26 t HISTORY OK 



its sportive fires, with variegated beauty, over tlie whole horizon. 

 Its appearance is almost constant in winter ; and at those sea- 

 sons when the sun departs to return no more for half a year, 

 this meteor kindly rises to supply its beams, and affords suffi- 



ingr in the cloud, a luminous train or column, of which the motion is at first 

 gentle and unifonn, and which increases in size as it advances. The dimen 

 eions and duration of these columns, however, vary considerably. Their 

 light is sometimes white, sometimes reddish, or even blood colour ; and, as 

 they advance, their colours change, till they form a kind of arch in the 

 heavens. When several of these columns, which have issued from different 

 places, encounter each other in the zenith, they intermingle with each other, 

 and form at their junction a small thick cloud, wliich seems as it were to 

 kindle, and sends forth a light considerably more brilliant than that of any 

 of the separate columns. This light changes to green, blue, and purple ; and 

 quitting its original situation, it directs itself towards the sout^h, under the 

 form of a small bright cloud. When no more columns are seen to issue, 

 the cloud assumes the appearance of the morning dawn, and insensibly 

 dissipates itself. The duration of the aurora is very various. Somecimes 

 it is formed and disappears in the course of a few minutes. At other 

 times, it lasts during the whole night, or even for two or three days toge- 

 ther. In high northern latitudes, as those of Sweden, Lapland, and Si- 

 beria, the aurora borealis are singularly resplendent, and even terrific. 

 They frequently occupy the whole of the heavens, and eclipse the splen- 

 dour of the stars, planets, and moon, and sometimes even of the sun 

 himself. In the north-eastern districts of Siberia, according to the descrip- 

 tion of Gmelin, cited and translated by Dr Blagden, (P/i(7. Trans, vol. Ixxiv. 

 p. 228,) the aurora is observed to " begin with single bright pillars, rising in 

 the north, and almost at the same time in the north-east, which, gradually 

 increasing, comprehend a large space of the heavens, rush about from place 

 to place with incredible velocity, and finally almost cover the whole sky up 

 to the zenith, and produce an appearance as if a vast tent was expanded in 

 the heavens, glittering with gold, rubies, and sapphire. .\s soon as the phe- 

 nomena of electricity, and the laws by which they are governed, were toler- 

 ably understood, philosophers very naturally had recourse to this agent, as 

 atfording a satisfactory explanation of the aurora borealis. The brilliancy o 

 its light, the rapidity of its motions, and the instantaneous changes of form 

 which it underwent, all seemed plainly to point to this powerful element as 

 the cause of these striking phenomena. A small quantity of electricity ex. 

 cited in a highly rarefied atmosphere, or in a medium approaching to a per- 

 fect vacuum, wiU exhibit luminous appearances entirely resembling thH 

 aurora borealis, for a very considerable space of time. With respect to the 

 variations of colour which we find in the aurora borealis, these seem fairly 

 ascribable to the difiereat degrees of rarefaction of the air ; for the same 

 electricity which appears wliitein a very rare medium, becomes blue, purple, 

 or red, in a medium of increased density ; as is fuijy evinced by the follow- 

 ing experiment. Let an electrical machine and an air pump be so disposed, 

 that while the machine is worked, a succession ot e.trong sparks shall be com- 

 municated from the prime conductor to a metallic knob attached to tie top 

 of the receiver of the air pump. l.et now the exhaustion of the receiv«t 



