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.