THE AURORA. 477 



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, perpen- 

 dicular 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 re- 

 turning 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 elec- 

 tricity 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 pro- 

 ducing 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 acci- 

 dental 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 



