Magnetic Poles of the Earth—Variation of the Magnetic
Needle—Phenomena of the Northern Lights.
Captain Ross, an English navigator, left England about twenty years ago,
in pursuit of the north magnetical pole. He followd the magnetical
needle, directing its course westwardly till he arrivd at Baffin’s bay
in America, where he left his ship and traveld about two hundred miles
still farther west, at which place he determind to be the north magnetic
pole. This point being several hundred miles south of the north
geographical pole, determines its opposite or south magnetical pole to
be an equal distance north of the south geographical pole, though on the
opposite side of the earth.
From the north magnetic pole southward, the line of coincidence, or line
of no variation as it is sometimes calld, passes through Hudson’s bay,
the state of Michigan, Ohio, and the several states lying south of
these. This line forms a perfect circle around the earth, and the
magnetic needle at any place on the earth within this circle will point
directly through the magnetical poles coincident with the extreme points
of the earth’s geographical axis. The magnetic needle along this line is
sometimes subject to slight vascillations, in consequence of the
fluctuations of electricity occasiond by the heat of the sun, or
geological differences of the earth’s surface.
The variation of the magnetic needle in Oregon, among other things, has
attracted my attention. Surveyors of this country tell me that the
magnetical needle varies to the right of the geographical pole about 19
deg. and 20 min. Some, however, have observd in different places a
variation of more than 20 degrees. This difference may be accounted for,
by the great amount of iron disseminated through the country.
I have taken the trouble to draw a diagram of a section of the earth, to
ascertain what the variation of the magnetic needle would be
geometrically, at Oregon city. This corresponds, to a considerable
degree of nearness, with experiments made with the compass. Concerning
the depth to which the magnetic pole is seated in the earth, I have no
means of ascertaining. This could best be done with a dipping needle
along the line of no variation, at a suitable distance from the equator,
so that the north end of the needle may have a perfect freedom of dip.
Let the distance be ascertaind from the observer to the north magnetic
pole, as manifested at the surface, which differs not much from two
hundred miles, west of Baffin’s bay. This distance may form the base
line of a right-angled triangle. Let the surface angle at the magnetic
pole be the right angle, and the dipping needle will show the angle at
the place of the observer between the line on the surface of the earth
and the line made by the dip. The observer then has a right-angled
triangle, with sufficient data to ascertain at what depth the magnetic
pole lies below the surface of the earth.
It would be gratifying to me, if some philosopher along the line of
coincidence would take the trouble to ascertain the depth of the
magnetic pole, and publish his experiments. Possibly, some day, a
knowledge of that fact may add to the light of science.
The reasons why the magnetic needle at some places on the earth has a
stationary variation from the geographical pole, and at others an annual
movable variation, seems by some to be not easily accounted for, but I
am of opinion that the same reasons may be assigned for the stationary
position of the needle that are assigned for the stationary appearance
of a planet in its orbit around the sun.
The motion of the magnetic pole around the earth, to an observer at any
one point on the earth, as at London at present, presents during its
whole circuit two stationary points or extremes to the left in its
forward, and to the right in its retrograde movement. The extreme
slowness of the magnetic pole round the earth, causes the stationary
variation of the needle to remain nearly the same for a great many
years. Hence so long as the magnetic pole continues to revolve around
the earth, every other place on the earth will give in its turn a
stationary and an annual variation.
Soon after my arrival at Oregon city, in the fall of the year, I observd
that the central portion of that body of light calld Aurora Borealis,
was as much to the right of the north geographical pole as the variation
of the magnetic needle. I was then led to conclude that it was a
constant attendant upon the north magnetical pole, moving westward
gradually as the magnetic pole advances in that direction.
Very evidently the Northern Lights are occasiond by emissions of
electrical light flowing from the great amount of electricity
concentrated at the north magnetical pole.
[Illustration: Decorative glyph]