the Magnetic 'Needle and on Aurora Boreales. 29S 



pecting the analogy, of the noise which the voltaic arc pro- 

 duces in the action of the magnetism. 



The magnetic disturbances which always accompany the 

 appearance of an aurora borealis are now easily explained. 

 This accidental union of a greater proportion of the accumu- 

 lated electricities must derange the normal action of the regu- 

 lar current; with respect to the directions of the disturbance, 

 it will depend on the portion of the current acting upon the 

 needle, and consequently on circumstances impossible to fore- 

 see, since they depend on the extent of the phaenomenon 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 phaenomenon takes place, it will be 

 either the current circulating on the earth or that travelling 

 in the air (currents which proceed in a contrary direction) 

 which will act upon the needle ; even during the same aurora, 

 it may be sometimes one sometimes 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 Annates 

 de Chimie et de Physique and in several scientific voyages. 

 The remarkable effect observed by M. Matteucci in the appa- 

 ratus of the electric telegraph between Ravenna and Pisa, 

 during the magnificent aurora of the 17th of last November, 

 fully proves the existence of a current circulating on the sur- 

 face of the earth, and which, ascending the wire of the tele- 

 graph, 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 circumstances, 

 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 observations which would enable us to demon- 

 strate and to measure the electric currents which probably 

 traverse them ; it would be easily accomplished by means of 

 a multiplying galvanometer, by completing the communication 

 of these wires with the earth at one of their extremities. The 

 comparison of the results obtained in this manner with those 

 furnished by the simultaneous observation of the diurnal va- 

 riations of the needle, would certainly present considerable 



