416 FIRST REPORT UPON MAGNETIC WORK

attractive force of the earth's magnetism is the greatest. There are
four foci of maximum intensity, two in the northern hemisphere and
two in the southern, none of which, as stated, fall together with the
foci of vertical dip. The stronger of the two intensity foci in the
northern hemisphere, according to Lefroy's observations in 1843-44,
lies somewhat south of Hudson's Bay, in latitude 52° 10' N. and in
longitude 99° 59' W. of Greenwich, whereas the point where the
direction assumed by the dipping needle coincides with that of gravity
is, according to Sir James Ross's observations of 1831, in Boothia
Felix, in latitude 70° 05' and longitude 90° 46' west of Greenwich.
The " magnetic poles " of the earth are not to be compared with
those of a bar magnet. There are no points on the earth's surface
which possess the same properties as the poles of a bar magnet. Some
time ago the writer received a letter in which a seemingly plausible
argument was advanced to overthrow Gilbert's theory that the earth
is a great magnet, the fallacy in the line of reasoning being due to
the overlooking of the very fact just pointed out. The earth is a
great spherical magnet and, as well known, the external action of
such a magnet is analogous to that of a bar magnet at the centre of
the sphere—a bar magnet whose magnetic moment is equal to that
of the magnetized sphere, whose poles are infinitesimally close to each
other and whose axis coincides with the axis of magnetization of the
sphere. ¹ The equivalent magnetic poles of the earth would therefore
both lie near the centre of the earth.

¹ To obtain some slight conception of the magnetic moment of the
earth, the figures below are given. Suppose we take as our unit, a bar mag-
net of the hardest steel, magnetized as strongly as possible, which shall be
14 inches long, one and a quarter inch wide—such a bar magnet would
weigh one pound. According to Gauss, it would take the following number
of such bar magnets placed at the earth's centre in order to produce the
same external effect as the earth:

8, 464, 000, 000, 000, 000, 000, 000.

Or if we assume that the earth's magnetism is uniformly distributed
throughout the earth, then will the magnetic intensity of each cubic yard
be equal to six of the one-pound steel magnets.

To put the same fact in still another form. The radius of a soft iron
sphere magnetized to saturation, and concentric with the earth, which shall
have the same magnetic effect as that of the earth, is, according to Over-
beck, 243. 2 kilometers, or 132. 4 geographical miles, or 151 statute miles, or
_;, V of the earth's radius.