480 FIRST REPORT UPON MAGNETIC WORK

ary 1st, 1900, is 4°.73 W., then the value on January 1st, 1750, for
example, with a fair degree of approximation, was

D₁₇₃₀ = + 4°.73 — 2°.17 = + 2°.56 = 2° 34' W.

TABLE XI A.
Specimen of auxiliary table giving the secular changes in Maryland between 1700 and 1900.

[See page image for table]

On glancing over this table one must be impressed with the gen-
eral agreement of the tabular quantities among themselves, derived
as they were from such various sources. Yet it will be noticed that
if we compare the quantities for any particular year, for example,
1820, the figures may differ among each other by about 0°.8. Such
differences should not be ascribed entirely to defective data; they very
likely represent in a large degree an actual physical fact. In other
words, the amount of secular change, c, at any particular time will
not be quite the same in all parts of Maryland. If we subtract series
II (Western Maryland) from series Y (Eastern Maryland) we no,tice
that the differences vary systematically with the lapse of time (see
figures in last column).