ELECTRICAL CURRENTS OF THE EARTH. 209 



a manner of repeating, by using- the earth as an intermediate conductor, 

 the experiments tried, especially by Nobili, many years ago, on the 

 reciprocal chemical actions of ditferent liquids. The same may be said 

 of electric cnrrents obtained with electrodes in which may be formed the 

 so-called secondary polarizations. 



These difl'erent modes of obtaining electric currents in a mixed circuit 

 have nothing to do with the study in which we are engaged, if not on 

 account of the very important and indeed indis]^ensable knowledge of 

 the causes of error which are apt to intrude into the experiments by 

 which v,-e seek to discover and to study the electric currents of the 

 earth, independently of those causes. 



I believe that the first case of electric currents proper of the earth, 

 which may be called, as has been done by Airy, spontaneous terrestrial 

 electric eurrents — at any rate, if not the first observed, certainly the first 

 described and i)ublished — was that which was discovered on the night 

 of the 17th of November, 1847, at Pisa, Isy means of the telegraphic 

 wires, and which was described in a letter directed to Arago and pub- 

 lished in the Comptes-rcndus of the Academy of Sciences of Paris. This 

 fact consisted in the existence of an extraordinary electric current which 

 circulated with such intensity and constancy as to keep the armatures 

 of the electro-magnets of the apparatus in a state of attraction during 

 the whole time that a magnificent aurora borealis was apparent in the 

 heavens. The same iihenomenon was soon afterwards observed in the 

 United States, and since that epoch the observations have been frequent 

 of electric currents in the wires of the telegraph associated with the 

 appearance of the aurora. When it is considered that we know, on the 

 other hand, the constant relation which exists between the aurora 

 borealis and the indications shown by the instruments which serve to- 

 measure the magnetic force of the earth, it is impossible not to recog- 

 nize all the importance of these studies. And in fact, after these 

 observations, there was no delay in directing special researches to the 

 existence of electric currents of the earth and their laws, independently 

 of the apparitions of the aurora borealis. 



We must be content, on the present occasion, with brietl}' referring 

 to the researches made previously to our own, and which are due to- 

 Baumgarten, Barlow, Lloyd, and Walker, but especially to Lamont, 

 who, beyond the rest, has extended iind varied these investigations. 

 Whoever has studied the memoirs of these observers with the attention 

 due to the importance of the subject, and to the authority of tLieir 

 authors, will find it difficult to avoid the conclusion that the uncertain 

 results obtained, results so little accordant among themselv(^s, are prin- 

 cipally attributable to the method of experimenting and to the (listurbiug 

 causes necessarily introduced by that method. The greater part of the 

 experiments were executed with telegraphic circuits, and therefore with 

 a metallic line established in conjunction with other metallic lines 

 worked for the purpose of correspondence, and traversed by the electric 

 currents of the offices at the moment of the experiment. We know 

 that the wires of telegraphic lines are never so perfectly insulated from 

 one another and from the earth that there shall not be signs of the 

 current in the lateral wires when the circuit is closed with one of the 

 same wires. Moreover, the communications of the metallic lines with 

 the earth are now made with a lamina of copper immersed in pits, the 

 wires being at one time connected with the iron tubes of pumps, at 

 another with iron railways. In the memoirs which we have r^iferred tO' 

 there is generally no indication of the mode in which the lines were 

 constructed, nor as regards their insulation or their connection with the 

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