FJeciro -Magnetic Expsrimenis, 43 



the current, or from right to left ; in the first case, tlie sfldith polf 

 of ithe needle, corresponding to the north pole of the terrestrial 

 magnet, will be on the side of the zinc plate of the apparatus ; in 

 the second case, it is the north pole of the needle which is on that 

 side. 



M. Arago, following the theory of M. Ampore, conceived the 

 idea of twisting a conjinictive wire in the manner of two symme- 

 trical spirals placed one after the other ; these spirals differed 

 from each other only as to the direction in which their generating 

 points turned round their hollow spindles : by putting a needle in 

 each spindle, the two needles became magnetised at the same 

 time, so that their poles of the same name were contiguous. In 

 transmitting a discharge of a Leyden phial through a copper wire 

 twisted in the same manner, in the manner of two consecutive 

 symmetrical spirals, M. Arago has further observed, that the steel 

 needles placed on these spirals became magnetised by the electric 

 fluids of ordinary machines, as well as by the Voltaic apparatus. 



Other facts have been long known, which prove the mutual in- 

 fluence of the two fluids, magnetic and electric. The points of 

 })aratonnerres become naturally magnetised by the electricity cf 

 the atmospheric air. M. Arago, author of an article on tlte 

 magnetic forces, which is inserted in the Annuary of 1819, re- 

 ports, as from an eye-witness, that a Genoese ship, on its way to 

 Marseilles, was struck by the thnnder at a little distance from 

 Algiers ; that the needles of the compass made all a half revolu- 

 tion, although these needles did not appear damaged, and the 

 ship struck on the coast at the moment that the pilot thought he 

 made the North Cape. 



Ritter had concluded {Jo7irnalde Physique, t. 57. year 1803,) 

 from some experiments, which have not been since verified, 

 that the earth has electric poles, as it has magnetic meridians. 



M. Desormes and myself had attempted in 1805 to ascertain 

 the direction which a horizontal electric pile would take, com- 

 po'-ed of 1480 thin plates of copper, tinned with zinc, of the di- 

 ameter of a five-franc piece. We placed this pile upon a boat, 

 which floated on the water of a large vat. We knew that a mag- 

 netised steel bar, of a weight nearly equal to that of the pile, and 

 placed like it upon the boat, would turn, after some oscillations, 

 into the magnetic meridian. The pile, placed in the same situ- 

 ation, did not take any determinate direction. The only satisfac- 

 tion which this pile procured us, was the recognising of the ten- 

 sion of the electric fluid at its extremities, without the aid of the 

 condenser. (See the Correspondence of I he polytechnic Schoc!, 

 tonic 1. p. 151.) 



M. Ampere has confirmed, by experiment, the conclusions of 



the Memoir which he read on the 2.>th September, 1820, to the 



- . F2 Koyal 



