Having thus illustrated the effects of the aurora, let us now return to
the discussion of its causes.

The intimate and constant connection between the phenomena of the
aurora borealis and terrestrial magnetism led Humboldt to class under
the head of Magnetic Storms all disturbances in the equilibrium of the
earth's magnetic forces. The presence of such storms is indicated by
the oscillations of the magnetized needle, the disturbance of the
currents upon the telegraph-wires, and the appearance of the aurora, of
which these oscillations and disturbances are, as it were, the
forerunners, and which itself puts an end to the storm,--as in electric
storms the phenomenon of lightning announces that, the electrical
equilibrium, temporarily disturbed, is now restored.

The atmosphere is constantly charged with positive
electricity,--electricity furnished by the vapors that rise from the
sea, especially in tropical regions,--and, on the other hand, the earth
is negatively electrized. The recomposition or neutralization of the
two opposite electricities of the atmosphere and of the terrestrial
globe is brought about by means of the moisture with which the lower
strata of the air are more or less charged. But it is especially in the
polar regions, where the eternal ice that reigns there constantly
condenses the aqueous vapors under the form of haze, that this
recomposition must be brought about; the more so, as the positive
vapors are carried thither and accumulated by the tropical current,
which, setting out from the equatorial regions, where it occupies the
most elevated regions of the atmosphere, descends as it advances
towards the higher latitudes, until it comes in contact with the earth
in the neighborhood of the poles. It is there, then, chiefly, that the