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361
MAGNETISM, TERRESTRIAL


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 TABLE IV.-Declination at Kolaba (Bombay).

an expression of the type c+a(l -lo)-l-b() -ko), where a, b, c are constants, lo and 7° denoting some fixed latitude and longitude which it is convenient to take as point of departure. Supposing observational data available from a series of stations throughout the area, a, b and c can be determined by least squares! As an example, we may take the following slightly modified formula given by Ad. Schmidt 15 as applicable to Northern Europe for the period 1890 to 1900. AD, AI and AH represent the mean annual changes during this period in westerly declination, in inclination Year Declina- Change since Year Declina- Change since and in horizontal force:- tionEast. previous year. ' tionEast. previous year. 1 1 1 AD = -5-24-o-o71(I-5o)+o-03301-10),

° I I ff ° f ff I ~ AI = -1-58+o-010(l-5o)+0~o36(>-Io), 1876 0 55 037 E 1881 0 57 12 0 3 E AH = '|'23'5"0'59 (l'50)”“0'35 (1-10)-1877 56 39 041 E 1382 0 55 50 0 22 W Longitude A is here counted positive to the east. The central 1878 57 6 0 27 E 1383 57 2 0 12 E position assumed here (lat. 5o°, long. 10° E.) falls in the north of 1879 57 30 0 24 E 1884 55 3Q 1 23 W

1880 57 9 0 2I W 1885 55 3 0 36 W TABLE V.-Declination at St Helena and Cape of Good Hope. rapid secular change of inclination towards the end of the 18th St Helena. Cape of Good Hope. century in western Europe; for observations in Paris show a fall , of 56' between 1780 and 1791, and of 90' between 1791 and 1806. Date. D€C11Hat10I1- Date- D€C1111Ht1011-Between ISOI and 1901 inclination in London diminished by - . H-3° 26'-5, or on the average by 2'-I per annum, while between 1857 o 1 c 1 and 1900 H increased on the average by 227 a year. These 1610 7 1 E 1605 0 30 E values differ but little from the secular changes given in Table I. 1677 0 43 1609 0 12 W as applying at Kew for the epoch Jan. I, 1901. Since the beginning, 1691 1 0 W 1675 8 14 however, of the 20th century a notable change has set in, which 172 7 30 1691 11 0 seems shared by the whole of western Europe. This is shown in a 177; 12 18 1775 21 14 striking fashion by contrasting the data from European stations 178 15 30 1792 24 31 in Tables I. and II. There are fifteen of these stations which give 1792 15 48 1818 26 31 secular change data for H in both tables, while thirteen give secular 1806 17 18 1839 29 9 data for I. The mean values of the secular changes derived from 1839 22 17 1842 29 6 these stations are as follows z- 1840 22 53 1846 29 9 1 H 1846 23 II 1850 29 I9

From Table I. -2'-35 -l-21~o'y 1890 23 57 1357 29 34 From Table II. -1-12 +I'6'Y 1874 30 4

The difference in epoch between the two sets of results is only 1390 ' 29 32 about 5 years1 and yet in that short time the mean rate of annual 1903 28 44 increase in H iell to a thirteenth of its original value. During 1908-1909 H diminished throughout all Europe except in the extreme west. Whether we have to do with merely a temporar phase, or whether a general and persistent diminution in the' valiiie of H is about to set in over Europe it is yet hardly possible to say. § 13. It is often convenient to obtain a formula to express the mean annual change of an element during a given period throughout an area of some size. The usual method is to assume that the change at a place whose latitude is l and longitude A is given by Bavaria. In the case of the horizontal force unity represents I-y. Schmidt found the above formulae to give results in very close agreement with the data at the eight, stations which he had employed in determining the constants. These stations ranged from Pavlovsk to Perpignan, and from Stonyhurst to Ekaterinburg in Siberia. Formulae involving the second as well as the first powers of l-lo and A-ko have also been used, e.g., by A. Tanakadate in the Magnetic Survey of japan.

TABLE VI.-Secular Change of Declination in the United States (+ to the West). Place. Epoch 1760 70 80 Q0 1800 10 20 30 40 50 60 1 70 80 90 1900 50 I I I I I I I I I I I I I I I

