210 ESSAYS BIOGRAPHICAL AND CHEMICAL 



soft-iron bar covered with an insulating composition, and 

 projecting through the opposite neck of the flask. By 

 touching the exposed end of this rod with an electro- 



DE LA RIVE'S APPARATUS FOR REPRODUCING THE AURORA 



magnet D, it also became a magnet. The air in the globe 

 having been rarefied, a brush discharge took place between 

 the ring B and the end of the rod (7; on making C a 

 magnet, the discharge became more luminous and regular, 

 and revolved round the magnet, sending streamers towards 

 the' end of the rod. In De la Rive's model the copper 

 ring represents the atmosphere, from which electricity 

 discharges to the rod C, representing the earth. He con- 

 cluded that the aurora forms a luminous ring round the 

 magnetic poles as centres, with a greater or less diameter, 

 the ring rotating on account of the earth's magnetism; 

 that it is due to a discharge from the positively electrified 

 atmosphere to the negatively electrified earth, the separa- 

 tion of the two kinds of electricity being caused by the 

 action of the sun, chiefly in equatorial regions; that 

 discharges are of constant occurrence, though with very 

 varying intensities, and that the cirro-cumulus clouds are 

 also illuminated by such discharges. 



When the spectroscope was turned on the aurora the 

 presence of a green line of wave-length about 5570 units 



