the latter producing on a Holtz machine electric discharges which electrified rarefied air in Geissler tubes.
Another theory, proposed a few years ago by Edlund, seems nearer the truth than any yet propounded. He began with the phenomena of unipolar induction, discovered by Weber. This is the name given to those currents which arise in each half of a metallic sheath which surrounds a magnet when the sheath is rapidly revolved around the magnet. It is known that the general phenomena of magnetism can be satisfactorily explained on the hypothesis that the earth is a magnet with two poles. The earth rotating on its axis is similar to the sheath, and unipolar action is induced.
Edlund shows that a molecule charged with positive electricity, taken on the surface of the earth, is subjected to two forces: one, from below, driving it upward into the air; the other force, perpendicular to the first, drawing it toward the nearest pole. The first movement is in full force at the equator and nil at the poles; the other is nil at the equator, increases with the latitude, and is again nil at the poles. The tendency is then for the electrified molecule to rise in the atmosphere, thereby accumulating a store of electricity in the higher atmosphere, the movements thence tending toward the poles. As rarefied air and gases are good conductors, being like a vacuum, the electricity will find little resistance at the poles, in returning to the earth. It can either return in disruptive discharges like storms, or in slow discharges in the form of continuous currents, as in the polar aurora. The vertical force is here nil. The electricity of the atmosphere generally re-enters the earth before it reaches the poles, producing local auroras. In polar regions the tension or attraction is much stronger, the flow downward more rapid, having only to overcome the resistance of the air, hence producing greater displays.
Edlund's theory satisfactorily accounts for (1) the direction of the rays of the aurora; (2) the frequency of auroral crowns; (3) the zone of greatest frequency being in polar regions; (4) the deviation of the summit of the auroral arcs from the magnetic meridian (which does not coincide with the geographical meridian); (5) the accidental deviations caused by atmospheric conditions, clouds, humidity, and temperature, that cause curtains, draperies, cirrus effects, etc. This theory seems of all others the most tenable and credible, and should be retained, at any rate for the present, until a more plausible and satisfactory one can scientifically be adopted. It explains clearly the occurrence of polar auroras which are simple, and less reasonably the auroras of immense extent covering two thirds of the globe. These are always accompanied, as we have seen, by marked magnetic disturbance and telluric phenomena, which are probably
