Diurnal Variations of the Magnetic JSeedle. 151 



a result which again perfectly agrees with what I have stated re- 

 specting the increase in number of the currents towards the polar 

 regions. In these regions themselves, the variations may be very 

 irregular, and may be entirely absent if the magnetic needle 

 happens to be placed in those very localities where the electric cur- 

 rents traverse the atmosphere to reach the earth ; in fact, a needle 

 surrounded thus on all sides by currents, is no longer affected by 

 them, or at least is 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 espe- 

 cially 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 occurs 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. In fact, in the first case, as I 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 equatorial regions ; and, in the second case, it forms 

 part of the region 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 Helena 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 temperature 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 circum- 

 stance explains certain apparent anomalies exhibited by the 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 magnetic needle, 

 and not of the cause of this direction itself; that is to say, of terres- 

 trial 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 mag- 

 netic 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 atmosphere towards the north 



%ole. an effect due to the combination of certain conditions which 



