290 M. A. de la Rive on the Diurnal Variations of 



no longer affected in a regular manner. This remark may 

 explain certain observations, especially those made at Port 

 Bowen, which appeared rather exceptional. 



On examining carefully all the magnetic observations I was 

 able to consult, and in particular those of Colonel Sabine, I 

 was especially struck by the remarkable manner in which they 

 agreed with my theory. I will cite but one example — the 

 observations recently made at St. Helena, and just published, 

 by Colonel Sabine. At St. Helena the diurnal variation oc- 

 curs to the west as long as the sun is to the south of the island, 

 and to the east as soon as the sun is to the north. lu fact, in 

 the first case, as 1 have previously observed, St. Helena must 

 form part of the region in which the electric currents proceed 

 on the surface of the earth from the north pole to the equato- 

 rial regions ; and, in the second case, it forms part of the re- 

 gion in which these currents pass from the south pole to the 

 equator. The hour of the maximum of the diurnal variation 

 is not the same at the island of St. Flelena as in the continental 

 countries, which is owing to the temperature of the surface of 

 the ocean not following the same laws in its diurnal variations 

 as the temperature of the surface of the earth. Now the tem- 

 perature of the lower stratum of the atmospheric column is 

 always that of the surface of the ocean, or of the soil on which 

 it rests. This same circumstance explains certain apparent 

 anomalies exhibited by tlie diurnal variations in some parts of 

 the globe, as for instance at the Cape of Good Hope, which 

 is surrounded almost on every side by a vast extent of 

 ocean. 



I wish it to be understood that in the preceding I have only 

 taken notice of the causes disturbing the direction of the mag- 

 netic needle, and not of the cause of this direction itself, that is 

 to say of terrestrial magnetism — a cause which I do not at all 

 believe to be of the same nature, but upon which I at present 

 express no opinion. I am content to consider the terrestrial 

 globe as a large spherical magnet, and to study the external 

 causes capable of modifying the direction which it tends to 

 impart in its quality of magnet to magnetic needles. 



Now what is the aurora borealis according to the theory 

 which I have just expounded? It is the luminous effect of 

 electric currents travelling in the high regions of the atmo- 

 sphere towards the north pole — an effect due to the combina- 

 tion of certain conditions, which are not always exhibited in 

 the same manner, nor at all seasons of the year. 



It is now well proved that the aurora borealis is an atmo- 

 spheric phaenomenon, as you long ago suspected. The name 

 of magnetic stormy by which Von Humboldt designates it in 



