12G 



ON THE CAUSE OF THE AURORA BOREALIS. 



[1855. 



Holland, who found that ho had been making a wrong course 

 from following his needle ; there had been no storm, and the 

 compass had not been touched. Bvit on the evening of the 

 same day, there was a brilliant aurora, and to this he attributes 

 the deviation — a conclusion which could not have been dic- 

 tated by theory, since at the time (in 1818) the relations be- 

 tween electricity and magnetism were not known. 



The intimate connection between the aurora and terrestrial 

 magnetism, lias led Humboldt to designate as a magnetic storm 

 a succession of disturbances of equilibrium in the magnetic 

 forces of the earth. The presence of this storm is indicated 

 by the oscillations of the magnetic needle, and afterwards, by 

 the aurora, of which the oscillations are precursors, and which 

 also put an end to the storm, just as the lightning in an ordi- 

 nary electric storm announces that the equilibrium, before dis- 

 turbed, is again established in the normal distribution of the 

 electricity. Humboldt finds proof, amounting to experimental 

 certainty, in the discovery of Faraday, who produced light by 

 the action of magnetic forces alone, that the earth, by virtue 

 of its magnetism, has the property of emitting light quite dis- 

 tinct from that which is afforded by the sun. 



While recognizing the tnith of the analogy which Hum- 

 boldt here traces out, we should recollect, that it is not of 

 itself, but because it produces electric currents, that magnetism 

 gives out light; the light is purely electrical in origin. Mag- 

 netism produces luminous phenomena only because it can 

 disengage electricity, and it is probably in this point of view 

 that Humboldt says in a general way that it is a source of 

 light. 



It is hence in electricity, and in the influence which this 

 agent in a state of motion, and magnetism, mutually exert, 

 that we must look for the cause of the aurora borealis. This is 

 the view which I would sustain, and to the force of my 

 demonstration, I propose to bring some direct experiments, as 

 well as the results of numerous observations through past 

 years. 



2. Proposed Theory. 



The atmosphere in its normal state is constantly charged 

 with a considerable quantity of positive electricity, which 

 increa.=es as we ascend, starting at the earth's surface where it 

 is zero. 



I will not inquire into the origin of this electricity : what is 

 certain is that its production is connected with the action of 

 the .sun, since its intensity is subject to diurnal variations. It 

 may be a question whether the sun acts directly, either through 

 its light or its heat, on the constituents of our atmosphere, and 

 so produces the electricity ; or whether it is an indirect effect 

 of the solar rays causing evaporation from the waters of the 

 seas, orthe vegetation of the land. It is probable that both cauess 

 act : yet I am inclined to regard the first as most general and 

 most constant. But this is of little importance here : the fact 

 of the constant charge of positive electricity in the atmosphere 

 and of negative electricity in the earth, is abundantly proved, 

 and this is sufficient for our explanations. 



This constant production of the two electricities must 

 necessarily be attended by a recomposition or neutralisation ; 

 otherwise the contrary electric states would acquire an infinite 

 tension, which is contrary to observation. This recomposition 

 or neutralisation takes place in two ways, one irregular and 

 accidental, the other normal and constant. 



The first method is exhibited under various forms. Gener- 

 ally it is the simple humidity of the air, or the fall of rain or 

 sn9W, which causey the neutralisation- At other times, it is 



the thunder-bolt, which exhibits in an energetic manner the 

 tendency to union in the two accumulated electricities, one in 

 the air, the other in the ground. The winds in certain cases, 

 by mixing the air from the earth's surface which is negative 

 like the earth, with the positive air of a region more elevated, 

 leads to a neutralisation of the two electricities, causing cither 

 storms or an exhibition of heat lightning. In winter, the air being 

 constantly more saturated with moisture, the direct neutralisation 

 is effected through the aqueous vapors and there are therefore 

 fewer great disturbances and consequently fewer storms ; and 

 at the same time, as Arago has remarked considering the num- 

 ber of storms, the lightning strikes the earth more frequently 

 in winter than in summer. 



In general, the influence of the hygrometric state of the air 

 on the manifestations of atmospheric electricity is almost as 

 great as that of the cause itself which produces this electricity; 

 for this influence makes itself felt both in the production of the 

 accidental phenomena just enumerated, and in the indications 

 of the electrometer by which we ascertain the normal electric 

 state of the air for the hours of the day, and days of the year. 

 Hence it is diflacult to deduce from these observations even the 

 intensity of the atmospheric electricity for any given moment, 

 seeing that it is impossible to separate this original intensity 

 from the degree more or less decided which the electric regis- 

 ters may manifest. 



Let us now pass to the second mode of neutralisation of the 

 two electricities, which I regard as normal and regular. 



The positive electricity, with which the upper beds of the 

 atmosphere are charged, will traverse them fieely, because of 

 their high state of rarefaction. But in the polar region, where 

 the intense cold constantly condenses the aqueous vapors, it 

 finds a portion of the atmosphere saturated with humidity, giv- 

 ing rise to mists; and by this means it may easily pass to the 

 earth and combine with the negative electricity with which the 

 earth itself is charged. It consequently results that there are 

 constant currents of positive electricity rising from different 

 points of the earth's surface into the upper regions of the 

 atmosphere, which pass towards the poles, and then return 

 beneath the earth's surface towards each of the points whence 

 they have started. The currents of the northern hemisphere 

 should go to the north pole, and those of the southern, to the 

 south pole. In the equatorial regions, the position of the sun 

 will determine the dividing line between the two systems. We 

 may add that the experiments made with the electric telegraph 

 have demonstrated that the terrestrial globe is an almost per- 

 fect conductor of electricity, compensating bj' its mass, for 

 what it wants in the conductibility of the materials which con- 

 stitute it. Thus the existence of the currents, whose course I 

 have traced, rests on well established principles, with a foun- 

 dation of simple experiment. 



But more than this : their existence is demonstrated by facts 

 long studied and established, — those pertaining to the diurnal 

 variation of the magnetic needle. 



I do not examine here into the origin of the earth's mag- 

 netism, a subject to which I shall have occasion to return in 

 another work ; for the present, I only say that I do not legard 

 the disturbing causes of the direction of the magnetic needle 

 as of the same nature with those which determine this direc- 

 tion. I content myself now with regarding the earth as a 

 large magnet having its two poles ; and I study only the causes 

 that modify the direction which, in this quality of a rcrgnet, it 

 tends to impress on the magnetic needle. Ihcse causes are 

 the electric currents, whose existence I have just shown ; they 

 well explain the diurnal variations. These variations, in faqt, 



