JJe La Rive on the Aurora BoreaUs. 39 



neighbourhood of the poles, which takes place under the form 

 of more or less frequent discharges as soon as their tension has 

 reached a limit which cannot be maintained. These discharges 

 ought to take place simultaneously at both poles, since, as the 

 conducting power of the earth is perfect, its electrical tension 

 ought to be sensibly the same, with some slight differences 

 arising solely from accidental variations in the stratum of air 

 interposed between the two electricities. There are thus upon 

 the earth during the appearance of the auroras two currents 

 proceeding from the poles to the equator ; but if the discharge 

 only takes place at one pole — the southern for example — there 

 is no longer in the northern hemisphere a current directed 

 from north to south, but a weaker current directed from south 

 to north. This change gives an eastern declination to the 

 compass-needle instead of the western declination which occurs 

 when the boreal discharge takes place and the current is directed 

 from north to south." 



" It is known that auroras are accompanied by more or less 

 intense currents in telegraphic wires. Mr. Walker in England, 

 and Mr. Loomis in America, have made them the subjects of 

 special study, and they have found that they vary constantly 

 not only in intensity but likewise in direction, coming alter- 

 nately from north to south, and from south to north. If we 

 remember that the currents propagated by telegraphic wires 

 are derivative currents gathered by means of large metallic 

 plates sunk in the moist soil, it will appear that these plates are 

 not slow to polarize themselves under the chemical action of 

 the current which they transmit, and that they ought to deter- 

 mine in the wire which unites them an inverse current as soon 

 as that which occasioned their polarization ceases or diminshes 

 its force; and all observers know that the auroras exhibit a 

 very variable and perpetually oscillating light." 



" The change which occurs in one terrestrial current when 

 the discharge passes from one pole to another — from the north 

 to the south, for example — determines also a change in the 

 direction of the currents of the telegraphic wires, which in that 

 case flow from south to north, instead of from north to south ; 

 but the new current is much weaker than the old one, except 

 when it unites with the secondary currents arising from the 

 plates." 



" There is, however, a great difference in the results obtained 

 when, instead of observing the currents collected by telegraphic 

 wires, we study the perturbations of the magnetic needle which 

 accompany auroral manifestations, as in the latter case there 

 are neither electrodes nor secondary currents, but only one 

 direct action of the principal current. This current may vary 

 in intensity, but it must always operate in the same way (meme 



