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474 FIRST REPORT UPON MAGNETIC WORK
namely, l. °4, is the quotient of 360° divided by twice the time in-
terval between epochs of maximum and minimum decimation. How
closely the formula represents the existing data is seen from the next
table.
TABLE VIII.
Comparison between observed and computed values of the magnetic declination at Baltimore
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Date.
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Obs'd D.
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Weight.
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Comp'd
D.
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C. -O.
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Date.
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Ob3'd D.
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Weight.
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Comp'd
D.
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C. — O.
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1640. 5
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+9°. 00
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1/4
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+5°. 28
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—3°. 72
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1780. 5
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+0°. 77
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1
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+1°. 02
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+0°. 25
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1679. 0
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5. 25
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1/2
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6. 07
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+0. 82
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1787. 5
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0. 37
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1
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0. 82
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+0. 45
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1683. 5
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6. 25
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1
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6. 01
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—0. 24
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1808. 5
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0. 21
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1
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0. 70
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+0. 49
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1703. 5
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5. 12
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1
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5. 38
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+ 0. 26
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1840. 7
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2. 27
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1
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1. 81
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—0. 46
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1720. 5
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4. 21
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1
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4. 47
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+0. 26
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1847. 3
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2. 31
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1
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2. 18
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—0. 13
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1729. 2
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4. 02
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1
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3. 92
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—0. 10
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1856. 7
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2. 49
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1
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2. 77
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+0. 28
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1754. 5
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2. 28
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1
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2. 27
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—0. 01
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1875. 5
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3. 74
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1
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4. 01
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+0. 27
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1756. 9
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2. 88
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1
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2. 13
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—0. 75
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1877. 8
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4. 18
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1
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4. 16
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—0. 02
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1771. 0
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1. 11
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1
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1. 39
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+0. 28
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1885. 6
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4. 49
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1
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4. 64
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+0. 15
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1776. 1
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+ 1. 75
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1
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+ 1. 17
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—0. 58
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1895. 7
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+5. 34
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1
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+5. 19
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—0. 15
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Fig. No. 6 gives a graphical representation of the formula and
of the observed values as indicated by the dots. The formula must
not be regarded as actually representing the real law governing the
secular variation, but simply as a mathematical approximation to
the law. On the other hand, the differences between values com-
puted from formula and the observed values must not be regarded
as representing in every instance a real fact. The observations,
especially the older ones, are themselves more or less defective. The
formula should not be used for many years before or after the extreme
dates of the series of observations.
The time when the maximum or minimum declination is reached
and N. W. of it. These surveys were made by order of the Baltimore County
Circuit Court in consequence of disputed land boundaries. Other values
Mr. Kelbaugh copied from the record books of the county surveyor and his
assistants, between 1805 and 1825. "—C. and G. S. Report, 1888, p. 215.
These 52 different values were carefully scrutinized by Mr. Schott and
finally combined to 12 mean results, as given in the above table, Nos. 2-14
and No. 17. To these several differences Mr. Schott added the respective
values adopted by him for the declination at the time of the resurvey, as
resulting from his 1877 formula.
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