58 



NORTHERN LIGHTS. 



pnenomena — those of the dip, variation, and in- 

 tensity, according to the hour of the day and 

 even of the night, according to the season and 

 the lapse of whole years, permit us to suspect 

 the existence of very dissimilar partial sys- 

 tems of electrical currents in the crust of the 

 earth. Are these currents, as in Seebeck's ex- 

 periments, thermo-magnetical, and immediate- 

 ly excited by unequal distribution of heat 1 Or 

 shall we not rather regard them as induced by 

 the position of the sun, and through the influ- 

 ence of his heat 1{^^^) Has the rotation of our 

 planet and the accident of the diflTerent veloci- 

 ties impressed upon the several zones, accord- 

 ing to their distance from the equator, any in- 

 fluence upon the distribution of magnetism 1 

 Shall the seat of the currents, in other words, 

 of the electricity in motion, be sought for in 

 the atmosphere, in the interplanetary spaces, 

 or in the polarity of the sun and moon 1 Gali- 

 leo, in his celebrated Dialogo, is disposed to 

 ascribe the parallel direction of the earth's axis 

 to a magnetic point of attraction in space. 



When the interior of the earth is regarded 

 as molten and subjected to an enormous pres- 

 sure, as raised to a degree of temperature such 

 as we have no means of estimating, then must 

 the idea of a magnetical nucleus of the earth 

 be abandoned. All magnetism is certainly lost 

 at a white heat(^3*) ; it is still manifested when 

 iron is raised to a dull red ; and however dif- 

 ferent the modifications undergone by the mole- 

 cular condition, and the coercive force of mat- 

 ter dependent on it, may be in experiments, 

 there still remains a considerable thickness of 

 the crust of the earth which might be assumed 

 as the seat of magnetic currents. In what re- 

 gards the old explanation of the horary varia- 

 tions of the deflection, by the progressive heat- 

 ing of the earth in the apparent course of the 

 sun from east to west, it must be owned that 

 we are here limited to the very outermost sur- 

 face ; inasmuch as the thermometers now sunk 

 in the ground in so many places, and so care- 

 fully observed, show us how slowly the sun's 

 heat penetrates even to the moderate depth of 

 a few feet. And then the thermal state of the 

 surface of the ocean, covering two-thirds of 

 the globe, is little favourable to such an expla- 

 nation, when the question is one of immediate 

 mean influence, not of induction from the ae- 

 rial and vaporous covering of our planet. 



To all questions as to the ultimate physical 

 cause of phenomena so complicated, there is 

 no satisfactory answer to be given in the pres- 

 ent state of our knowledge. It is only in ref- 

 erence to the three-fold manifestations of the 

 earth-force, to that which meets us as mensu- 

 rable relations of Space and of Time, as the 

 Normal or conformable to laws in the Variable, 

 that brilliant advances have lately been made, 

 through the determination of numerical mean 

 values. Since the year 1828, from Toronto, in 

 Upper Canada, to the Cape of Good Hope and 

 Van Dieman's Land, from Paris to Pekin, the 

 earth has been covered with magnetical observ- 

 atories(^"), in which uninterrupted and simul- 

 taneous observations are made of every reg- 

 ular and irregular excitement of the earth-force. 

 A decrease of the magnetic intensity amount- 

 ing to the ^^»^^th part is measured ; at cer- 

 tain epochs, observations are noted every 2^ 



minutes through an entire period of 24 hours. 

 An illustrious English astronomer and natural 

 philosopher(^3') has calculated that the mass of 

 observations accumulated in the course of three 

 years, which remain for discussion, amounts to 

 1,958,000 ! Never has there been so grand, so 

 delightful an eflfort made to get at the root of 

 the Quantitative in the laws of a natural phe- 

 nomenon. We may therefore be permitted to 

 entertain a well-grounded hope, that these 

 laws, compared with those which prevail in 

 the atmosphere, and still more distant spaces, 

 will gradually bring us nearer and nearer to the 

 Genetical in magnetic phenomena. Until now 

 we can only boast that a greater number of 

 ways which might possibly lead to information 

 have been opened up. In the physical doctrine 

 of terrestrial magnetism, which must not be 

 confounded with the purely mathematical one, 

 as in the doctrine of the meteorological pro- 

 cesses of the atmosphere, some completely 

 satisfy themselves by conveniently denying as 

 realities all the phenomena which cannot be 

 explained in conformity with their views. 



Terrestrial magnetism, the electro-dynamic 

 forces which have been calculated by the able 

 Ampere("^), stands at the same time in inti- 

 mate relationship with the Earth- or North- 

 ern-Lights [Aurora borealis], as with the in- 

 ternal and external temperature of our globe, 

 whose magnetic poles must be regarded as poles 

 of cold("«). If Halley('39), some 128 years ago, 

 gave it out as a mere bold conjecture that the 

 northern light was a magnetic phenomenon, 

 Faraday's brilliant discovery of the evolution of 

 light through magnetic power has raised that 

 conjecture to the rank of an empirical certainty. 

 There are heralds or harbingers of the northern 

 lights. In the course of the day on which the 

 lights are to appear, irregular horary movements 

 of the magnetic needle usually indicate an in- 

 terruption of equilibrium in the distribution of 

 the terrestrial magnetism. When this disturb- 

 ance has attained a great intensity, the equilib- 

 rium of the distribution is restored by a dis- 

 charge, accompanied with an evolution of light. 

 " The northern light itself is not, therefore, to 

 be regarded as an external cause of the disturb- 

 ance, but rather as a terrestrial activity raised 

 to the pitch of a luminous phenomenon, one of 

 the sides of which is the light, the other the 

 oscillations of the needle"(**°). The splendid 

 phenomenon of coloured northern lights is the 

 act of discharge, the conclusion of a magnetic 

 storm ; in the same way as, in the electrical 

 storm, an evolution of light — lightning — indi- 

 cates the restoration of the disturbed equilib- 

 rium in the distribution of electricity. The 

 electrical storm is usually limited to a small 

 space, beyond which the state of the electricity 

 remains unchanged. The magnetic storm, on 

 the contrary, reveals its influence on the march 

 of the needle over large portions of continents, 

 as Arago first observed, and far from the place 

 where the development of light is visible. It is 

 not improbable that, as in the case of heavily 

 charged and threatening clouds, and of frequent 

 transitions of the atmospheric electricity into 

 opposite states, it does not always come to dis- 

 charges by lightning, so also may magnetic 

 storms produce great disturbances in thft^iorary 



