372 SCIENTIFIC EECORD FOR 1884 



ward during the nigbt and southward during the day. This explains 

 at once the daily period which is especially marked in middle latitudes 

 where the maximum frequency of auroras occurs some hours before 

 midnight. 



15. At the time of the aurora maximum the frequency at Godthaab 

 exceeds not only the absolute but also the relative frequency for the 

 zenith for this locality, or others more to the north of it; a similar 

 statement is true for the time of minimum. Therefore, in the course of 

 the eleven-year period, there is a periodic movement of the auroral zone 

 which lies further north during the sun-spot minimum than during the 

 maximum. 



16. We see thus that a perfect harmony exists between the eleven- 

 year period and the annual and diurnal i^eriodicity. so that these three 

 j)eriods have one and the same cause, namely, the oscillation of the 

 auroral zone. 



17. Considering each of Kleinschmidt's seven types separately, Trom- 

 holt finds that during the auroral maximum not only the frequency, 

 extent, and intensity, but also the variety of kinds surpasses those 

 of the minimum. 



18. Between the periodicity of the cirrus clouds and that of the 

 aurora a connection exists, such that these phenomena have their max- 

 ima and minima almost at the same time. 



Tromholt concludes that we are not able to establish a satisfactory 

 theory as to the nature and cause of the aurora, but that it is not a 

 cosmic phenomenon is shown by the want of agreement in the auroral 

 periods lor the whole earth. For the present, thereibre, observatii.us 

 are of the first importance. He has orjianized a network of observers 

 for Scandinavia, Finland, Denmark, England, and Iceland, and has 

 already shown that over so small a region as l!^orway no day j)asses 

 without an aurora at one or more points. (Z. 0. O. M., xviii, p. oUG.) 



363. Pri»f. S. Lemstrom, of Helsingibrs, gives the results of his own 

 observations on artificial auroras at Oratunturi and SodankylaandPie- 

 tarintuntuti. {Xaiurc, XXA^III, p. GO.) 



364. Prof. S. Tromholt communicates a general account of his Litest 

 resi'arclics, during 1<SS2 an<l 1883, into the aurora and its ])iienonie!ia in 

 Northern Fiuujark, where the aurora is seen ahnost every night in nu- 

 merous forms and frequently very low down, although his own estimate 

 of its height is 150 kilometers. {Nature, xxvii, p. 394.) 



365. xViter concluding his special observations in Kautokeino, Nor- 

 way, Tromholt took up his lesidence in Redykjavik, Iceland, in October, 

 1883, and communicates to Nature some of the results of his work dur- 

 ing the winter ]88i-'84. He has never as yet seen any auroral light de- 

 scend to the low level of the mountain top of Esja, namely, about l.',500 

 feet. The weather, however, has been very unfavorable, and he has made 

 but 40 observations on 83 thoroughly clear evenings ; apparently Ice- 



