550 M. A. dc la Rive on the Aurora; Borealcs, 



entirely surrounds its equator, and the other, which crosses the 

 first, extends from one pole to the other in such a manner that 

 its extremities are respectively in contact with the iron cylinders. 

 On the last-mentioned band, small plates of copper, of from 1 

 to 2 centimetres square, are placed, on both sides of the equa- 

 torial band ; these are to be fixed on by small screws of the same 

 metal, which penetrate the wood of the sphere, and to be placed 

 equidistant from each other on the same meridian. A metallic 

 communication may be established between them by means of a 

 galvanometer wire, which is placed at 10 or 12 metres distance, so 

 as not to be directly influenced by the electro-magnets. The 

 apparatus thus arranged, moisten the blotting-paper bands with 

 salt water ; to keep these bands in the necessary state of humidity, 

 it will be sufficient to dip the two ends of the equatorial band in a 

 saline solution contained in a small metal capsule fixed, by means 

 of a rod fastened in the wooden sphere, a little below the lower 

 part of the sphere. The metal capsule is connected with the nega- 

 tive electrode of a Ruhmkorff apparatus, of which the positive elec- 

 trode is put in communication, by means of a bifurcate conductor, 

 with the metallic disks whence proceed the rings placed in the 

 interior of the tubes. As soon as the Ruhmkorff apparatus is 

 put in action, the discharge is seen, if care has been taken to 

 make a sufficient vacuum in the interior of the tubes, issuing in 

 the shape of a luminous jet between the end of the soft-iron 

 cylinder and the ring ; but it is sometimes in one tube and some- 

 times in the other that the jet bursts, and rarely in both together, 

 notwithstanding that care has been taken to place the two media 

 in condition apparently identical, and particularly to have exactly 

 the same vacuum in both by connecting, during this operation, 

 the interior of one of the tubes with that of the other. 



Immediately the soft irons are magnetized, the jet spreads and 

 assumes a rotatory motion, the direction of which depends upon 

 that of the magnetization and of the direction of the discharge. 

 This latter circumstance influences not only the direction of the 

 rotation, but gives rise to other curious changes. Thus, if the 

 direction be altered so that the positive electricity is received by 

 the soft iron, and the negative by the ring, the jet will be observed 

 to spread round the magnetized end of the soft iron so as to form 

 a true ring 1 to 2 centimetres in thickness, of a beautiful rosy red, 

 with a very regular rotatory motion ; whilst the metallic ring is 

 surrounded with a luminous violet envelope, forming a sort of 

 sheath about it, and having likewise a rotatory motion. These 

 two rotatory movements are perfectly synchronous, although the 

 rose-coloured ring and the violet ring, separated by an obscure 

 space of several centimetres in extent, appear to have no con- 

 nexion the one with the other. 



