15-4 Diurnal Variations of the Magnetic Needle. 



derange the normal action of the regular current; with respect to 

 the directions of the disturbance, it will depend on the portion of the 

 current acting upon the needle, and consequently upon circumstances 

 impossible to foresee, since they depend on the extent of the pheno- 

 menon, and the position of the needle in relation to it. In fact, 

 according as the horizontal plane in which the declination needle 

 moves, comprises above or below some of the region in which the 

 greatest activity of the phenomenon takes place, it will be either the 

 current circulating on the earth, or that travelling in the air (cur- 

 rents which proceed in a contrary direction), which will act upon the 

 needle : even during the same aurora, it may be sometimes one, some- 

 times the other of these two currents which will act. The variable 

 directions in which the needle is deflected during an aurora borealis 

 agree very well with this explanation, at least as far as I have been 

 able to judge from the different observations published in the An- 

 nates de Chimie et de Physique, and in several scientific voyages. 

 The remarkable effect observed by M. Matteucci in the apparatus of 

 the electric telegraph between Ravenna and Pisa during the magni- 

 ficent aurora of the 17th of last November, fully proves the exist- 

 ence of a current circulating on the surface of the earth, and which, 

 ascending the wire of the telegraph, passed in part through this 

 better conductor. The sounds which long iron wires, strung in the 

 direction of north to south, give out under certain meteorological cir- 

 cumstances, are undoubtedly a proof that they are traversed by a 

 current which is probably derived from the currents circulating on 

 the surface of the earth from north to south in our hemisphere. 



It would be highly interesting and important to profit by those 

 telegraphic wires, which are found to have a direction more or less 

 approaching to that of the declination needle, in order to make with 

 them, when they are not in use for ordinary purposes, some obser- 

 vations which would enable us to demonstrate and to measure the 

 electric currents which probably traverse them ; it would be easily 

 accomplished by means of a multiplying galvanometer, by com- 

 pleting the communication of these wires with the earth at one of 

 their extremities. The comparison of the results obtained in this 

 manner with these furnished by the simultaneous observation of the 

 diurnal variations of the needle, would certainly present considerable 

 interest, and might lead to meteorological results of a remarkable 

 nature. 



I cannot conclude this abstract without drawing attention to the 

 circumstance, that M. Arago had already pointed out in 1820, 

 shortly after (Ersted's discovery, the possibility of acting upon the 

 voltaic arc by this magnet, and the analogy which might result 

 between this phenomenon and that of the aurora borealis. 



