AURORA BOREALIS. 



11T 



observed to follow its direction in the heavens, and, 

 as it were, to be attracted towards it, and regulated 

 by its influence in their course. 



But it appears, by calculation, that during the 

 scientific age of Rome, the only period at which ac- 

 curate observations of the phenomena of the heavens 

 could be made and recorded, the line of no variation 

 run across the continent of Europe ; and consequent- 

 ly the aurora borealis, or stream of electrical matter 

 that passes from the north pole towards the equator, 

 would find a ready vent from the earth into the sky, 

 through the mountains, and other pointed bodies 

 which are so plentifully scattered over the land. By 

 thus flowing constantly, and so greatly subdivided, 

 it could not give rise to any striking atmospheric 

 phenomena. At present, the direction of this line is 

 through the Atlantic ocean ; and of consequence, the 

 electricity of the polar regions is not imperceptibly 

 transmitted to the atmosphere, but proceeds in such 

 masses, and at such irregular intervals, as to exhibit 

 itself in the beautiful phenomena of the northern 

 lights. A thousand years ago the line of no varia- 

 tion, no doubt, occupied the same situation as it does 

 at present; and the aurora then shone forth in all its 

 splendour; but, at that period, the nations of Eu- 

 rope were sunk in ignorance and barbarism; and 

 'whatever phenomena the heavens presented were lost 

 to posterity, from the rudeness and want of know- 

 ledge of the people of that age. Thus, if this theory 

 be true, when the line of no variation shall again pass 

 over the land, the aurora borealis will become invi- 

 sible for a time ; and when this line reverts to its pre- 

 sent situation over the ocean, the aurora will once 

 more shine forth with its wonted lustre. (>) 



A new theory of the aurora borealis has lately 

 been proposed by M. Monge. He imagines that this 

 phenomenon is merely clouds illuminated by the sun's 

 light, which falls upon them after numerous reflec- 

 tions from other clouds placed at different distances 

 in the heavens. If we suppose that clouds placed in 

 the atmosphere are enlightened by the direct rays of 

 the sun, and reflect the light which they receive to 

 other clouds, situated in a part of the heavens de- 

 prived of the direct light of the snn, and if we sup- 

 pose this light to be necessarily reflected to other 

 clouds, we shall have some idea of the possibility of a 

 mass of thin clouds being illuminated by the sun 

 when this luminary is considerably depressed below 

 the horizon of the spectator. The intensity of these 

 radiations will depend on the dispersion and absorp- 

 tion of the hglit 10 its successive reflections, and it 

 will be more distinctly.perceived when the rest of the 

 atmosphere is involved in darkness. Upon this hy- 

 pothesis, Monge has explained why the phenomenon 

 is perceived near the poles, and why it is most fre- 

 quently seen between the vernal and autumnal equi- 

 noxes. Se'eLecons de Physique, par Pujoulz, 1805.. 



Another theory of this singular phenomenon has 

 been recently proposed by M. Libes. It is founded 

 on the following principles. 1. If the electric spark 

 d through a mixture of azotic and oxygen 

 gas, nitric acid, nitrous acid, or nitrous gas, will be 

 produced, according to the proportion which exists 

 betwtea the a/ot jnd oxygen. 2. The nitric acid 

 d to the sun, becomes more coloured and vola- 



tile. If a receiver is placed over a vessel containing 

 this acid, exposed to the rays of the sun, the acid 

 will in a few minutes become coloured, and the re- 

 ceiver will be filled with red and volatile vapours, 

 which continue for a long time, and exhibit a bril- 

 liancy resembling the aurora borealis. 3. In the 

 flasks which contain the nitrous acid, there is always 

 above the acid, a red and volatile vapour, which is 

 never condensed. 4. Nitrous gas, in contact with 

 atmospheric air, exhales red vapours, which fly away 

 in the atmosphere. 5. The hydrogen gas which dis- 

 engages itself from the surface of the earth, rises to 

 a height in the atmosphere corresponding with its 

 specific gravity. 6. The heat of the sun is extreme- 

 ly feeble in the solar regions. From these principles, 

 M. Libes concludes, that there is very little hydrogen 

 produced in the polar regions, and that therefore 

 there is almost none of this substance in the higher 

 regions of the atmosphere. The electric fluid, there- 

 fore, passing through a mixture of azot and oxygen, 

 will produce, nitric acid, nitrous acid, or nitrous gas, 

 and these substances, acted upon by the solar rays, 

 will exhibit those red and volatile vapours, which 

 form the aurora borealis. A more complete account 

 of thft ingenious theory may be found in the Traile 

 de Physique, par Libes, or in the Dictionhaire de 

 Physique, of the same author ; and in Rozier's Jour- 

 nal, June 1790, Feb. 1791, and vol xxxviii. p. 191. 



The latest theory which has been employed to ac- 

 count for the aurora borealis is that of our ingenious- 

 countryman Mr Dalton, who considers it as a mag 

 netic phenomenon, whose beams are governed by the 

 magnetism of the earth. He supposes that these 

 beams are cylindrical portions of a magnetic fluid, 

 which are actually parallel to the dipping needle, and 

 therefore appear to converge to the magnetic pole ; 

 and that the light is produced by the transmission of 

 electricity, winch disturbs their magnetic properties. 

 Mr Dalton observes, that the luminous arches are al- 

 ways perpendicular to the magnetic meridian ; and 

 that, from the permanency of their form, they afford 

 an opportunity of determining the height of the me- 

 teors. From an observation on a base of 22 miles, 

 he found its altitude to be about 150 miles. See^ 

 Dalton's Meteorological Observations and Essays, 

 1793, p. 54. 153. 



The Abbe Bertholon ascribes the aurora borealis 

 to a-^hosphorico-electric light. A full account of 

 this theory has been given by its author in the Encyc. 

 Method, art. A u no he. 



Much interesting information respecting the aurora 

 borealis, will be found in the following works : 

 Muschenb. Instit. Phys. c. 41. Tract. Phys. et Hist, 

 de I'Aur. Bor. par M. de Mairan,. Paris, 1754, 

 9to. Beccana Dell. Elect ricismo Artif. et Nat. 

 p. 221. Smith's Optics, . 69. D'Alembert'* Opus- 

 cule* Mat/iemali./iics, vol. vi. p. 334. Phil. Trans. 

 1716, p. 406 ; 1717, p. 584, 586 ; 1719, p. 1099, 

 1101, 1104, 1107; 1720, p. 21 ; 1721, p. 180, 186; 

 1723, p. 300 ; 1724, p. 175 ; 1726, p. 128, 132, 150 ; 

 1727, p. 245, 253, 255, 301 ; 1728, p. 453 ; 1729, 

 p. 137; 1730, p. 279; 1731, p. 53, 55; 1734, 

 p. 243, 291 ; 1736, p. 211 ; 1740, p. 368 ; 1741, 

 p. 744, 839, 840, 813 ; 1750, p. 319, 345,346, 499 ;' 

 1751, p. 39, 126 ; 1762, p. 474, 479 ; 1764, p. 326, 

 332 ; 1767, p. 108 ; 1769, p. 86, 367 ; 1770, p. 532 ; 



Aurora 

 Boreairs. 



