§ 11. Secular changes of declination have been particularly interesting in the United States, an area about which information is unusually complete, thanks to the labours and publications of the United States Coast and Geodetic Survey.[13] At present the agonic line passes in a south-easterly direction from Lake Superior to South Carolina. To the east of the agonic line the declination is westerly, and to the west it is easterly. In 1905 the declination varied from about 21° W. in the extreme north-east to about 24° E. in the extreme north-west. At present the motion of the agonic line seems to be towards the west, but it is very slow. To the east of the agonic line westerly declination is increasing, and to the west of the line, with the exception of a narrow strip immediately adjacent to it, easterly declination is increasing. The phenomena in short suggest a motion southwards in the north magnetic pole. Since 1750 declination has always been westerly in the extreme east of the States, and always easterly in the extreme west, but the position of the agonic line has altered a good deal. It was to the west of Richmond, Virginia, from 1750 to about 1772, then to the east of it until about 1838 when it once more passed to the west; since that time it has travelled farther to the west. Table VI. is intended to show the nature of the secular change throughout the whole country. As before, + denotes that the north pole of the magnet is moving to the west,—that it is moving to the east.

The data in Table VI. represent the mean change of declination per annum, derived from the period (ten years, except for 1900-1905) which ended in the year put at the top of the column. The stations are arranged in four groups, the first group representing the extreme eastern, the last group the extreme western states, the other two groups being intermediate. In each group the stations are arranged, 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 1830, in Des Moines (Iowa) about 1840, in Santa Rosa (New Mexico) about 1860 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 greatly 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 west lasting for about sixty years. Further west the period shortens. At Pueblo (Colorado) it is about forty years, at Salt Lake under thirty years, at Prescott (Arizona) about twenty years. Considering how fast the area throughout which the secular change is easterly has extended to the east since 1885, one would be tempted to infer that at no distant date it will include the whole of the United States. In the extreme north-east, however, the movement of the needle to the west, which had slackened perceptibly after 1860 or 1870, is once more accelerating. Thus the auspices do not all point one way, and the future is as uncertain as it is interesting.

Table II.—Recent Values of the Magnetic Elements and their Rate of Secular Change.

