6i6 



NA TURE 



[Oct. 27, 1 88 1 



down in part by the negative electricity about C, which it in- 

 duces. In another portion of the atmosphere — it may lie at 

 some considerable distance from the former — you may have the 

 atmosphere charged in an opposite way, and of course if this were 

 negative lliere w ould be induced positive electricity below, or it 

 might be that the whole of the atmosphere from c h> D is 

 charged positively, but at D the negative charge is much feebler 

 than at c. The end-result would be the same ; but for facility 

 of explanation I will suppose that the upper portiini in one place 

 is actually charged witli electricity of the opposite kind to what 

 the charge is at the other. If the tension were sufficient, then 

 there might be a striking across of the electricity of this name in 

 the atmo-phere from c to u, and in the earth in the reverse direc- 

 tion. Coaipared witii the atmosphere, the earth would be an 

 exceedingly gooJ conductor, so that the electromotive force 

 concerned in sending the currents from one part of the earth 

 to the other would be, comparatively speaking, trifling, and there- 

 fore the electromotive force represented perhaps by a few scores 

 of the elements of a Daniell's battery. Well, then, in atmospheric 

 electricity we appear to liave the tension requisite to send the 

 discharge through a considerable space of rarefied air. Now if a 

 discharge took place, and if it were night, and the sky were clear, 

 it would, at least where sufficiently concentrated, be visible to us 

 just in the same way as the discharge pa-sing through the ex- 

 hausted tube i^ visible, by the light it produces. It would produce 

 in fact an aurora. The air is not a comparatively good con- 

 ductor, like a thunder-cloud, from which a great quantity of 

 electricity strdies in one moment, but is, after all, a bad con- 

 ductor, so that the electricity can only pass in a spitting sort 

 of way. We may cjnceive here that we have a sort of 

 double current, yet not forming a complete circuit ; nevertheless 

 a discharge would go on nearly of the same nature as if the 

 circuit were complete, and the effect of such a discharge on tlie 

 magnetic needle would be nearly the same as tlxat of a circuit 

 which was complete. I will endeavour to produce a discharge 

 in a circuit v hich is doubly incomplete. 



[An experiment was here shown in which two Leyden jars 

 were charged, one positively, and the other negatively, and were 

 laid on the same wire re-ting on a table. Un connecting the 

 knobj the jars were discharged, and that almost completely, as 

 it happened that the charges were almost exactly equal. The 

 inner coatings here represent two portions of the upper atmo- 

 sphere, the outer coatings the opposed portions of the earth's 

 surface, and the glass of the two jars the intervening portions of 

 the lower atmosphere.] 



As I said, although the circuit is not complete, the elec- 

 tro-magnetic effect of the whole .system would be nearly the 

 same as if you had a complete circuit with an electric current 

 passing through it. Now if there be only a sufficient quantity 

 of electricity, we have here the elements necessary for producing 

 a disturbance of the magnetic needle. Moreover, those disturb- 

 ances, as the instruments show, are of a most fitful and appa- 

 rently capricious character. They resemble in that the fitful 

 character of electric discharges through air. I need hardly say 

 that according to this theory the earth-current consists in the 

 return currents produced by the statically-induced change on the 

 surface of the earth, induced by the charged atmosphere above. 

 When there is a neutralisation of the electricity from one part 

 to another of the atmosphere above, the induced electricity in 

 the earth is set free, and w-e have earth-currents to bring about 

 the redisti'ibution of the electricity on the surface of the earth. 



It seems to me that this theory not only accounts for the con- 

 nection between the phenomena, which could be otherwise ac- 

 counted for, bat enables us to conceive how it is that electricity 

 strikes across such enormous distances in the upper regions 

 of the air, and I think, further, it will account for some 

 interesting features of the electric discharge which CDnstitutes 

 no doubt itself the aurora. I have here a sheet of blotting- 

 or filtering-paper, and I will suppDse this to represent an electri- 

 fied tract of air lying over, it may be, an extensive tract of 

 country, say somewhere to the nortli of us. Suppose that this 

 air is charged positively, it will induce negative electricity on the 

 earth below. This metallic coating on this sheet of glass [over 

 which the blotting-paper was held hoiizontally at a little distance] 

