M. A. De la Rive on the Cause of Aurora Boreales. 449 



not only with the season, with the time of the clay, and witli 

 the latitude of the place where it is observed, but also with 

 the nature of the surface of the globe on which it reposes. 

 When, therefore, this surface is the sea, the hours of maxima 

 and minima of temperature are not the same as when it is terra 

 Jirma, all other circumstances behig the same ; it results 

 necessarily that the hours of maxima and minima of inten- 

 sity of the electric currents, and consequently of the diurnal 

 variations to which they give rise, must be equally different. 

 Now, St. Helena and the Cape of Good Hope may be con- 

 sidered as places enveloped in atmospheric columns, which 

 have almost their entire base resting on the sea and not on the 

 land ; thence the anomalies pointed out by Colonel Sabine 

 are very easily explained, and, in particular, it is easily under- 

 stood how there is no agreement, in direction, which must in 

 every case be different, between the diurnal variations ob- 

 served at the Cape of Good Hope and those observed at Al- 

 giers, which is equally distant from the equator, but to the 

 north. An excellent paper by M. Aime on terrestrial mag- 

 netism, inserted in the Annales de Chimie et de Physique, 3rd 

 series, vol. xvii., in which he discusses comparatively the 

 observations made at St. Helena, the Cape, and Algiers, has 

 singularly facilitated the explanation of the anomalies pre- 

 sented as objections by Mr. Sabine. 



I, however, do not pretend that there does not exist any 

 anomaly ; my explanation is not more free than others from 

 those which result from certain local and exceptional causes. 

 I am not further from admitting that the currents of induc- 

 tion determined on the surface itself of the globe, by its rota- 

 tion under the influence of its magnetic poles, cannot have 

 any part in the phaenomenon of the diurnal variations and 

 auroras boreales, and account for the connexion which these 

 variations appear to have with the absolute direction both in 

 declination and in inclination of the magnetic needle, and with 

 the absolute intensity of the terrestrial magnetism. But this 

 subject would require, for elucidation, to be treated more at 

 length than can be done in a letter; I shall therefore stop, 

 and beg to refer those persons who may be interested in this 

 question to a memoir which I am on the point of completing, 

 and which will be published forthwith. 



Phil, Mag, S. 3. Vol. 35. No. 238. Dec, 1849. 2 G 



