ot aid us materially in the inquiry.

CHAPTER X.

We are yet ignorant of the true nature of magnetism. We trace its lines,
as in the diagrams, upon and around the magnet; but we can only do this
with soft iron, or other substance, in which magnetic action may be
induced. We know that these lines are currents, or lines of force, for
that force produces sensible effects, and we measure it by the movements
of the needle. We know that these lines may be _deflected_ by other
magnetic bodies, and concentrated upon them. We know that the earth, and
the smallest magnets, exhibit properties in common. The poles of the
magnet are some distance from its extreme ends--so are those of the earth.
The intensity increases, from the center, or near it, to the poles of the
magnet, as shown by its attraction; and the same increase of magnetic
intensity, from the magnetic equator to the magnetic poles, or near them,
is traced upon the earth.

We know that there are two lines, or rather _areas_, of greater intensity
upon the globe. One extending from the American magnetic pole,
south-eastwardly, to a corresponding pole in the southern hemisphere; and
another, the Asiatic, extending from the Siberian pole to a corresponding
southern one, in like manner. We know that, from those lines or areas,
the intensity, east and west, on the same parallel of latitude, decreases
each way, to about midway between them. Thus, calling the intensity where
Humboldt found the magnetic equator over South America, in 7 deg. 1' south
latitude, 1, or unity--the least intensity known is, .706, found at the
magnetic equator, over the South Atlantic, and at its most southern
depression; and it increases to 1.4 in the West Indies, and to 2.0099 upon
one or more points of the North American continent, south of the magnetic
pole, and about the meridian of 92 deg.. That it is 1.805, at Warren, Ohio, in
latitude 41 deg. 16', and longitude 72 deg. 57', and decreases to 1.774 at New
Haven, Connecticut, in latitude 41 deg. 18'. That it is but 1.348 at Paris,
nearly one third less than on the same latitude in some portions of this
continent. That the line of equal intensity, or "_iso-dynamic_" line, of
1-8/10, is a closed curve of an oval shape, extending somewhat below 40 deg.,
in the longitude of Cincinnati, and reaches off nearly to Bhering's
Straits, on the west; rising in a similar manner, though not so abruptly,
on the east; including the great northern lakes a