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359
MAGNETISM, TERRESTRIAL

TABLE I.-Magnetic Elements and the ir Rate of Secular Change for January 1, 1901.

Table V. contains some data for St Helena and the Cape of Good Hope, " both places having a long magnetic history. The remarkable feature at St He ena is the uniformity in Absolute Val€S- Secular Change- the rate of secular change. The figures for the Cape show PlHC€- a reversal in the direction of the secular change about 1840, D- I- H- D~ I- H- but after a few years the arrested movement to the west again became visible. According, however, to ]. C. Beattie's ° f ° I I I .Y Magnetic Survey of South A frica the movement to the west pavlovelr 0 30.35 70 36.3N .16553 4.1 0.3 + 7 ceased shortly after 1870. A persistent movement to the Ekatarinburg 10 6.3E 70 40.5N .17733 4.6 +0.5 13 east then set in, the mean annual change increasing from Copenhagen 1g10.4 l y 33.5% .17525 1330 between 1873 and 1890 to 3'-8 between 1890 and 1 10- -o -1 o - -0 22%6%T3;i1nli;i.itaven

12 26.gW 67 § 0.7N .1g%g3 1.1 2.1 zi| 2o § 11. Secular changes of declination have been parporsdani 9 54.2W 66 24.5N .13352 4.2 1.6 +16 ticularly interesting in the United States, an area about lrlrursln 2 1.013 70 15.'3N .20122 + 0.5 +1.6 14 which information is unusually complete, thanks to the de Bile 13 43.3W 66 55.5N .13516 4.4 2.2 +14 labours and publications of the United States Coast and Kew 16 50.3W 67 10.6N .13440 4.2 2.2 +25 Geodetic Survey.l3 At present the agonic line passes in a Greenwich 16 27.5W 67 7.3N .13465 4.0 2.2 +23 south-easterly direction from Lake Superior to South Ueeie 14 11.0W 66 3.3N .13954 4.2 2.1 +23 Carolina. To the east of the agonic line the declination is palrnourh 13 27.3W 66 44.0N .13705 3.3 2.7 +26 westerly, and to the west it is easterly. In 1905 the prague 0 4.4W .10956 4.4 +20 declination varied from about 21° W. in the extreme north-Sr Helier 16 53.1W 65 44.1N 3.5 2.7 east to about 24° E. in the extreme north-west. At present pare Sr Maur 14 43.4W 64 52.3N .10755 the motion of the agonic line seems to be towards the west, Val Joyeux 15 13.7W 65 0.0N .10670 4'° '“2'2 +23 but it is very slow. To the east of the agonic line westerly Niunich, 10 25~8W 63 I8~IN -20629 4.3 2.7 +21 declination 1s increasing, and to the west of the line, with O'Gyalla 7 26.1W .21164 4.3 +13 the exception of a narrow strip immediately adjacent to pola ' 9 22.7W 60 14.5N .22216 4.0 +23 it, easterly declination is increasing. The phenomena in Toulouse 14 16.4W 60 55.0N .21945 3.9 2.5 +25 short suggest a motion southwards in the north magnetic Perpignan 13 34.7W 50 57.6N .22453 pole. Since 1750 declination has always been westerly in Cano di M0nt€ 9 3.0W 56 22.3N 5.2 2.3 the extreme east of the States, and always easterly in the Madrid 15 30.0W extreme west, but the position of the agonic line has Coimbra . 17 18~1W 59 22-ON -22786 3.7 4.3 +34 altered a good deal. It was to the west of Richmond, Lisbon 17 15.7W 57 53.oN .23548 Virginia, from 1750 to about 1772, then to the east of it Athens 5 33.2W 52 7,5N 26076 until about 1838 when it once more passed to the west; San Fernando 15 57.5W 55 3.3N .24643 since that time it has travelled farther to the west. Table VI. Tokyo 4 34.9W 40 0.3N .29032 is intended to show the nature of the secular change Zi lra wei 2 23.5W 45 43.5N .32375 + 1.5 1.5 +37 throughout the whole country. As before, + denotes that Helwan 3 30.7W 40 30.8N .30136 7.0 0.4 7 pglehofeglge magnet is moving to the west, -that it 0 I . . . . .§

