MARYLAND GEOLOGICAL SURVEY 453

circle can be turned in azimuth and set in the plane of the magnetic
meridian by means of the horizontal circle. Both circles are about
5 inches in diameter and the vertical circle is provided with two
opposite verniers reading to minutes, while the horizontal circle can
be read to minutes with a lens. These are the main features of the
dip circle.

THE VARIATIONS OF THE MAGNETIC DECLINATION.

THE SPECIAL PURPOSE OF THIS CHAPTER.

The mysterious force of the earth's magnetism is forever changing
its direction and magnitude. It seems quite safe to assert that at no
two periods of the earth's history has the state of its magnetism been
precisely the same and that at no future period will the magnetic
condition of the earth return to precisely the same condition prevail-
ing to-day. The tides, the trade winds, while subject to definite-
periodic fluctuations, nevertheless will not change their general char-
acter for thousands of years, but a half-century will suffice to change
materially a cartographical representation of the earth's magnetism.

A survey run on the bearings taken but a comparatively short
time ago will result in mapping out a totally different area from the
previous one. In this way arise the overlappings of areas and the
presence of gores and quadrangles between areas surveyed by the
magnetic needle at different times, when the proper allowance for the
change of the magnetic meridian during the interval, for some reason
or another, was not made.

It is the purpose of this chapter to endeavor to put this matter of
proper allowance, for secular change in Maryland on a better basis
than has hitherto existed. While it is not professed that the tables
presented in this connection are perfect or not subject to improve-
ment, nevertheless the assertion can be made that they are very much
better than the rules of thumb practised quite generally by surveyors