and tlte Aurora Borealis. 43o 



by these currents. All these circumstances have a marked 

 influence upon the declination. Thus, although it may be im- 

 possible beforehand to determine exactly the action of these 

 currents upon the declination-needle, we can compare the va- 

 riations of the declination with the results given by the obser- 

 vations. 



The relative humidity of the air is in general greater during 

 the night than during the day. It may be taken as a rule that 

 the diurnal variation of that humidity increases with the varia- 

 tion of its temperature : it is consequently greater in summer 

 than in winter. The conductivity of the air, and, in consequence, 

 also the intensity of the currents in question, have therefore a 

 diurnal and an annual period. Now, if it be admitted that 

 the diurnal variations of the declination depend chiefly on the 

 variations in the intensity of the currents we are considering, 

 it follows that the diurnal variations of the declination should 

 be greater in summer than in winter, and should moreover in- 

 crease with the distance from the equator — a deduction con- 

 firmed, as we know, by the observations. To this may be 

 added that the action upon the declination-needle does not de- 

 pend exclusively on the nature of the air and the intensity of 

 the current at the locality in which the needle is placed ; the 

 intensity of the currents in localities the most distant acts also, 

 although in a less degree. Besides, it is evident that here we 

 have to do with the humidity of the air not merely at the sur- 

 face of the earth, but also in the upper strata of the atmo- 

 sphere. We ought not, then, to expect that the variations of 

 declination at a given place will be in direct proportion to the 

 humidity of the air at the same place. 



When, for one cause or another, the current descending. to 

 the earth from the upper regions of the atmosphere has ac- 

 quired sufficient intensity, it produces a luminosity in the rare- 

 fied air, aud we then have the aurora borealis. If the current 

 is endowed with an invariable intensity (as appears to be the 

 case in some of the feebler aurorse boreales), the needle remains 

 pretty steady ; but when there are rapid variations in the in- 

 tensity of the current, or when its maximum of intensity shifts 

 from one point to another (in which case the aurora becomes 

 sparkling and changes its aspect continually), the needle, as 

 can be readily understood, becomes agitated and moves vio- 

 lently. This agitation extends over a considerable portion of 

 the earth's surface, and thus indicates what is going on in the 

 atmosphere, even when no aurora borealis is perceived. 



The thesis that the aurorse boreales are produced by electric 

 currents is not new; the majority of physicists have long been 

 agreed in regard to this, with good reason. But hitherto no 



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