Place. Geographical position. Absolute Values of Elements. Secular change (mean per annum).
Latitude. Longitude. Year. D. I. H. V. Interval
in years.		 D. I. H. V.
°  ′ °  ′ °  ′ °   ′ ′ ′
Pavlovsk 59 41N  30 29E 1906 1  4.2E 70 36.6N .16528 .46963 5 −4.5 +0.1 − 6 −14
Sitka (Alaska) 57  3N 135 20W 1906 30  3.3E 74 41.7N .15502 .56646 4 −3.0 −1.6 +18 −38
Ekatarinburg 56 49N  60 38E 1906 10 31.0E 70 49.5N .17664 .50796 5 −4.5 +1.7 −23 +18
Rude Skov (Copenhagen) 55 51N  12 27E 1908 9 43.3W 68 45N .17406 .44759
Stonyhurst 53 51N  2 28W 1909 17 28.6W 68 42.8N .17424 .44722 5 −5.9 −1.1 + 6 −25
Hamburg 53 33N  9 59E 1903 11 10.2W 67 23.5N .18126 .43527
Wilhelmshaven 53 32N  8  9E 1909 11 46.8W .18129 5 −5.2 − 7
Potsdam 52 23N  13  4E 1909 9 10.6W 66 20.0N .18834 .42971 5 −5.8 +0.1 − 9 −19
Irkutsk 52 16N 104 16E 1905 1 58.1E 70 25.0N .20011 .56250 5 +0.6 +2.0 −24 +39
de Bilt 52  5N  5 11E 1907 13 19.0W 66 49.9N .18559 .43368 5 −4.7 −0.6 + 2 −16
Valencia 51 56N  10 15W 1909 20 50.3W 68 15.1N .17877 .44812 5 −5.0 −1.2 + 7 −25
Kew 51 28N  0 19W 1909 16 10.8W 66 59.7N .18506 .43588 5 −5.4 −1.1 + 2 −35
Greenwich 51 28N  0  0 1909 15 47.6W 66 53.9N .18526 .43432 5 −5.5 −0.7 + 1 −20
Uccle 50 48N  4 21E 1908 13 36.7W 66  1.6N .19061 .42867 4 −5.3 −0.8 − 3 −35
Falmouth 50  9N  5  5W 1909 17 48.4W 66 30.6N .18802 .43266 5 −4.7 −1.4 + 9 −30
Prague 50  5N  14 25E 1908 8 20.9W 5 −6.5
Cracow 50  4N  19 58E 1909 5 35.1W 64 18N 3 −7.3
St Helier 49 12N  2 5W 1907 16 27.4W 65 34.5N 5 −5.3 −1.2
Val Joyeux 48 49N  2 1E 1909 14 32.9W 64 43.9N .19727 .41792 5 −5.4 −1.7 + 1 −51
Vienna 48 15N  16 21E 1898 8 24.1W
Munich 48  9N  11 37E 1906 9 59.5W 63 10.0N .20657 .40835 5 −4.8 −1.3 + 4 −31
O’Gyalla 47 53N  18 12E 1909 6 43.9W .21094 5 −5.0 −10
Odessa 46 26N  30 46E 1899 4 36.7W 62 18.2N .21869 .41660
Pola 44 52N  15 51E 1908 8 43.2W 60  6.8N .22207 .38640 5 −5.5 −0.6 − 4 −23
Agincourt (Toronto) 43 47N  79 16W 1906 5 45.3W 74 35.6N .16397 .59502 4 +3.4 +0.9 −23 −24
Nice 43 43N  7 16E 1899 12  4.0W 60 11.7N .22390 .39087
Toulouse 43 37N  1 28E 1905 13 56.3W 60 49.1N .22025 .39439 5 −4.5 −1.5 + 2 − 2
Perpignan 42 42N  2 53E 1907 13 4.4W 7 −4.7
Tiflis 41 43N  44 48E 1905 2 41.6E 56 2.8N .25451 .37799 7 −5.2 +1.7 −26 + 2
Capo di Monte 40 52N  14 15E 1906 8 40.3W 56 13.5N 5 −5.1 −1.5
Madrid 40 25N  3 40W 1901 15 35.6W
Coimbra 40 12N  8 25W 1908 16 46.2W 58 57.3N .22946 .38120 5 −4.6 −2.9 +17 −45
Baldwin (Kansas) 38 47N 95 10W 1906 8 30.1E 68 45.1N .21807 .56081 4 −1.7 +1.8 −36 − 8
Cheltenham(Maryland) 38 44N 76 50W 1906 5 22.0W 70 27.3N .20035 .56436 4 +3.8 +1.2 −38 −45
Lisbon 38 43N  9  9W 1900 17 18.0W 57 54.8N .23516 .37484
Athens 37 58N  21 23E 1908 4 52.9W 52 11.7N .26197 .33613 5 −5.5
San Fernando 36 28N  6 12W 1908 15 25.6W 54 48.4N .24829 .35206 5 −4.6 −2.8 +26 −24
Tokyo 35 41N 139 45E 1901 4 36.1W 49  0.0N .29954 .34459
Zi-ka-wei 31 12N 121 26E 1906 2 32.0W 45 35.3N .33040 .33726 5 +1.5 −1.3 +30 + 6
Dehra Dun 30 19N  78  3E 1907 2 38.3E 43 36.1N .33324 .31736 4 +0.8 +5.5 −26 +77
Helwan 29 52N  31 21E 1909 2 49.2W 40 40.4N .30031 .25804 5 −5.7 +1.2 − 6 +13
Havana 23  8N  82 25W 1905 2 25.0E 52 57.4N .30531 .40452
Barrackpore 22 46N  88 22E 1907 1  9.9E 30 30.2N .37288 .21967 3 +4.2 +3.4 +21 +62
Hong-Kong 22 18N 114 10E 1908 0  3.9E 31  2.5N .37047 .22292 5 +1.9 −1.8 +43 − 1
Honolulu 21 19N 158  4W 1906 9 21.7E 40  1.8N .29220 .24545 4 −0.9 −3.2 −19 −62
Kolaba 18 54N  72 49E 1905 0 14.0E 21 58.5N .37382 .15084 5 +2.1 +7.2 −11 +86
Alibagh 18 39N  72 52E 1909 1  0.3E 23 29.0N .36845 .16008 3 +1.7 +6.8 −10 +82
Vieques (Porto Rico) 18  9N  65 26W 1906 1 33.2W 49 47.7N .28927 .34224 2 +7.2 +6.8 −49 +66
Manila 14 35N 120 59E 1904 0 51.4E 16  0.2N .38215 .10960 5 +0.1 −3.9 +47 −34
Kodaikanal 10 14N  77 28E 1907 0 40.7W 3 27.2N .37431 .02259 4 +4.3 +5.5 +16 +61
Batavia  6 11S 106 49E 1906 0 54.1E 30 48.5S .36708 .21889 4 +2.1 −7.7 − 2 +110
Dar es Salaam  6 49S  39 18E 1903 7 35.2W
Mauritius 20  6S  57 33E 1908 9 14.3W 53 44.9S .23415 .31932 5 −0.3 +2.9 −53 −131
Rio de Janeiro 22 55S  43 11W 1906 8 55.5W 13 57.1S .24772 .06164 5 +9.1 −6.8 −42 +44
Santiago (Chile) 33 27S 70 42W 1906 14 18.7E 30 11.8S 3 +6.1 +9.9
Melbourne 37 50S 144 58E 1901 8 26.7E 67 25.0S .23305 .56024
Christchurch, N.Z. 43 32S 172 37E 1903 16 18.4E 67 42.3S .22657 .55259

