Page:Popular Science Monthly Volume 36.djvu/825

Rh through our globe, here appearing, there receding, but forever seeking to attain its equilibrium. The instrument by which these occurrences are observed is the most simple conceivable—the magnetic needle. It is known that the direction and the inclination of the needle point out the magnetic location of a place. From its movement the degree of magnetic attraction of any point may be accurately determined. If sudden deviations and fluctuations of the needle occur, this evidently is an indication that the magnetic condition of the earth has been disturbed. If such variations always occur simultaneously with the appearance of northern lights, this is to be regarded as proof that an intimate relation exists between these phenomena and terrestrial magnetism. Strong northern lights, in fact, exert so great an influence on the magnetic condition of the earth, that the appearance of the phenomenon in some northern latitude may be inferred from the sudden oscillations of the needle in places where the lights are not visible. But the most striking connection between terrestrial magnetism and northern lights is shown by the position of the light-crown in the heavens. The center of this is always to be found at that point where the dipping-needle, if prolonged, would meet the aurora. The rays emanating from the arc have the direction of the earth's magnetic power; they are therefore parallel, and only apparently converge to a point. The crown of light has in truth no fixed place in the heavens, but like the rainbow its position depends on the point from which it is observed, and thus moves with the observer. But the dipping-needle in every place points to the center of the crown.

It may hence be asserted that the northern lights hold an intimate relation to the changes of terrestrial magnetism as indicated by the oscillation of the needle, and that both phenomena must have one and the same cause. The explanation of this cause has been sought in various ways, but an interpretation which would be universally accepted has thus far not been advanced. One of the first attempts to solve the problem was made by the physicist De la Rive, of Geneva, in 1862. It was based on an experiment devised by him, in which he attempted an imitation of these phenomena on a small scale. He held that vapors charged with positive electricity rise into the higher regions of the atmosphere, while the earth remains charged with the negative fluid. When the vapors are driven by the trade-winds to the poles, as soon as the tension is sufficient to overcome the non-conducting property of the air, which like an insulator lies between the earth and these vapors, the positive and the negative electricity come together. This process is accompanied by the appearance of light. The earth and the upper layer of the atmosphere must hence be regarded as an electrical condenser, with the lower layers