464 FIRST REPORT UPON MAGNETIC WORK

meridian during a given interval of time is generally of greater con-
cern to him than the knowledge of the absolute value of the magnetic
declination. Unfortunately the problem of proper allowance of
change of declination is frequently complicated by other questions,
such as, for example, the date of the early survey or the error of the
compass used in the original survey; or, again, whether the bearings as
recorded are those taken from some previous survey without allowing
for secular variation; or, again, whether they are true bearings or
magnetic ones. The early land records are frequently faulty in all
the details that are absolutely necessary for the proper allowance of
the secular change. No rules can, of course, be given for supplying
such omissions. The surveyor must be guided entirely by the experi-
ence gained in the treatment of analogous cases. My purpose is
simply to give tables enabling the surveyor to determine the change
in the compass direction between any two years during the eighteenth
and nineteenth centuries.

Whether the secular variation is of a strictly periodic character,
that is, whether the needle will at some future time return to the
very same position from which it started out, has not as yet been
definitely settled, for the reason that we do not possess at any one
station records of a complete swing of the needle. The researches
thus far made would seem to indicate that after the lapse of many
centuries the needle may return approximately to its original position,
but that it ever again reaches the identical position does not seem
probable.

At a number of stations we possess records of the magnetic declina-
tion for over three centuries. The table below will show how the
declination has changed at some of these stations during this interval.

Thus at London, for example, we find that the needle pointed east
of north during the interval 1540 to 1658, the easterly declination
reaching its maximum value of 11° in 1580. About 1658 the needle
bore due north, and thereafter westward of due north, the westerly
declination reaching its maximum value of 24° 12' in about 1812.
Beginning with 1812, the westerly declination has been steadily
diminishing, amounting in 1890 to 17°. 57 or 17° 34'. Consequently,