. Soc. Vaud., Sc. Nat._ 1906, 42, p. 225.

[91] _Comptes rendus_, 1905, 141, p. 567.

[92] _National Antarctic Expedition 1901-1904_, "Magnetic
Observations."

[93] _The Norwegian Aurora Polaris Expedition 1902-1903_, vol. i.

[94] (1) p. 163.

(C. Ch.)