Table III.—Declination at London.

Date.Declination.Date.Declination.Date.Declination.
°   ′ °   ′ °   ′
158011   15E177321   9W186021   38.9W
1622 6    0 178723   191865 20   58.7
1634 4    6 179523   57187020   18.3
1657 0    0 180224    6187519   35.6
1665 1   22W180524    8188018   52.1
1672 2   30 181724   36188518   19.2
1692 6    0 181824   38189017   50.6
172314   17 181924   36189517   16.8
174817   40 182024   34190016   52.7
190516   32.9

§ 12. Table VII. gives particulars of the secular change of horizontal force and northerly inclination at London. Prior to the middle of the 19th century information as to the value of H is of uncertain value. The earlier inclination data[14] are due to Norman, Gilbert, Bond, Graham, Heberden and Gilpin. The data from 1857 onwards, both for H and I, refer to Kew. “London” is rather a vague term, but the differences between the values of H and I at Kew and Greenwich—in the extreme west and east—are almost nil. For some time after its discovery by Robert Norman inclination at London increased. The earlier observations are not sufficient to admit of the date of the maximum inclination or its absolute value being determined with precision. Probably the date was near 1723. This view is supported by the fact that at Paris the inclination fell from 72° 15′ in 1754 to 71° 48′ in 1780. The earlier observations in London were probably of no very high accuracy, and the rates of secular change deducible from them are correspondingly uncertain. It is not improbable that the average annual change 0′.8 derived from the thirteen years 1773-1786 is too small, and the value 6′.2 derived from the fifteen years 1786-1801 too large. There is, however, other evidence of unusually rapid secular change of inclination towards the end of the 18th century in western Europe; for observations in Paris show a fall of 56′ between 1780 and 1791, and of 90′ between 1791 and 1806. Between 1801 and 1901 inclination in London diminished by 3° 26′.5, or on the average by 2′.1 per annum, while between 1857 and 1900 H increased on the average by 22γ a year. These values differ but little from the secular changes given in Table I. as applying at Kew for the epoch Jan. 1, 1901. Since the beginning, however, of the 20th century a notable change has set in, which seems shared by the whole of western Europe. This is shown in a striking fashion by contrasting the data from European stations in Tables I. and II. There are fifteen of these stations which give secular change data for H in both tables, while thirteen give secular data for I. The mean values of the secular changes derived from these stations are as follows:—

IH
From Table I.−2′.35+21.0γ
From Table II.−1.12+1.6γ