THE AURORA BOREALIS 223 



regions of the atmosphere. It is indeed known that such 

 rays are deviated by the neighbourhood of a magnet ; and 

 also that the presence of large solar spots is always 

 accompanied by magnetic ' storms ' on the earth and the 

 appearance of frequent and brilliant auroras. 



The theory of Arrhenius is that the corpuscles emitted 

 by the sun are not inconceivably minute bodies, but have 

 an appreciable size ; that they are, say, the thousandth of 

 a millimetre, or the 25,000th of an inch in diameter, and 

 that they are expelled from the sun by the repulsive 

 action of light. 



Whichever theory be correct, it is probable that nega- 

 tively electrified gaseous molecules are present in the 

 upper regions of the atmosphere, and it is also probable 

 that these molecules receive their charge most readily 

 where they are most exposed to a vertical sun, that is, at 

 and near the equator. We have seen that Professor James 

 Thomson's upper aerial currents would carry these and 

 other molecules towards the poles; they would move 

 spirally northwards and southwards with an easterly 

 trend. As they approach the poles their number per unit 

 area will obviously increase ; for the terrestrial parallels of 

 latitude decrease in circumference the nearer they are to 

 the poles. It is to be expected that before the actual poles 

 are reached, the potential of the upper air should increase 

 to such an extent as to produce a luminous discharge, in 

 the form of a ring or halo, with the magnetic poles as 

 their centres. It is conceivably this ring which we see as 

 an arch in the sky; it may not be so high-as the coloured 

 streamers, and may well give the nitrogen spectrum. It 

 must be remembered, however, that the earth is a huge 

 magnet; and that lines of force connect the poles in a 

 fashion shown in the figure. The halo, exposed to these 

 magnetic forces, will send out streamers towards the poles 

 as well as towards the zenith; as they approach the 



