153 SECTIONPHYSICS. 



Rive shows the action of the magnet upon the electric discharge when 

 the latter radiates round the pole of the magnet which then forms one 

 of the electrodes, the other electrode being a concentric ring at the 

 pole. The electric discharge revolves, in this case, round the magnet, 

 like a needle on a dial, and with a velocity which varies considerably 

 according to the nature of the gas, and which is nearly in an inverse 

 ratio to its density. MM. de la Rive and Sarasin have, moreover, 

 satisfied themselves with regard to the fact that the electric spark 

 carries along with it, in its rotation, the gas itself which transmits it 

 and any sufficiently light movable body, such as a little pendulum, or, 

 still better, a small mill having its fulciment concentric to the annular 

 electrode. In hydrogen at a pressure of i moa the discharge, revolving 

 under the action of the magnet, has imparted to this mill a velocity of 

 120 and even 140 revolutions per minute. If the direction of the 

 rotation be changed, the discharge very quickly stops the mill, and 

 then causes it to revolve in the opposite direction at a speed which 

 very soon equals the former. 



Finally, M. Sarasin drew attention to the fact, that this experiment 

 of De la Rive, upon the rotation of the electric spark around the pole 

 of a magnet, has given him new and important evidence in support of 

 the electric theory of the aurora borealis, of which he has been one of 

 the most earnest defenders. Indeed this experiment can perfectly be 

 compared to the well-known fact of the rotation of the arcs of the 

 aurora round the poles of the earth. M. Sarasin gave a rapid sketch 

 of the theory of the aurora borealis such as De la Rive had stated it, 

 and explained the apparatus which he devised in order to reproduce the 

 phenomenon, for the sake of experiment. 



The PRESIDENT : We must now convey our thanks to M. Sarasin- 

 Diodati for his interesting communication. He has explained very 

 clearly the origin of the pieces of the apparatus which are happily now 

 well-known to us ; thanks to Professor de la Rive and himself. 



I will now call on M. Lemstrom to read his paper on " The 

 Aurora Borealis." 



Professor Lemstrom, of Helsingfors, Finland, then delivered the 

 following Address on his Polar-light Apparatus, and on the Theory 

 of the Polar Light. 



Professor LEMSTROM: This apparatus serves to prove that the 



