ected line
to which it is perpendicular may be deduced from Brewster's formula
for the temperature, by differentiating and putting the differential
equal to zero.

"3. As a consequence, the laws of the terrestrial distribution of the
physical principles of magnetism and heat must be the same, or nearly
the same; and these principles themselves must have, toward one
another, the most intimate physical relations."

The magnetic elements, of which Professor Norton speaks, are the
declination, dip, and horizontal and vertical forces or intensities.

I have said, that toward the areas of greatest magnetic intensity, the
needle every where declines. So as intensity increases, from the magnetic
equator toward the poles, the needle, when so suspended as to permit of
the motion, _dips_, inclines downward, and the dip is greatest, on the
same parallel, where intensity is greatest. To my mind, the magnetic
elements are very intelligible. They are all attributable to attraction,
and attraction is greatest where intensity is greatest. There is nothing
in the earth or atmosphere to make the needle point northerly rather than
in any other direction, except magnetic intensity. Thus, the greater
intensity of magnetism near the northern and southern points of the globe,
attracts the corresponding ends of the needle in those directions. And, as
magnetism increases in quantity or intensity, and the poles are
approached, the attraction increases, and the needle dips more and more,
till the focus of intensity and attraction is reached, and then it becomes
perpendicular. So magnetism is unequally diffused, meridionally, in or
over the earth, and there are two equidistant areas where its quantity or
intensity is greatest. These exert a lateral attraction upon the needle;
it yields to this attraction, and hence its declination. If it is carried
on to one area of intensity, and to the center of it, it will point to the
northern focus of intensity or magnetic pole; and, if carried a trifle
further west, it will yield to an eastern attraction, and point directly
north. If carried still further west, its declination _east_ will
increase. Thus its normal direction is to the pole, on the central focus
of intensity, and when it points directly north it is west of the central
line of intensity. And thus, it seems to me, all the magnetic elements may
be resolved into the one element of attraction by excess of intensit