486 FIRST REPORT UPON MAGNETIC WORK

THE DISTRIBUTION OF THE MAGNETIC DECLINATION

IN MARYLAND.

EXPLANATORY REMARKS.

At no two periods of the earth's history is the distribution of mag-
netism within the earth's crust precisely the same. Any graphical
representation of that distribution will therefore apply only to a
particular time. In the present instance the isogonic chart or chart
of the lines of equal magnetic declination (variation) for Maryland
has been constructed for the year 1900. These lines, as is well known,
connect all the places in Maryland where the angle of deviation of
the compass direction from the true north and south direction is the
same. They should not be confounded with the lines which would
be traced if we were to start out, for example, from some point in
southern Maryland and always follow the direction indicated by the
north end of the compass. Such lines have been termed magnetic,
meridians.

Future reports will contain likewise the charts showing the distri-
bution of the magnetic inclination and the intensity of the magnetic
force. In order to supply the practical demands of the surveyor it
was necessary to publish first the isogonic chart. It has been
termed "preliminary," as in its construction only the 1896 results
of the magnetic survey could be utilized. Still, though it is regarded
as preliminary, it is nevertheless far superior to any chart embracing
the same region hitherto published. The values of the declination
given in the table below may undergo slight modifications in future
reports, when the more precise reductions to the epoch of the chart
are known. These changes, however, will be of such a nature as not
to affect the practical use which the land surveyor may make of the
present values.

Besides the data resulting from my last year's work and given in
Table XIII, I have made use of all the material observed and collected
by the Coast and Geodetic Survey in Maryland and vicinity. This