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460 FIRST REPORT UPON MAGNETIC WORK
If the monthly values of the magnetic declination, as given at the
bottom of Table J, be corrected for the secular change in the course
of the year, they likewise exhibit a slight variation, having the year as
the period. This is termed the annual variation of the magnetic
declination. This is not to be confounded with the annual change
of the declination, which means the change in one year due to the
secular variation. The latter is a progressive change, so that the
needle at the end of the year does not point the same way as it did
at the beginning, while the annual variation is a cyclical change, that
is, as far as the annual variation is concerned, the needle returns to
the same position virtually at the end of the year that it had at the
beginning. The next table shows how minute a quantity this annual
variation is and that it can be neglected for all practical purposes.
TABLE V.
Annual variation of the magnetic declination at several places in the northern magnetic
hemisphere. 1
[A + sign denotes a deflection of the north end of the magnet to the eastward, a —
sign, the contrary direction. ]
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Month.
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Los
Angeles,
Cal.
l882-'89.
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Key West.
Fla.
1862-'65.
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Washing-
ton, D. C.
1840--42.
1867--68.
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Philadel-
phia, Pa.
1840-'45.
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Toronto,
Canada.
1846-'B1.
1866-'64.
1865--71.
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Dublin,
Ireland.
1841--60.
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Kew,
England.
1858-'62.
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January....
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+0'. 6
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— 0'. 6
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+0'. 6
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— 0'. 5
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0'. 0
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+0'. 4
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0'. 0
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February....
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+0. 2
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—0. 6
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+0. 3
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—0. 4
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+0. 2
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+1. 6
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—0. 6
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March......
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—0. 4
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+0. 1
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+0. 2
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+0. 1
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+0. 1
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+ 1. 7
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—0. 5
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April.......
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—0. 4
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+0. 3
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—0. 1
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+0. 1
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0. 0
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+1. 9
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0. 0
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May........
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—0. 4
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--0. 3
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—0. 4
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—0. 2
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+0. 3
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+1. 3
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+0. 7
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June........
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—0. 4
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--0. 2
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—0. 1
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+0. 6
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+0. 5
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0. 0
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+0. 8
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July........
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—0. 4
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--0. 3
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+0. 2
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+1. 0
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+0. 4
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—1. 2
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+1. 2
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August....
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—0. 1
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—0. 8
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+0. 7
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+0. 9
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0. 0
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—2. 2
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+0. 3
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September.
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+0. 2
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+0. 7
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—0. 4
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0. 0
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—0. 4
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—2. 1
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—0. 2
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October....
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+0. 4
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—0. 5
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—0. 2
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+0. 2
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—0. 6
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—1. 4
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—0. 8
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November.
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+0. 5
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—0. 5
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—0. 2
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—0. 9
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—0. 4
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—0. 3
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—0. 6
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December..
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+0. 6
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—0. 3
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—0. 3
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—0. 7
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—0. 1
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+0. 2
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—0. 7
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It is seen that the total range of the annual variation is a very
small quantity, about 1' for the North American stations. The char-
acter of the variation appears to be different for each station. This
1 From Coast and Geodetic Report for 1890, p. 249. The matter contained
in Tables III and IV was taken from the same source.
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