MARYLAND GEOLOGICAL SURVEY 407

THE DEVELOPMENT AND PURPOSES OF MAGNETIC

SURVEYS.

THE MAGNETIC DECLINATION.

" True as the needle to the pole " is an old and familiar saying.
How untrue it is even within the borders of our own country may be
judged from the fact that in the extreme northeastern part of Maine
the compass needle points twenty-one degrees to the west of north,
while in the extreme northwestern part of the state of Washington
it points twenty-three degrees to the east of north; hence a change of
forty-four degrees from one end of our country to the other! And
even over so comparatively small a territory as that of Maryland the
pointing of the needle varies from six degrees west to three and one-
half degrees west.

There are portions of the earth's surface where the needle points
due east and west and still others where the north end actually points
south. We are thus made acquainted with one element involved in a
magnetic survey, viz., the magnetic declination, or " variation, " as the
mariner and the surveyor are accustomed to say. Scientifically de-
fined, the magnetic declination is the angle between the true north
and south line and the magnetic north and south line as pointed out
by a compass needle, i. e. _} a magnetized needle so mounted as to swing
freely about a vertical axis or pivot.

It was several centuries after the introduction of the compass in
Europe before this deviation of the magnetic meridian from the true
meridian was discovered, and the discoverer was no less a man than
Christopher Columbus. We all doubtless remember reading of the
consternation caused on board ship when it was observed that the
compass had shifted its direction from east of north to west of north.
Columbus had, in fact, crossed the line, on September 13th, 1402,
along which the needle pointed true north, i. e., the line of no magnetic
decimation or variation, or the so-called agonic line. To the east of
this line the needle pointed east, and to the west the needle bore west.