De La Rive on the Aurora Borealis. 41 



copper squares are arranged in the same meridian. Between 

 two of the squares a metallic communication is established 

 with the thread of a galvanometer placed about twelve yards 

 off, so that its needle shall not be directly influenced by the 

 electro-magnet. Having thus arranged the apparatus, the 

 paper bands are moistened with salt and water, and the equa- 

 torial band is connected with the negative electrode of a Euhm- 

 korff's coil, which has its positive electrode brought into com- 

 munication, by means of a bifurcated wire, with the two 

 metallic rings which are inside the glass vessels, and in highly 

 rarefied air. The discharge is soon seen as a luminous jet 

 between the rings and the extremity of the iron bar, sometimes 

 in one vessel, sometimes in the other, but rarely in both at 

 once, although both are placed under apparently the same 

 circumstances.'" 



" As soon as the soft iron is magnetized, the jet spreads and 

 forms an arc round the central wire, animated by a rotary 

 movement, the direction of which depends on the character of 

 the magnetization. It is evident that it depends also on the 

 direction of the discharge, but we have supposed this direction 

 constant, and resembling that of nature, that is to say, directed 

 from the circumference towards the centre. It is important to 

 notice that if the air be not too rarefied, a moment is observed 

 in which, when the iron bars are magnetized, the rotation 

 begins, and the jet not only expands into an arc, but darts 

 brilliant rays that remain quite distinct from each other, and 

 turn round with greater or less velocity like the spokes of a 

 wheel. In this we see an exact representation of what occurs 

 in the aurora borealis, when the luminous arcs being all im- 

 pressed with a movement of rotation from west to east, dart 

 luminous jets in the higher regions of the atmosphere. These 

 jets do not occur unless the iron is magnetized, and they may 

 be stopped if the air is highly rarefied, by introducing avapor- 

 izable liquid, such as a drop of water. It is impossible to pro- 

 duce them if the discharge, instead of being directed, as in 

 nature, from the circumference to the centre, passes in an 

 opposite direction." 



M. de la Rive adds, that on examining the galvanometer 

 with which the two wires previously mentioned are in com- 

 munication, a secondary current will be indicated, its character 

 and direction being determined by whether the discharge takes 

 place at one pole or the other ; and he states that he can imi- 

 tate the disturbances which the magnetic needle experiences 

 when the auroras occur. 



