account of the preponderance of metal in the composition of that body, generates positive electricity on the earth by induction, while the negative electricity passes over into the atmosphere surrounding the earth. The constitution of the air at different times and at different places favors this process more or less. The conducting metallic rod of our electrical experiments may be considered here as being replaced by mountain-peaks projecting high into the air.
If the equatorial currents, already mentioned by De la Rive, are borne in mind, it is evident that the air at the poles must be highly electrified, and that an exchange must then and there take place between the negative electricity and the positive (induced) electricity. This exchange gives rise to phenomena of light identical with those observed as northern lights at the north pole and as southern lights at the south pole.
This theory is strengthened by the observation that northern lights are closely connected with the appearance of sun-spots and protuberances on the sun. Already in former times a certain periodicity of northern lights was noticed. Besides the annual period in which they appear most frequently at the times of the equinox, and least frequently at the times of the solstices, a period of eleven years has been observed, corresponding closely to that of the sun-spots, the maximum of which coincides with the maximum of northern lights.
The appearance of sun-spots and protuberances, the connection between which was pointed out by Tacchini in 1885, may be regarded as signs of changes occurring on the sun, probably involving increased combustion.
This increased activity must influence the induced terrestrial electricity, and in consequence also the phenomena of northern lights. The variations and declinations of the magnetic needle, moreover, give indubitable proof of the connection between the periods of sun-spots and the electrical condition of the earth.
It should not seem strange that the terms electric and magnetic condition have been here used as synonymous. Since Oersted 's discovery of the influence of the electric current on the magnetic needle, and Ampère's theory of magnetism, electricity and magnetism are regarded as merely two different forms of one and the same force of nature.—Translated for the Popular Science Monthly from Ueber Land und Meer.
