264 On the Aurora Borealis. 



frozen, then electricity will be evolved, and by induction a 

 luminosity will appear ; such clouds meeting with others of 

 opposite electricity, would communicate by means of streamers, 

 these also being- luminous. In other words, a vaporous cloud, 

 passing through a region where the air is of lower temperature, 

 becomes condensed, and, if the temperature be sufficiently low, 

 composed of minute frozen spiculge, which induce recomposition 

 between other clouds of different electricity near them, causing- 

 streamers and bands to flash out light. These appearances 

 will present themselves wherever there are clouds composed of 

 frozen particles, acted upon by the surrounding atmosphere or 

 by neighbouring clouds, so that no altitude will be too great 

 or too inconsiderable for the appearance of Aurora so long as 

 the atmosphere contains the necessary conditions for the 

 evolution of this light. Oftentimes in this country, and in 

 crossing the Atlantic, I have seen Auroras which at times as- 

 sumed simply the appearance of cirrous clouds. The wind may 

 occasion a pulsation in the body of an Aurora, and even a 

 greater degree of brilliancy, the friction produced by it perhaps 

 causing an increase in the electricity evolved. 



I believe Aurora is never seen, except when clouds or other 

 similar vapours are exposed to the process of congelation. We 

 know by Mr, Glaisher's last balloon ascent that a temperature 

 of — 20° occurs at a height of six miles above the earth, at the 

 same height clouds exist ; here, then, according to this " con- 

 gelation " theory, Auroras may appear, or at any other heights 

 where similar circumstances are to be found. It may be argued 

 that Auroras are often seen on a clear night when no clouds 

 are visible, but there is no proof that vapour-masses do not 

 exist at the same time ; in fact, often when no such masses are 

 seen in the sky, a halo round the moon or sun will exhibit 

 irrefragable evidence that such are present, though they be 

 otherwise undistinguishable. 



This theory would go far to account for the more frequent 

 appearance of Aurora in this country lately, the amount of cold 

 having been greater during late winters : last winter, however, 

 being mild, very few Auroral displays were noticed. 



