ioS 



NA TURE 



[A/ay 31, 1883 



contrasted sharply with that of the latter, particularly 

 where the Milky Way stood out of the same. In the 

 yellow-white light it was difficult to make out the Milky 

 Way. This phenomenon lasted far into the night. Later 

 in the evening, between seven and nine, there appeared an 

 aurora of great intensity, of which I shall speak below. 

 This luminosity gave no reaction in the speccroscope 

 at our disposal, but no doubt it would have been obtained 

 had this been less absorbing. Thus, for instance, the 

 larger Wrede's spectroscope (four prisms) did not give 

 the reaction of the auroral phenomenon at Oratunturi, 

 whereas the smaller, as stated above, really showed the 

 line. 



There is not the least reason for assuming that this 

 luminosity is of any but an auroral nature, and the 

 result of these observations is that the -whole of northern 

 Lapland is during most winter nights illuminated by a 

 phosphorescent luminosity, whose intensity varies greatly 

 according to period and place, but which is undoubtedly 

 0/ an auroral nature. 



On the same day, viz. December 8, the expedition was 

 enable \ to make the first measurement in the magnetic 

 meridian of the elevation of the auroral arc. The wire, 

 which was laid out north and south for the study of the 

 terrestrial current, was used as a t lephone line, and the 

 observations thus made by signals. Two theodolites 

 with the necessary instruments were employed, viz. one 

 at Sodankyla and the other about 4/5 kilometres distant 

 to the north, near the mouth of Kalujoki. The observa- 

 tions at the observato y were made by Herr Biese, and at 

 tie northern end of the telephone line by Herr Petrelius. 

 The auroral arc appeared in the north and shone with a 

 quiet, subdued light, while a streamer now and then shot 

 forth into the sky. Six measurements were made with 

 the following result : — At the northern station the line of 

 sight formed, with the under rim of the arc and the hori- 

 zontal plane, an angle of 9 , and at the southern station 

 one of 12 , i.e. an angle 3 larger at the southern than 

 at the northern station ! Even assuming that both obser- 

 vers saw the same arc, the result is absurd, as however 

 great the distance between the two might be, the differ- 

 ence of the angle would be very small indeed, and, if a 

 difference at all, the angle of the northern station should 

 have been the greatest. As, however, the reverse was the 

 case, I have come to the conclusion that the two observers 

 did not see the same aurora. A corroboration of this 

 opinion is that on one occasion Herr Biese telephoned, 

 " Turn the instrument to where the red column is," while 

 at the northern station no such colour could be traced. 

 This was proved still further during the return journey 

 from Kultula to Sodankyla by the following circumstance. 

 At K6ng;is,6o kilometres north of Sodankyla, on January 

 3, 1883, at 4 p.m., the whole horizon was flooded with a 

 yellow-white luminosity of great intensity. At the same 

 time an auroral arc formed in the south about 25 over 

 the horizon, and a similar one was at the same moment 

 observed at the same elevation in the north from 

 Sodankyla. The departure from Kongas took place just 

 after 4 p.m., and during the journey this arc gradually 

 disappeared, while the luminosity and the arc seen at 

 Sodankyla were seen all through the evening. Here 

 there was an opportunity of measuring the elevation of 

 the auroral arc, but as I was convinced that the two 

 phenomena were not the same, I did not attempt it. 



It was clear that we were within an auroral discharge 

 which extended considerably east and west, but the main 

 strike of which was north and south. It is very probable 

 that the electric current which caused this light some 

 thousand metres above the surface of the earth also 

 produced the above-described intense luminosity in a 

 layer some 20 metres in depth, running parallel with the 

 earth. It was this layer which was projected from both 

 points into the sky in the shape of an arc. But it is 

 clear that the auroral " drapery " did not penetrate far 



into the horizontal plane, but as it is generally produced 

 in the centre of a weak discharge of great penetration 

 its appearance from various places in the line north and 

 south would be very variable according as the layer lends 

 its light to the drapery. 



The measurements and results described above exactly 

 correspond with those of Mr. Fritz in Greenland (Bul- 

 letin de la Commission polaire Internationale ; Mittheil. 

 der Intcrnationalen Polar commission, Heft 3), where he 

 obtained an auroral drapery of 650 feet, 1700 feet distant 

 from the observer, and another one of 170 feet, 350 feet 

 distant. 



Without further discussing this question here I must state 

 that I consider that all measurements of the height of the 

 aurora, calculated on those with a longbase north and south, 

 are always erroneous, as the two observers never see the 

 same aurora. And even those calculations which are 

 based on the measurments of the height and length ot 

 an arc from one point, and the hypothesis that the arc 

 extends around the magnetic pole, must be considered very 

 unreliable,as no satisfactory answer can be given as to what 

 results would have been obtained a little further north or 

 south. This is also the case with aurorae with long bases 

 east and west, as only on a shorter distance is it possible 

 to say if it is the same phenomenon which is seen. 



That the height of the aurora borealis is very variable 

 I fully admit, but in my opinion it has been greatly over- 

 estimated. 



Researches with the Terrestrial Current. — During my 

 expedition to Lapland in 187 1, we examined the terrestrial 

 current in two places, viz. Kittila, lat. 67 40', and Enare 

 Vicarage, lat. 68 55', with wires \\ kilometres long — east 

 and west, north and south — of copper 0^4 mm. in diameter, 

 and finishing in platina disks 10 cm. by 5 cm., buried in 

 the earth at a depth of 07 to 09 metre. The deflexion 

 was measured by a galvanometer with astatic needles with 

 telescope and scale (Weber's magnetometer, of Edlund's 

 improved construction). The remarkable result obtained 

 here was that the galvanometer at Kittila, with the current 

 east and west, gave a deflexion equal to 60 to 100 parts of 

 the meter scale, whereas the current at Enare only gave a 

 fraction of one part of the meter. With the current north 

 and south, the difference was not so great, although even 

 here the deflexions were smaller at Enare. It was unfortu- 

 nately impossible to ascertain if this remarkable pheno- 

 menon was due to latitude or season, the researches at 

 Kittila being made in October, and those at Enare in the 

 latter half of November, while on the former occasion the 

 ground was not frozen, which it was on the latter. 



The Finnish expedition this year to Sodankyla has also 

 examined the terrestrial current, viz. during certain periods 

 of the phenomenon every five minutes, at other times once 

 every hour, with a wire 5 kilometres long, terminating in 

 small platina disks in the earth. During my visit to 

 Kuttala — Decern er 22 to January 4 — I also tested the 

 terrestrial current, but with a wire only 1 kilometre in 

 length, running east and west. Here, too, no deflexion was 

 shown, while in Sodankyla the current was just as strong 

 as ever. At Kuttala the galvanometer was certainly not 

 so sensitive as at Sodankyla, still, the experiments of 

 1871 are even in this respect not without importance. 



I have, therefore, from these researches drawn the 

 inference that, while the condition of the giound is of 

 some influence, the terrestrial current ceases at a certain 

 latitude. In 1 87 1 already I maintained that the terres- 

 trial current was caused chiefly by the electricity which 

 descends from the atmosphere in the belt around the 

 Pole, in which the aurora borealis attains its maximum, 

 and my recent researches at Sodankyla have greatly 

 confirmed this theory. 



I now intend to discuss the conclusions I have come to 

 from the above detailed researches. 



Although the general belief as to the nature of the 

 aurora borealis certainly is that it is of electric origin, 



