408 Description of the Aurora Borealis 



observation in the third, that, in the Shetland Islands, it is the 

 constant attendant of clear evenings, will seem to suggest, 

 what, indeed, will probably be easily agreed to, that the Au- 

 rora, in itself, is peculiar neither to clear evenings nor to 

 evenings at all ; but is in activity during the twenty-four hours, 

 or without intermission ; though, to be visible to human eyes, 

 first, the atmosphere must be dark, and, secondly, it must 

 be more or less clear. It may also be thought apparent, 

 from the terms of the twelfth and thirteenth sentences, that 

 too much has not been said by the present writer, of the 

 degree in which the peculiar spectacle, upon each separate 

 occasion, depends, not alone of the proper and really uniform 

 features of the x\urora itself, but also of the atmosphere 

 through which it is seen, with the appearance of which its 

 own appearance is combined ; and of the consequent value 

 of a careful separation of the real phenomena of the Aurora, 

 from the adventitious phenomena of the intervening and sur- 

 rounding atmosphere. That the colours which, whether visibly 

 connected with the atmosphere or otherwise, are displayed dur- 

 ing the appearance of the Aurora Borealis, are wholly derived 

 from the atmospherical medium through which we behold it, 

 and that the Aurora itself exhibits only a pure white light, is 

 what the writer greatly inclines to suspect, and what may 

 seem to be rendered still more credible by that which is re- 

 ported by those who have obtained a partial glimpse of the 

 Aurora Australis, or corresponding phenomenon of the south. 

 This is described, by Mr. Forster, who sailed round the world 

 with Captain Cook, as consisting in ^' long columns of clear 

 white hght;" but the whiteness, in the eyes of the narrator, 

 seemed to establish a difference, instead of a similitude, be- 

 tweeti the Aurorae Austrahs and Borealis, Mr. F. wholly over- 

 looking the explanation which his own account supplies ! 

 ** These columns," says he, '' though in most respects similar 

 to the Northern Lights (Aurora Borealis) of our hemisphere, 

 yet differed from them in being always of a whitish colour, 

 whereas ours assume various tints, especially those of a fiery or 

 purple hue. The sky was generally clear when they appeared, 

 and the air sharp and cold, the thermometer standing at the 

 freezing point." Now this text is its own commentary. The 



