and the Phenomena which attend them. 551 



But what is still more remarkable is what takes place in the 

 contrary case, that in which, as happens in nature, negative 

 electricity comes in by the soft iron which represents the terres- 

 trial pole, and the positive by the metal ring which represents the 

 atmosphere. In order that the discharge may be well seen, the 

 air must not be so rarefied as in the preceding case ; if it is too 

 much so, a little air may be introduced by means of a cock 

 placed for this purpose, or better still a few drops of water, which 

 evaporate, until a single jet is at first observed; then at the 

 moment the soft iron is magnetized the jet commences to rotate 

 and to throw off a multitude of brilliant little jets issuing from 

 the luminous ring which surrounds the end of the soft iron, and 

 terminating in the metallic circle. These jets are equidistant, 

 and perfectly distinct from each other, like the spokes of a wheel, 

 and turn with more or less rapidity. We have here a perfectly 

 faithful representation of what takes place in the auroras boreales 

 when the auroral arcs send out luminous jets into the high regions 

 of the atmosphere. It is a very remarkable fact that the produc- 

 tion of these jets can only occur when the current is directed from 

 the circumference to the centre, as in the natural phenomenon, in 

 which the discharge takes place from the atmosphere to the earth. 

 None are to be seen when the current is directed from the centre 

 to the circumference; there are then but two concentric arcs, 

 one of a rosy red round the soft iron, the other violet which 

 envelopes the metallic ring ; the two separated by a perfectly 

 obscure annular space. 



Looking now at the galvanometer 10 or 12 metres off, where 

 terminate the wires which separate the metallic plates fixed on 

 the moistened bands of the sphere which represents the earth, we 

 shall make the following observations. 



The sphere is divided by the equatorial band of paper into two 

 hemispheres, each of which has a magnetic pole, and which we 

 will style northern hemisphere and southern hemisphere. We 

 at first place the two metallic plates whence proceed the wires 

 connected with the galvanometer, at a little distance from each 

 other, on the same side of the equator, on the moistened band 

 which extends like a meridian from one pole to the other. If 

 the discharge takes place at the north pole, the needle of the gal- 

 vanometer indicates by its deviation a derived current of 70 to 

 80 degrees, which shows the presence of a strong current tra- 

 velling on the sphere in its northern hemisphere from N. to S. 

 If the discharge takes place at the south pole, a much weaker 

 derived current is remarked (from 25 to 30 degrees) directed a 

 contrary way, and which indicates the presence on the sphere, 

 still in its northern hemisphere, of a current much weaker 

 than the preceding, and moving from the S. to the N., and not 



