118 ANNUAL OF SCIENTIFIC DISCOVERT. 



place simultaneously at both poles, since, as tlie conducting power of 

 the earth is perfect, its electrical tenison ought to be sensibly the 

 same, with some slight differences arising solely from accidental varia- 

 tions 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 dis- 

 charge 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 in- 

 stead of the western declination which occurs when the boreal dis- 

 charge 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, of 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 alternately 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 cnemical action 

 of the current which they transmit, and that they ought to determine 

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

 occasioned their polarization ceases or diminishes its force ; and all 

 observers know that the auroras exhibit a very variable and perpet- 

 ually 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 (jneme sens'), while the discharge 

 takes place at the same pole, whether it be strong or weak, and it 

 will not change its character until the discharge nearly ceases at the 

 nearest pole, in order to operate almost exclusively at the other ; 

 whilst by reason of the effect of secondary polarities a change in 

 intensity suffices to produce a change of direction in the currents of 

 telegraphic wires. This difference is strikingly shown by comparing 

 the graphic representations of perturbations in the magnetic needle 

 observed by Mr. Stewart at Kew, during the auroras of the 2d Sep- 

 tember, 1859, with the results of Mr. Walker's observations of the 

 currents exhibited by telegraph wires at the same time. I have suc- 

 ceeded in experimentally verifying these observations by transmit- 

 ting the discharge of a Ruhmkorff's coil through rarefied air, placing 



