and the Aurora borealis, • 433 



I am not, however, acquainted with any results of observations, 

 freed from accidental perturbations, of a nature to confirm or 

 refute this assumption. 



Wilcke* had already observed that the place of the auroral 

 crown is in the zenith of the magnetic meridian, or in its vi- 

 cinity, The correctness of his observations has been many 

 times confirmed since then by other physicists. The above- 

 mentioned French Expedition made 43 determinations, the 

 mean result of which was, that the situation of the crown de- 

 viates less than 1° from the magnetic zenith. The difference, 

 however, between the two positions amounted to 15° on one 

 occasion, and to 12° on two others. 



In explaining the annual period of frequency of the auroras 

 boreales shown by the observations, the following circum- 

 stances must be considered : — The electric fluid accumulated 

 in the earth's atmosphere by the unipolar induction of the ter- 

 restrial magnet descends to the surface of the earth either by 

 disruptive discharges (thunder-storms), or in feeble continuous 

 currents. The former have their principal frequency between 

 the tropics, and the latter in high latitudes. The fluid which 

 does not flow to the earth in the first of these two ways, is 

 conducted by the magnetic force to higher latitudes, where it 

 flows down in the form of continuous currents. From this it 

 follows that the rarer and weaker the tempests, the more in- 

 tense and numerous must the auroras boreales be, and vice 

 versa. In the zone of calms, immediately to the north of the 

 equator, thunder is heard throughout the year; but the limits 

 of this zone vary from one season to another. Outside of that 

 zone, but between the tropics, thunder-storms travel, like the 

 rainy seasons, with the sun. We can therefore assume that 

 the quantity of the electric fluid which within the tropics de- 

 scends to the earth in tempests is not the same all through 

 the year. As is known, the auroras boreales present two 

 maxima, viz. at the spring and autumn equinoxes. According 

 to our view the tempests would consequently be the weakest, 

 or, rather, the least quantity of electric fluid would descend in 

 lightning from the atmosphere to the surface of the earth, 

 within the tropics, when the sun crosses the equinoctial line. 

 We have not sufficient materials of observation for deciding 

 whether this view is correct or notf. 



It cannot be admitted that the electric fluid flows down upon 

 the earth only on the occasion of auroras boreales. Beyond all 

 doubt the flow is continual, although mostly the currents do 



* Kongl. Vetenskaps-Akademiens Handling ar for 1777 (vol. xxxviii.). 

 t It appears to follow from the observations made upon the aurora 

 borealis by the Austro-Hungarian Arctic Expedition of 1872-1874, that 

 Phil. Mag. S. 5. Vol. 6. No. 39. Dec. 1878, 2 F 



