AURORA BOREALIS. 



the north-northeast. His studies of this phe- A = 5,569 was observed, with soft variable 



nomenon led him to the conclusion that the intensity. The galvanometer gave the detiec- 



earth is provided with a permanent luminous tions, extremely variable in intensity, but never 



corona, about 400 kilometres from the surface, ceasing, of a positive current from the " ut- 



having its center correspondent with the mag- stromnings " apparatus to the earth. Ari 

 netic pole, and its plane perpendicular to the 



terrestrial radius at that point. Its light is so 

 feeble that the slightest rival luminous mani- 

 festation the light of the moon, for example, 

 or the presence of moisture or frost in the 

 air is sufficient to extinguish it. Hence it is 

 not likely to be visible in inhabited lands, and 

 can be seen, even in the polar regions, only in 

 such favorable seasons as the one he enjoyed, 

 which was a season of minimum of auroras, 

 and then only rarely. 



Prof. Lenstrbm's Experiments in Lapland. Di- 

 rect and definite experiments to ascertain the 

 cause of the auroral displays have been made 

 by Prof. Selim Lenstrom, of the Fin- 

 nish Meteorological Station at Sodan- 

 kyla, Lapland. They were directed 

 especially to the variety of the mani- 

 festations which takes the form of tiny 

 flames or a phosphorescent luminosity 

 appearing around projecting objects, 

 such as mountain cones and ridges. 

 Prof. Lenstrom's first experiments 

 were made in 1871, when, with an 

 apparatus similar to the enlarged one 

 with which he produced the same re- 

 sults on a grander scale in 1882, he 

 succeeded in artificially inducing an 

 aurora on the top of the Luosmavaara 

 mountain, 520 feet above the surface 

 of Lake En are, in Lapland. Toward 

 the end of November, 1882, Prof. Len- 

 strom laid out on the summit of Mount 

 Oratunturi (lat. 67 21', long. 27 17' 

 32"), about 540 metres above the level 

 of the sea and twelve miles from the 



On an- 

 other mountain, Pietarintunturi (lat. 68 32' 

 5", long. 27 17' 32"), 950 metres above the sea, 

 a smaller utstromnings apparatus was erected 

 in two parts, so arranged that the inner one 

 covered about 80 square metres, and the outer 

 one 320 square metres. On the 29th of Decem- 

 ber a single column of aurora, 120 metres in 

 height, appeared above the apparatus. The 

 current, as shown by the galvanometer, was 

 found in the case of this mountain to be " pro- 

 portionate to the surface-area laid out " ; and 

 observations of comparison between the two 

 mountains led to the conclusion that u the elec- 

 tric current from the atmosphere increases 



observatory at Sodankyla, an instru- 

 ment which he called an " utstrom- 

 nings" or "discharging" apparatus. 

 It consisted of a bare copper wire 

 two millimetres m diameter, fitted at every 

 half-metre with points or nibs soldered upon 

 it. The wire was laid out in entwined squares, 

 or in the form of a rectangular helix, in such 

 a way that each innar coil was about a metre 

 and a half from the outer one, and was raised 

 on poles 2| metres high ; and the whole appa- 

 ratus covered a superficial area of 900 square 

 metres. From the inner end of this wire, an 



PROFESSOR LENSTRSM'S UTSTRO'MNINGS APPARATUS. THE Con op 

 WIRES. The insulators are indicated by the letters'. The open 

 end of the wire is shown at 0, while the inner end is connected 

 with the galvanometer. 



rapidly with the latitude." Other researches 

 led to the inference that, while the condition 

 of the ground is of some influence, the terres- 

 trial current ceases at a certain latitude. Si- 

 multaneous measurements of the angles of ele- 

 vation were made, at Sodankyla, and at a sta- 

 tion four and a half kilometres north of that 

 place, for the purpose of determining the height 

 of the aurora. The measurements made the 



insulated copper wire on poles, with telegraph angle at the southern station three degrees 



insulators, led to the foot of the mountain, 

 where a connection was made at the station 

 with a galvanometer, whence another wire led 

 to a zinc disk in the earth. From the day the 

 apparatus was finished, a yellowish-white lumi- 

 nosity appeared almost every night around 

 the summit of the mountain, while nothing of 

 the kind was seen around any of the other 

 mountains. When tested with the spectro- 

 scope, the light gave a faintly continuous spec- 

 trum from D to F, in which the auroral line 



larger than that at the northern one, a result 

 inconsistent with the supposition that the dif- 

 ference in angle represented a parallax of a 

 single object seen from two places; for in -that 

 case the larger angle would have been observed 

 at the northern station, and the difference 

 would have been slight. Prof. Lenstrom came 

 to the conclusion that the two observers did 

 not see the same aurora ; and comparing this 

 observation with others, that " measurements 

 of the height of the aurora calculated on those 



