40 Dc La Rive on the Aurora Borealis. 



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 purturbations in the magnetic needle 

 observed by Mr. Balfour Stewart at Kew, during the auroras 

 of the 2nd September, 1859, with the results of Mr. Walker's 

 observations of the currents exhibited by telegraph wires at the 

 same time. I have succeeded in experimentally verifying these 

 observations by transmitting the discharge of a Ruhmkorff's 

 coil through rarefied air, placing in the circuit some water 

 holding a little salt in solution, and in which two plates of 

 metal were immersed. As soon as the principal current was 

 weakened or stopped the inverse current was exhibited by the 

 plates." 



" In order to reproduce all the details of the natural pheno- 

 mena, I caused an apparatus to be constructed composed of a 

 sphere of wood about ten inches in diameter, which represented 

 the earth, and carried at each pole a bar of soft iron about two 

 inches long, and about one inch in diameter. Each bar rested 

 on a vertical cylinder of soft iron to which it was united, and 

 thus the sphere was supported. So arranged, the sphere had 

 a horizontal axis terminating in two appendages of soft iron 

 which could be magnetized by bringing the two cylinders on 

 which they rested in contact with the poles of an electro-mag- 

 net, or by surrounding the cylinders with coils of wire traversed 

 by electric currents. Each of the iron bars was surrounded by 

 a glass cylinder (manchon) between five and six inches in dia- 

 meter, and about seven inches long, and in which it occupied 

 an axial position projecting into the middle of the glass. The 

 two vessels were hermetically sealed by two metallic caps, one 

 of which was traversed by the iron bar, while the other earned a 

 metal ring upon two arms, the centre of the ring coinciding 

 with the end of the iron bar, and having its plane perpendicular 

 to the axis of one bar. The diameter of the ring is a little less 

 than that of the glass. Stopcocks were conveniently placed to 

 allow of a vacuum being formed in the glass vessels, and any 

 kind of gas introduced. 



" To use this apparatus, the wooden ball is covered with two 

 strong bands of bibulous paper, one occupying its equator and 

 the other crossing it from pole to pole, and making contact 

 with the two bars of iron. On this last band, pieces of copper 

 about one or two-thirds of an inch square are fixed at equal 

 intervals with copper tacks that penetrate the wood. All the 



