MARYLAND GEOLOGICAL SURVEY 415

The dip of the needle or the magnetic inclination if the second
element involved in a magnetic survey. Scientifically defined the
magnetic inclination is the angle, measured in the vertical plane
passing through the magnetic meridian, which a magnetized needle,
mounted so as to move freely about a horizontal axis, makes with the
plane of horizon. The dip measured in the plane at right angles to
the magnetic meridian would be 90°, i. e., the needle would stand
exactly vertical, while in the plane of the magnetic meridian it would
have a minimum value for the place at which the measurements are
made.

At the so-called " magnetic equator " the dip is zero, and hence the
dipping needle perfectly horizontal. As we advance northward along
a magnetic meridian of the earth the north end begins to dip down-
ward by an amount continually increasing until we reach that point
on the earth's surface usually designated as the north magnetic pole;
here the needle stands precisely vertical with the north end down, just
as it did at any place in the plane of the magnetic prime vertical—
in the plane at right angles to the magnetic meridian. In the south-
ern hemisphere the south end is the one which dips down, and pre-
cisely the same phenomena are exhibited by the south end in the
magnetic southern hemisphere as by the north end in the northern.

In Maryland the magnetic dip or inclination varies from about 69 1/2°
in the extreme southern portion to about 71° in the extreme northern.

The magnetic dip, like the declination, is also subject to a secular
variation.

Reference has been made above to the magnetic poles of the earth.
This term is so generally misunderstood—the term being really an
unfortunate one—that it will not be amiss to explain here what is
meant. The so-called magnetic poles of the earth are those points
on the earth's surface where the dipping needle stands exactly vertical.
At these points the compass needle has no fixed direction, the declina-
tion having any value from 0° to 360°. This is due to the fact that
at these places the part of the earth's magnetic force which acts on a
compass needle has dwindled to nothing. Thus far two such points
are known to us, one in the northern hemisphere and one in the
southern. These points do not coincide with those points where the