 may be supposed to represent the surface of the earth on which 

 this negative electricity is induced. The two may be quietly in 

 equilibrium. Suppose, ho" ever, that from some cau--e or other 

 the tension becomes sufficient to enable the electricity from some 

 point in this stratum of air to strike across higher up — because 

 the streamers are found to be parallel to the direction of the 



dipping-needle, and parallel accordingly to the lines of magnetic 

 force —higher up in the first instance ; from thence I do not know 

 where the discharges go, but I should suppose that in our country 

 they generally go somewhere to the south of us. Now if a 

 tract of air were pretty uniformly electrified, it would induce 

 electricity of the opposite name underneath it, pretty uniformly 

 distributed except about the edges, where the electrified air which 

 was the inducing body would tend rather to overlap the electrified 

 portion of the earth bel jw, and where accordingly, if the charge 

 were the same throughout, there would be ihe greatest tendency 

 for the electricity to strike off and pass into the upper regions of 

 tlie atmosphere, and thence probably to the south. Well, sup- 

 pose no A" that a discharge begins anywhere, say somewhere 

 along this edge of the paper, which I will suppose to be the 

 northern edge. The paper which I hold in my hand is really 

 toucli-paper (such as boys use for amusement), and I will light 

 the edge of it. Now this smouldering away of the tojch-paper 

 I conceive to represent the mode in whicli the rarefied air 

 becomes successively discharged. Suppose that a discharge 

 takes place somewhere about the edge of this sheet of electrified 

 air covering a large tract of country, then if once a hole (50 to 

 speak) were formed, the tendency would be for the discharge to 

 continue along that edge, because, as I said, as soon as the 

 electricity at the edge was discharged the electricity of opposite 

 name which had been induced on the surface of th'j earth below 

 would be set free, the earth-current would be set up ; and then 

 again, what now is the edge of the electrified tract of air would 

 be left exposed, no longer protected in the same manner as 

 bef jre by the induction of the electricity of op,josite name be- 

 neath ; the electricity would fly off from it in turn, and so on, so 

 that there would be formed a sort of cm'tain composed of auroral 

 rays, and gradually advancing, in our country usually in the 

 direction from north to south ; because we live in a sort of 

 neutral region not too far south to see the aurora from time to 

 time, and not far enough north to be exempt from thunder- 

 storms. This auroral discharge, which takes the place of 

 thunder-storms in lower latitudes in some way or other, usually 

 occurs to the north of us, and accordingly the aurora is called the 

 Northern Lights ; but when there is a fine display it sometimes 

 reaches down to us and goes south of us. So I say the discharge 

 would usually begin from a place north of u-, and would creep 

 along the edge of the electrified stratum of air, forming a sort of 

 luminous curtain, and passing from norlh to south, just as the 

 smouldering edge of the touch-paper passes along the paper 

 gradually. When we are just under the ed^e at which the dis- 

 charge takes place we have, as I conceive, an auroral arch 

 passing, it may be, through the zenith, generally stretching also 

 east and wes', aud generally moving with a slow motion from 

 north to south. 



Now suppo-ing that that is the explaiiation of the three phe- 

 nomena — magnetic disturbances, earth-currents, and aurora; — 

 can we in any way connect their occurrence with changes going 

 on at the surface of the sun? I think we can. We know that 

 a tube containing rarefied air, supposing the density of the air 

 in it is given, opposes less resistance to the electric discharge 

 through it when it is warm than when it is cold. The conducting 

 power of a wire for electricity decreases if you heat the wire, but 

 it is the reverse with air. The passage of electricity through 

 rarefied gases is very different in its natm'e from the passage of 

 electricity through a wire or through an electrolyte. Mr. De La 

 Rue has shown in the course the researches made by means of his 

 splendid battery, that in these highly exhausted tubes the electric 

 discharge, be it ever so steady to all appearance, obeys laws 

 csinnecting it rather with a series of di-ruptive discharges,, with a 

 rapid succession of sparks, than with a discharge passing throitgh 

 a wire. Now connected with that difference, or at least accom- 

 panying it, there is that opposite action of he.at which, as I say, 

 in the case of gases renders the passage of electricity more easy 

 instead of less easy. We may imagine that if from any cause 

 the sun gives out greater radiation than u-ual, the upper regions 

 of the atmosphere may thereby become heated to a certain 

 extent ' and oppose less resistance to the passage of the electric 



* The rays of the visible speclrum, and even the invisible rays for some 

 considerable distance beyond the extreme violet, pass freely through clear 

 air, which could not therefore be sensibly heated by them But there is 

 reason to think that the atmosphere, or some of its constituents, are more or 

 less opaque to rays of very high refrangibility ; and it i- just for copious 

 emission of these that sources of radiation of an excessively high tempera- 

 ture are so remarkable. The substance, which is opaque to the rays of 

 excessive refrangibility, and consequently enables them to heat upper regions 

 of the atmosphere, is probably njt nitrogen or oxygen, but some gas or 

 gases'present in verj' small quantity. 



