448 M. A. De la Rive oji the Cause of Aurora Boreales. 



recur to the other circumstances which accompany this me- 

 teorological phaenomenon, the agreementof which I have shown 

 with the explanation I have given in a letter addressed to M. 

 Arago, which was communicated to the Academy, and inserted 

 in the Philosophical Magazine for April 1849, p. 286. .g 



But, having referred to this letter, in which the question 

 was also raised respecting the explanation of the diurnal varia- 

 tions of the magnetic needle, permit me to add, that 1 have 

 had occasion to prove, in England, both by my own observa- 

 tions, and still better by the more extensive ones of several 

 physicists*, the existence of electric currents having a direction 

 from the north-west to the south-east on the surface of the 

 earth. The presence of these currents can be easily proved 

 by means of the metallic wires which serve as telegraphic com- 

 munications : wires which are placed underground and at the 

 same time well-isolated, except at their two extremities which 

 dip into the ground, are best suited for this kind of observa- 

 tions. It is very curious to follow the agreement which exists 

 between the variations of intensity of these currents and the 

 variations of magnitude of the deviation of the magnetic needle 

 of declination ; a new proof to add to that drawn from their 

 direction, that they are the cause of the diurnal variations. 



Colonel Sabine has stated, in opposition to my explanation 

 of the diurnal variations, an objection drawn from the obser- 

 vation of these variations at the Island of St. Helena and at 

 the Cape of Good Hope f. I do not think it well-founded, 

 and, without entering into the details which will better find 

 a place elsewhere, I shall limit myself to one single remark. 

 I attribute the origin of the currents which give rise to the 

 aurora borealis and to the diurnal variations, to the rupture 

 of the electric equilibrium occasioned, in each atmospheric 

 column, by the difference of temperature which exists between 

 its base which reposes on the surface of the globe and its upper 

 part which is at the limit of the atmosphere. Each column 

 thus forms a kind of pile charged at its two extremities with 

 contrary electricities, which unite in part by the pile itself, in 

 part by a circuit formed of the upper regions of the atmo- 

 sphere, of the atmospheric polar regions, and of the surface 

 of the earth. Meteorological circumstances determine the 

 greater or less proportion of the two electricities which unite 

 by one or other of these ways. 



Now, the temperature of the base of the column must vary 



* See the observations of Mr. W. H. Barlow on this subject, Phil. Mag. 

 vol. xxxiv. p. 344. 

 t Phil. Mag. vol. xxxiv. p. 4G6. 



