148 



On the Diurnal Variations of the Magnetic Needle, and on 

 Aurorai Boreales. By Auguste de la Rive; being an 

 extract from a Letter to M. Arago.* 



Allow me to communicate to you, with the request that you will 

 make it known to the Academie des Sciences, an extract of a memoir 

 recently read before our Societe de Physique et d'Histoire Natu- 

 relle, on the cause of the diurnal variations of the magnet needle, 

 and of Aurorse Boreales. In assigning successfully these two classes 

 of phenomena to the same origin, I have but followed the path you 

 have pointed out; for more than thirty years ago you established, 

 with indefatigable perseverance, by your numerous observations, the 

 remarkable agreement which prevails between the appearances of the 

 aurora borealis and the disturbance of the magnet needle. 



o 



The following is my theory, — you will observe that it rests solely 

 upon well ascertained facts, and on principles of physics positively 

 established. 



I had already, in 1836, in a notice upon hail,f attempted to shew 

 that the atmospheric electricity owes its origin to the unequal distri- 

 bution of temperature in the strata of the atmosphere. It is well 

 known that, in a body of any nature whatsoever, heated at one of its 

 extremities and cooled at the other, the positive electricity proceeds 

 from the hot part to the cold, and the negative electricity in the con- 

 trary direction; it thence results that the lower extremity of an 

 atmospheric column is constantly negative, and the upper one con- 

 stantly positive. This difference of opposite electric conditions must 

 be so much the greater, the more considerable is the difference of 

 temperature ; consequently more marked in our latitudes in summer 

 than in winter, more striking in general in the equatorial than in 

 the polar regions. It must be observed that the negative state of 

 the lower portions of the atmospheric columns must be communi- 

 cated to the surface of the earth on which they repose, whilst the 

 positive state of the upper portions is diffused, more or less, from 

 above downwards, through nearly the whole of each of the columns, 

 according to the facilities offered by the greater or less degree of 

 humidity of the air to the propagation of the electricity. An at- 

 mospheric column, therefore, resembles a high-pressure battery, on 

 account of the imperfect conductibility of the elements of which it 

 is composed. A battery, the negative pole of which is in constant 

 and direct communication with the terrestrial globe, discharges itself 

 upon the globe, whilst it becomes itself charged with the electricity 

 of its positive pole, which is distributed over it with an intensity de- 



* From the Annales de Ohimie et de Physique, for March 1849. 

 t Bibliothique Unwerselle, vol. iii. p. 217. Nonvelle serie. 