3£§ >ei<img. 0 1 +3-3 T43 The data in Table VI. represent the mean change of Manila 0 52.2E 16 13.5N .33064 + 0.1 5.3 +47 declination per annum, derived from the period (ten years, Batavia 1 7 3E 3o 35,55 36724 + 3, o 7 3 ri except for Ipoo-1905) which ended in the year put at the Mauritius 9 25.2W 54 9.45 .23320 4.7 +4.6 3o top of the co umn. The stations are arranged in four groups, Rio de Janeiro 3 2.0W 13 20.15 .2501 +10.4 2.3 the first group representing the extreme eastern, the last Melhourne 3 25 6E 67 24,65 23205 group the extreme western states, the other two groups eing intermediate. In each group the stations are arranged, The rate of movement of the needle to the east at London—and throughout Europe generally-fell off markedly subsequent to 1880. The change of declination in fact between ISSO and 1895 was only about 75% of that between 1865 and 1880, and the mean annual change from 1895 to 1900 was less than 75% of the mean annual change of the preceding fifteen years. Thus in 1902 it was at least open to doubt whether a change in the sign of the secular change were not in immediate prospect. Subsequent, however, to that date there was little further decline in the rate of secular change, and since 1905 there has been very distinct acceleration. Thus, if we derive a mean value from 'the eighteen European stations for which declination secular changes are given in Tables I. and II. we n

mean value from table I. -4-18

n 1| n n II- "5'21

The epoch to which the data in Table II. refer is somewhat variable, but is in all cases more recent than the epoch, January 1, 1901, for Table I., the mean difference being about 5 years.

§ 10. At Paris there seerns to have been a maximum of easterl declination (about 9°) about 1580; the needle pointed to true nortli about 1662, and reached its extreme westerly position between 1812 and 1814. The phenomena at Rome resembled those at Paris and London, but the extreme westerly position is believed to have been attained earlier. The rate of change near the turning point seems to have been very slow, and as no fixed observatories existed in those days, the precise time of its occurrence is open to some doubt. Perhaps the most complete observations extant as to the declination phenomena near a turning point relate to Kolaba observatory at Bombay; they were given originally by N. A. F. Moos, “ the director of the observatory. Some of the more interesting details are given in Table IV.; here W denotes movement to be west, and so answers to a numerical diminution in the declination, which is easterly. Prior to 1880 the secular change at Kolaba was unmistakably to the east, and subsequent to 1883 it was clearly to the west; but between these dates opinions will probably differ as to what actually happened. The fluctuations then apparent in the sign of the annual change may be real, but it is at least conceivable that they are of instrumental origin. From 1870 to 1875 the mean annual change was -1'-2; from 1885 to 1890 it was +I'-5, from 1890 to 1895 it was +2'-0, while from 1895 to 1905 it was -{-2'-35, the + sign denoting movement to the west. Thus, in this case the rate of secular change has increased fairly steadily since the turning point was reached. at least approximately, in order of latitude. The data are derived from the values of the declination given in the Geodetic Survey's Report for 1906, appendix 4, and Magnetic Tables and Magnetic Charts by L. A. Bauer, 1908. The values seem, in most cases, based to some extent on calculation, and very probably the secular change was not in reality quite so regular as the figures suggest. For the Western States the earliest data are comparatively recent, but for some of the eastern states data earlier than any in the table appear in the Report of the Coast and Geodetic Survey for 1902. These data indicate that the easterly movement of the magnet, visible in all the earlier figures for the Eastern States in Table VI., existed in all of them at least as far back as 1700. There is not very much evidence as to the secular change between 1700 and 1650, the earliest date to which the Coast and Geodetic Survey's figures refer. The figures show a maximum of westerly declination about 1670 in New Jersey and about 1675 in Maryland. They suggest that this maximum was experienced all along the Atlantic border some time in the 17th century, but earlier in the extreme north-east than in New York or Maryland.

Examination of Table VI. shows that the needle continued to move to the east for some time after 1750 even in the Eastern States. But the rate of movement was clearly diminishing, and about 1765 the extreme easterly position was"reached in Eastport, Maine, the needle then beginning to retrace its steps to the west. The phenomena visible at Maine are seen repeating themselves at places more and more to the west, in Boston about 1785, in Albany about 1800, in Washington, D.C., about 1805, in Columbus (Ohio) about 1815, in Montgomery (Alabama) about 1825, in Bloomington (Ill.) about 183O, in Des Moines (Iowa) about 1840, in Santa Rosa (New Mexico) about 186O and in Salt Lake about 1870. In 1885 the needle was moving to the west over the whole United' States with the exception of a comparatively narrow strip along the Pacific coast. Even an acute observer would have been tempted to prophesy in 1885 that at no distant date the secular change would be pronouncedly westerly right up to the Pacific. But in a few years a complete change took place. The movement to the east, which had become exceedingly small, if existent, in the Pacific states, began to accelerate; the movement to the west continued in the central, as in the eastern states, but perceptibly slackened. In 1905 the area throughout which the movement to the west still continued had greatl contracted and lay to the east of a line drawn from the west end of] Lake Superior to the west of Georgia. If we take a station like Little Rock (Arkansas), we have the secular change to the