But how, it may be asked, arc all the subordinate phenomena of the aurora borealis to be accounted for be the action of electricity ; and why is it cir cumscrilipi 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 tined 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 tempests. The electrical fluid is thus conveyed in great quantities from the upper parts of the atmo sphere between the tropics, to the lower stratum, and thence to the earth ; and the inferior and warm atmo sphere, having once exhausted itself, must necessarily be recruited from the upper and colder region.
These principles are greatly illusttated and con firmed by whathappenc-i to the French mathemati cians, when stationed on the top of one of the Antics.
They found themselves frequently involved in clouds, which, sinking down into the warmer 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 zone is continual ly bringing down vast quantities of electric matter from the cold air that lies directly above it, it follows, 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 ; and it is in the night and not in the day tine 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 Obserr. 1769, p. 49.) Mr Kirwan (Irish Trans. 178S) 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 aurorae, 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 aurorx, this meteor should abound in the tropical as well as polar regions.