1S55.3 



OP THE CAUSE OF THE AURORA BOREALIS. 



127 



Consist in this, thtit in our heniisptlere, the north pole of the 

 needle moves to the west, during the morning until half-past 

 one p. M., and then returns to the east during the rest of the 

 day, to remain stationary during the night. IJut this deviation 

 is precisely that which should be occasioned by currents pass- 

 ing along the surface of the globe from the north pole to 

 the equator, augmented, in intensity with the heat of the 

 day and diminishing as it decreases. The diurnal variation 

 is at its maximum (13' to 16') in those months in which the 

 sun is longest above the horizon, 31ay, June, July, August. 

 It is at its minimum (■!' to 5') during the winter months. 

 The variation is greater as we pass from the equator towards 

 the pole ; but it is evident, that if the currents, proceeding 

 from different points of the earth's surface heated by the sun, 

 rise in the atmosphere to redescend at the polar regions, and 

 thus traversing the globe, reach their points of departure, the 

 nearer the needle to the magnetic pole, the greater the number 

 of currents that will act upon it : near the equator, it will not 

 be subject to any influence from the currents which are formed 

 beyond the region around the needle. In winter these differ- 

 ences are less sensible, because the currents from the equato- 

 rial regions are the only ones whose effects will be very decided, 

 on aecount of the little difference of temperature which exists 

 in this season between the earth's surface and the upper regions 

 of the atmosphere in the temperate and especially the polar 

 zone. 



Finally, according to our theory, the same effects should be 

 manifested in the southern hemisphere, only that all is reversed; 

 and this is fully established by the various results of recent 

 observers, including those of Colonel Sabine and a large num- 

 ber of travellers. 



I should however acknowledge that there are some anoma- 

 lies, either in the hours or in the direction of diurnal variation, 

 at certain places, especially at St. Helena and the Cape of 

 Good Hope, anomalies whicli it is difficult to explain by the 

 theory proposed. But 1 am convinced that when further exa- 

 mined, thay will be found to be due to local and accidental 

 causes, such as the vicinity of the sea, which influences very 

 notably the diurnal variations of temperature and especially 

 their amplitude and the hours of the maximum and minimum 

 of heat. The question whether there are not places of no 

 variation, proposed by Arago, is of little importance in this 

 connection The points of the earth's surface without diurnal 

 variation, will be those where the two currents originate, and 

 whence they proceed from the right and left towards the two 

 poles : they are situated in the equatorial regions, but they 

 cannot well be laid down, as their position will vary with the 

 sun, the temperature, the winds, and other disturbing causes. 



But I do not dwell on this point, as my object is not to 

 treat of the diurnal motions of the needle. My end is simply 

 to prove from the diurnal variations, the existence of the ter- 

 restrial currents. In continuation, we may obtain another 

 proof still more direct, although less general, of the presence of 

 these currents, by making use of the telegraph wires for col- 

 lecting them. This I have done in England, as has also Mr. 

 Barlow; and 51. Baumgartner has performed similar experi- 

 ments in Germany. In these trials, the currents have in all 

 cases been detected by means of the galvanometer. M. Baum- 

 gartner, having introduced a very sensitive galvanometer into 

 the circuit formed by the telegraph wire between Vienna and 

 Prague, which h.as a length of about 01 miles, obtained the 

 foll)wing results when the two extremities of the wire were 

 buried in the earth. 



1. The magnetic needle never stood at zero, but was more or 

 less deviated. 



2. The deviations were of two kinds, some of large extent, 

 even 5U', others small, varying from 1° to 8° ; — the former not 

 common, and changing in direction and intensit\' , so that no 

 law can be discovered ; the latter on the contrary subject to a 

 simple law, and being very regular when the air is dry and the 

 sky serene, but with many anomalies when the weather is cold 

 and rainj". 



Mr. Barlow has made numerous observations, and obtained 

 results demonstrating the exactness of the principle, which I 

 have laid down. Four main lines starting from Derby, were 

 used in his experiments, two running towards the north and 

 northeast, and two towards the south and southwest. The 

 direction of the currents perceived on the first two lines, was 

 always contrary to that of the currents on the two others, as 

 ought to be the case, on the theory proposed. But the most 

 remarkable fact, is the perfect concordance which these obser- 

 vations have proved to exist between the movement of the 

 needle of the galvanometer placed in the circuit of the tele- 

 graph wire and the diurnal variations of the magnetic needle. 

 The diurnal movement of the needle of the galvanometer 

 is subject to disturbances in intensity more or less contin- 

 ued, during storms, and also when the aurora borealis is 

 visible ; and so also is this true of the compass needle. 

 There is this difference, that the currents acting on the latter, 

 circulating beneath the earth's surface, should not be subject 

 to disturbances like those which happen to the telegraph 

 wires through the influence of the electrical condition of the 

 atmosphere about them. 



The existence then of electric currents circulating beneath 

 the earth's surface appears to us to be well demonstrated ; and 

 once proved, it leads necessarily to the conclusion that it is a 

 consequence of the normal and regular reijstablishment of the 

 electric equilibrium between the earth and its atmosphere, 

 which is broken essentially in tropical regions ; whilst the elec- 

 tric discharges, more or less intense, which take place between 

 the earth and the air are the accidental and variable means for 

 the reestablishment of this equilibrium. We may now see how 

 the explanation of the phenomena of the aurora both north and 

 south, flows necessarily from the formation of these electric 

 currents circulating from the equator to the two poles in the 

 upper regions of the atmosphere, and from the two poles to the 

 equator along or beneath the surface of the globe. 



As we have said above, the positive electricity with which 

 the atmosphere is charged, especially in the upper regions, is 

 carried towards the two poles either by the greater conductibil- 

 ity of the upper and most rarificd strata of the atmosphere, or 

 by the currents of air in the upper regions which move i'rom 

 the equator to the two poles. It is consequent!}' through the 

 vapors which arc constantly condensed in the forming mists in 

 the polar regions that the positive electricity should find its 

 pMsagc into the earth, and also therefore its discharge. This 

 discharge when possessing a certain degree of inten.sity should 

 be luminous, especially if, as is almost always the case near the 

 poles and sometimes in the upper regions of the atmo.xpherc, 

 it encounters in its course icy particles of extreme minuteness, 

 which form the haze as well as the more elevated clouds. 



The formation of lunar halos which generally precede the 

 appearance of an aurora, and the fall of rain oi snow which 

 also is often a prelude to it, are a proof of the presence in the 

 atmosphere of these fine needles of ice, and of the part they 

 play iu the phenomenon bcfuro us. 



