and the Phenomena which attend them* 549 



order to produce the appearance of the aurora, through a slightly 

 saline solution, and by producing, by means of two copper plates 

 immersed in this solution, a derived current, that these plates 

 acquire secondary polarities which give rise to an inverse current 

 almost as strong as the derived, compensating by its duration 

 what it may lack in intensity. As regards the magnetic dis- 

 turbances, they are very easy to reproduce by suspending above 

 and very near a surface of mercury placed in the circuit of the 

 same discharge a sewing-needle, the lightest obtainable, very 

 strongly magnetized : the extent and direction of its deviations 

 show that it obeys all the variations of intensity and of direction 

 of the discharge. 



The better to realize this reproduction of the natural pheno- 

 menon in its entirety and in detail, I have had an apparatus con- 

 structed composed of a wooden sphere of from 30 to 35 centime- 

 tres in diameter, which represents the earth. At each extremity of 

 one of the diameters of this sphere is fixed a cylinder of soft iron of 

 from 3 to 4 centimetres in diameter, and from 5 to 6 long. The 

 two cylinders repose each, in the portion nearest to the sphere, 

 on a rod of soft iron to which they are solidly united by strong 

 screws ; the two rods being vertical, serve as a support to the 

 cylinders and to the sphere, which has thus a horizontal axis 

 terminated by two soft-iron cylinders, which maybe magnetized 

 by placing the two vertical supports respectively on the two poles 

 of an electro-magnet, or by surrounding them with a coil tra- 

 versed by a strong current. An excellent representation is thus 

 obtained of the earth with the two magnetic poles. 



The cylinders of soft iron covered with a non-conducting 

 coating, except at their extremity, are each surrounded with a 

 wide glass tube of which they occupy the axis, ending in the 

 middle of this axis. The tubes have a diameter of about 10 

 centimetres, and a length of about 15 ; they are hermetically 

 closed by two metallic disks, of which one is traversed by the 

 soft-iron cylinder, and the other bears, by means of two metal 

 branches covered with a non-conducting varnish, a ring, of 

 which the diameter is a trifle less than that of the tube, and of 

 which the centre coincides with the end of the soft-iron cylinder, 

 whilst its plane is perpendicular to the axis of the cylinder, and 

 consequently to that of the tube. The ring itself presents a 

 bright metallic surface, and its outer edge is about half a centi- 

 metre from the inner surface of the glass tube. Stopcocks 

 fastened to the disks which close the tubes externally, admit of 

 producing a vacuum, or of introducing at pleasure gases or 

 vapours in greater or smaller quantities. 



When operating with this apparatus, two broad bands of 

 blotting-paper are placed on the wooden sphere, one of which 



