AURORA BOREALIS. 



115 



Aurora ter, to abound with electricity when the aurora shines 

 iioreilis. forth . that the aurora, when strong, is accompanied 

 11 V"*"' with the whizzing or crackling sound of electricity ;_ 

 and that the magnetic needle is evidently disturbed by 

 the aurora, as well as by the action of an electrical 

 machine, or by the natural electricity of a thunder 

 storm. 



But how, it may be asked, are all the subordinate 

 phenomena of the aurora borealis to be accounted 

 for by the action of electricity ; and why is it cir- 

 cumscribe to the polar latitudes, and the more ele- 

 vated regions of the atmosphere ? Mr Canton con- 

 jectured, that the aurora borealis is occasioned by the 

 flashing of the electric fire from positive towards ne- 

 gative clouds, at a great distance, through the upper 

 part of the atmosphere, where the resistance is least. 

 But were this all, the aurora ought to be as abundant 

 in the tropical regions as in the polar ; and it ought 

 to dart in all directions, instead of uniformly point- 

 ing towards the zenith. Signior Beccaria, who paid 

 very great attention to atmospherical electricity, sup- 

 posed that there is a constant and regular circula- 

 tion of the electric fluid from the north pole to the 

 south ; and he thinks that the aurora borealis may 

 be this electric matter performing its circulation, in 

 such a state of the atmosphere, as renders it visible 

 on approaching nearer to the earth than usual. This 

 supposition, however, is altogether inconsistent with 

 Mr Forster's observations, already mentioned ; ac- 

 cording to which the columns of the aurora shot up- 

 wards from the horizon towards the zenith, as well in 

 the southern as in the northern hemisphere. 



The course of the aurora, therefore, is uniformly 

 from the poles towards the equator ; and supposing 

 it to consist in a stream of electric light, the follow- 

 ing reasons may be assigned for its constantly preser- 

 ving this course. Extreme cold renders almost all 

 bodies electric, or disposed to accumulate electricity ; 

 while heat and moisture occasion a conducting power. 

 Air, when dry and cold, is powerfully electric ; and 

 hence the beautiful phenomena of the aurora are con- 

 fined to the polar regions, and appear by night and 

 not by day, and in winter rather than in summer. 

 The inferior part of the atmosphere, between the 

 tropics, is violently heated during the day time, by 

 the reflection of the sun's rays from the earth, while 

 the superior parts retain their original cold. It is 

 also impregnated with moisture exhaled by the 

 powerful heat which then acts upon the earth. It is 

 therefore in the conducting state, and readily com- 

 municates the electricity of the superior regions to 

 the clouds which float in it, or to the body of the 

 earth. Hence the awful electrical phenomena of the 

 tropical regions, exhibited in thunder and lightning, 

 water spouts, whirlwinds, and the most tremendous 

 tempest-. The electrical fluid i3 thus conveyed in 

 great quantities from the upper parts of the atmo- 

 re between the tropics, to the lower stratum, and 

 thence to the earth ; and the inferior and warm atmo- 



lere, having once exhausted itself, must necessarily 

 be recruited from the upper and colder region. 



These principles are greatly illustrated and con- 

 firmed by what happened to the French mathemati- 

 cians, when stationed on the top of one of the Andes. 

 They found themselves frequently involved in clouds, 



which, sinking down into the wanner air, appeared 

 there to be highly electrified, and discharged them- 

 selves in violent tempests of thunder and lightning ; 

 while, in the mean time, on the top of the mountain, 

 they enjoyed a calm and serene sky. 



Thus, as the hot air of the torrid zOneis continual- 

 ly bringing down vast quantities of electric matter 

 from the cold air that lies directly above it, it follow.-, 

 that the upper parts of the tropical atmosphere will 

 continually require a supply from the northern and 

 southern regions. Hence the constant electric cur- 

 rent in the upper parts of' the atmosphere, from the 

 poles towards the equator ; which in the colder re- 

 gions, where the air is sufficiently rarefied, assumes ' 

 the form of the aurora borealis and australis ; and 

 hence, this meteor is more frequent in winter than in 

 summer ; because, at that time, the electric power of 

 the inferior atmosphere is greater, on account of the 

 greater degree of cold j and it is in the night and not 

 in the day ti.ne that it displays itself, because, during 

 the day, the heat of the sun is sufficient to impart to 

 every portion of the atmosphere a conducting power. 

 With respect to the perpendicular direction which 

 the streams of the aurora appear to assume, it need 

 not be considered as a material difficulty ; since, as 

 Dr Halley has observed, they must dart from the 

 pole in arches of circles of very great diameter, and 

 consequently appear erect to those who view them 

 from the earth's surface. The upper regions of the 

 atmosphere, on account of their superior rarity, afford 

 them the readiest passage, and hence they assume 

 the perpendicular direction rather than any other. 



Dr Franklin has given a different form to the elec- 

 trical theory of the aurora borealis, supposing that 

 the electricity which is concerned in this phenome- 

 non passes into the polar regions, from the immense 

 quantities of vapour raised into the atmosphere be- 

 tween the tropics ; and that the light appears first, 

 where it is first in motion, that is, in the most northern 

 part ; so that the appearance proceeds southward, 

 though the fluid really moves northward. ( Exper. 

 and Oliserv. 1769, p. 49.) Mr Kirwan (frith Trans. 

 1788) supposes, that the light of the aurora borealis 

 and australis is occasioned by the combustion of in- 

 flammable air, kindled by electricity. He is of opi- 

 nion, that a great quantity of this gas which is form- 

 ed by a variety of natural processes, occupies the 

 higher regions of the atmosphere, on account of its 

 extreme levity ; and is the cause of the aurorre, which 

 are the highest of all meteors. But as far as we may 

 trust to the observations of aeronauts, there is no evi- 

 dence whatever, that inflammable air is more abun- 

 dant in the upper, than in the lower regions of the 

 atmosphere ; and were it the cause of the aurorre, this 

 meteor should abound in the tropical as well as polar 

 regions. 



With respect to the observation of Dr Kirwan, 

 that the barometer commonly falls after an aurora, 

 this is no more than what takes place also after a 

 thunder storm ; and its being followed by high winds 

 from the south is as explicable on the electrical, as on 

 the inflammable ga:; theory. Mr Winn, in the 73d 

 volume of the Phil. Trans, makes the same remark, 

 and says, that in l l?> instances without fail, a stroiii; 

 gab from the south, or south-west, followed the ap- 