fEastp0rt, Maine . -1-2 0-o +1~2 +2'I +3-2 +4-0 +4-5 +4-9 +5-0 +5~6.+4-5 +3-o +2-1 +1-0 +1-8 -4 Boston, Mass.. . . -2-7 -1~9 -I'O 0-0 +1-1 +1-9 +2-7 +3-5 +4-2 +4'4 +4-0 +3~3 +3-1 +3-0 +3-2 -4 Alban;/, New York. '-4'2 -3-6 -2-7 -1~6 -o~6 +06 +1-6 +2-7 +3~6 +4-6 +4-6 +3-9 +4-7 +2~3 +3-4 -6 Philadelphia Penn. . -4-6 °-4'2 -3-5 -2~3 -1-3 +0-1 +1-3 +2~5 +3-4 +4-3 +4'2 +4-6 +4-4 +3-4 +3~5 -4 <Baltimore, Maryland . -3-9 -3-4 -2-7 -2-0 -0-9 0~o +0-9 +2-0 -|'~2'7' +3~4 +3-9 +4~0 +3-9 +3-6 +3-5 -2 Richmond, Virginia . -3-6 -3-2 -2-5 -1-8 -o~9 0-0 +o~9 +1-8 +2-5 +3-1'+36 +3~9 +3-8 +3~7 +3~4 -2 1Columbia, S.Carol1na.' +3-7 -3-4 -2°9 -2'2 -1-3 -0-5 +0-5 +1-3 +2-2 +2-9 +34 +3~8 +3-8 +3-8 +3-6 ~8

Mac0n, Georgia . -3-7 -3-6 -3-2 -2-5 -1-8 -o~9 o-o +0-9 +1-8 +2-5 +3-2 +3~6 +3-9 +3-5 +3-1 -2

LTampa, Florida . +3-o -2'5 -2-0 -1-1 -0-4 +0-4 +1-1 +2-o +2~5 +3-0 +32 +3~5 +3~7 +2-8 +2~9 -6 Marquette, Michigan. o-0 +1-4 +2-6 +3-7 +4-7 +5-1 +4~9 +3~8 -4 |C0lumbus, Ofhi0 . -o~9 0-0 +0-9 +2'O +2'9 +34 +3~6 +3-7 +3-9 +4-o -4 Bloomington, Illinois . -2-4 -1-5 -0-4 +o~4 +1-5 +2~4 +2-8 +4°2 +3-9 +2~9 -0 lLexingt0n, Kentucky, -o~9 0-0 +0-9 +1-8 +2-5 +3-2 +3~6 +3-8 +3-8 +3~4 -8 Chattan00ga, Tennessee -0-9 0-0 +0-9 +1-8 +2-5 +3-2 +3-6 +4-0 +3~5 +3-1 -6 Little Rock, Arkansas - +2-3 -1-5 -0~9 +0-1 +0-8 +1-7 +2-0 +3~6 +3-7 +2»3 -2 Montgomery, /Xlabama -3-6 -3-5 -3-1 -2-8 -2-2 -1-5 -08 +0-1 -|-o~8 +1~6 +2-2 +2-8 +3-8 +3-9 +2-6 -2 Alexandria, Louisiana. -2»1 -1-6 -0~8 +o~1 +0~8 +1-6 +2-2 +3-6 +3-3 +2-0 -4

  • N0rth0me, Minnes0ta ~1-7 -0-6 +0-6 +1 7 +2-8 3|-4'2 +4-4 +3~5 ~o

]amestown, N. Dakota +1-0 +1-9 +3-1 +4-8 -Q-I~9 ~2

Des Moines, Iowa. . -1-5 -0-6 +0-6 +I'5 +2-5 +3-8 +4-5 +2-7 -6 lDouglas, Wyoming . +0-8 o~0 +1~2 +2-3 +0-5 ~6

4Emporia, Kansas . +0-6 +1-6 +2~7 +3~8 +1-7 ~8

Pueblo, Colorado +0-3 +0-4 +I~5 +31 +0-7 -2

iOkmulgee, Oklahoma . +0-9 +1-5 -i-2~7 -l-3~9 +1~4 -4 SantaRosa, NewMexic0 -0-4 +0-4 +1~4 +2-6 +0-4 ~4

¥San Antonio, Texas . -'1-1 -o~5 -o»5 +1~1 +1-8 +2-7 +0-9 -4 (Seattle, Washington -3'3 -3'5 "'3'7 -3'7 -3'5 -3'3 *3'01-26 °-2'I -1°3 "'1'9 -2'0 '2 Wilson Creel<, Wasl1ington

-21 -1-5 -0°4 -°I°O -1-6 -2

-Detroit, Oregon . . -3-8 -3'9 -3~9 -3~7 -3-4 -29 -2~5 -I-8 -0-8 -1-8 -8 iSalt Lake, Utah . -I'I -0-4 +1-o +I-0 -0-8 -8

Prescott, Arizona -1~4 -0-7 +0-4 +0-4 -1~2 -2

San jose, California -2-6 -2-9 -2-9 -2-9 -2°7 -2-5 -2-3 -2-0 -1-5 -0-8 -0-4 -1-9 ~8 351-0SAnge1eS., , -3-4 “3°4 -3~5 -3'2 fs-0.—2-7 -2-1 -1-6 -H -0-9 -0-3 ~1-6 3-6