April 2,, 1884] 



A^A TURE 



537 



9'4. lis place for 18S50 is in R.A. igh. 27m. 35 •5'=., N.P.D. 

 72° 29' 54". 



Nos. 2577-78 of the Aslronoimsche Nachrichlcn contain the 

 late Prof. Julius Schmidt's results of observations of variable 

 stars made at Athens in 18S3, which were communicated about 

 a fortnight before his sudden decease. 



ON THE AURORA BOREALIS IN ICELAND 

 A S considerable doubt has hitherto prevailed as to the form 

 ■^ and nature of the aurora borealis in Iceland, I have 

 decided to pass the winter here in Reykjavik, in order to study 

 the phenomenon on principles which I followed during my 

 sojourn at Kautokeino la^t winter, 1SS2-83 (Nature, vol-. 

 xxvii. p. 394, and xxviii. p. 397). 



I arrived here about the middle of October last, and began 

 my regular observations on November 6 ; and although the 

 series of observations as yet is brief, and, through very un- 

 favourable weather, not so complete as might be desired, I 

 believe that a few preliminary remarks on this phenomenon may 

 not prove without interest, particularly as the appearance of the 

 aurora borealis here is somewhat different to what we might 

 expect and what is generally assumed. 



Weather more unfavourable than I have encountered since my 

 arrival it is impossible to imagine. A sky nearly always cloudy, 

 rain, snow, and storm following upon slorm — such have its chief 

 characteristics been. A clear sky is quite an exception, and 

 when it occurs there is a wind blowing so keen and cutting that 

 no human being can walk out of doors for any length of time. 

 Iceland is, no doubt, not favoured with very congenial weather, 

 but such a h inter as the present must, according to the dwellers 

 here, be considered as quite an exceptional one. 



I have shown in Tat)le I. the average cloud calculations of 

 each evening hoar (the observations begin generally at 5 p.m., 

 and continue until two or three hours after midnight) from 

 November 6 to January 28. Here 5 indicates the hour from 5h. 

 1 1 5h. Sgm., &c. The scale is the usual one, viz. fiom o (clear) 

 to 10 (cloudy). 



Tai!LE I. 



15 Average 

 ]8'o6;7'72l7'5i'7'83 7 9o8'io8-24 8 o,J7'39'8-29 866 7-91 



If an average of the nebulosity on each evening be taken, each 

 v.ilue of the scale will fall on the number of days shown in Table 

 1 1. The former are also calculated in per cents, of the total 

 days (83). 



Table II. 



These figures speak so plainly for themselves that any com- 

 ment is needless. 



Through Iceland being situated in the zone of the terrestrial 

 magnetism, it might be assumed that the aurora borealis attained 

 a high degree of development and splendour here ; but this has 

 not been the case this winter, in Reykjavik at all events, even 

 allowing for the unfavourable weather. The aurorse here are 

 generally faint and wanting in force ; it is only seldom that there 

 is any energy in the movements, and but rarely that the forms 

 are sharply defined, while the outlines are dim and vague. 



There have therefore only been a fcA' occasions on which I have 

 been able to effect somewhat satisfactory measurements with 

 the auroral theodolite of azinmth and the height. The aurora 

 djubtless often reaches far up on the sky, and even travels far 

 down on the southern horizon, but the force of light is very 

 small. In spite of the circumstance that Reykjavik lies — judging 

 by the appearance of tlie aurora borealis on the horizon — much 

 nearer to the auroral maximum zone than Kautokeino, the 

 appearance of the aurorje in the two places cannot be compared. 

 There was activity, force, and colour ; here is vagueness, un- 

 certainty, and want of character. Only once — on January 25 — 

 I observed an aurora during one hour which was a true Arctic 

 one, with defined, elegant outlines, intense play of colour, and 

 hold movements. 



The more extensive aurora; which I saw in Kautokeino gene- 

 rally finished by the bands or streamers changing into luminous 

 clouds, which again shortly afterwards assumed the wave-like 

 motion I have called "coniscation," and which often lasted for 

 hours, flooding the entire heavens. This form of the aurora 

 borealis / have not observed on a single occasion Jure, which 

 appears to me to be a very remarkable circumstance. Extensive 

 aurora; finish here through the simple vanishing of the light or 

 by the changing of the forms into faint, luminous clouds con- 

 sisting of stripes (north-east to south-west), or vague, cloudy 

 bands which by degrees lose in energy and finally die away. 



Any real corona 1 have not seen as yet, and the usual 

 colours, viz. red and green, I have only noted on six occasions. 



On forty of the eighty-three evenings I have effected observa- 

 tions there have been aurora;, which is rather a high figure when 

 the unfavourable weather condi'.ions are taken into consideration. 

 But the aurora is, however, not always present when the sky is 

 clear or nearly so ; on the contrary, it is not nearly as frequent 

 here as in Kautokeino. This will be understood from Table III., 

 which has been framed on the assumption that all observations 

 were equally divided over the twelve hours, viz. from 4h. to 

 iSh., which also shows that in every hour there was observed 

 one hundred times either .aurora or clear sky without aurora. 

 The lower figures show in per cents, when the sky was without 

 aurora. 



Table III. 



In consequence of the great magnetic declination in Iceland, 

 viz. about 40° N.W., the points of culmination of the arcs and 

 bands fall far outside the astronomical meridian, and their 

 direction is nearly north-east to south-west. From the measure- 

 ment of twenty arcs, partly on the north, partly on the south 

 horizon, I have certainly only obtained an azimuth of 22° "4 W., 

 but I do not accept this as any definite result before more com- 

 plete observations are in my hands. 



The intensity of the aurora borealis here I have defined 

 approximately in Table IV. by four degrees, viz. from one to 

 four. From the total determinations of intensity for every hour 

 when no aurora is visible, in spite of clear sky, being determined 

 by o, the following average figures are obtained : — 



Table IV. 



Intensity 





[■26 1-45 I'oso' 



I 1 



15 Average 

 ■59 0-40JO-44 0-03 0-95 



From these figures a decided maximum of intensity is manifest 

 between 9h. and loh. 



As regards the position of the auroroe on the sky and the rela- 

 tive frequency of the various forms, I append a tabular list 

 of observations. The abbreviations made in the same are 

 these : — 



H N. Aurora stands near the northern horizon, i.e. the mag- 

 netic north. 

 IN. ,, ,, low in the north. 



N. ,, ,, in the north (to a height of about 45°). 



N h. ,, ,, on the northern horizon (to a height from 



the horizon about 70°). 

 Nh — Z. ,, ,, on the northern horizon to zenith. 



Farther, Z indicates through or on both sides of the zenith 

 S, south ; S Z, south of zenith ; S h, south horizon ; /, over th e 

 whole sky ; -;-, with the exception of; A^-l- S, aurora; in the 

 north and south (but not in zenith); o, no aurora. Below N' 

 I have collected the values of H N, IN, N, Nh, and N h—Z ; 

 and under S' those of S Z, S h, and S ; and under i' the others, 

 with the exception of o. 



Table V. gives percentally, assuming an evenly divided time 

 of observation, a view of the position of the aurorte in the 

 sky. 



Table VI. shows the relative appearance of the various forms 

 calculated percentally on the same basis as in the previous tables. 

 Here / indicates one arc ; /=, several arcs ; //, a band ; //= , 

 several bands ; J, diffused ; s, streaming ; Js, simultaneously 

 diffused and streaming, or a variety between the two ; /// 

 isolated streamers, or bunches of streamers ; K, luminous clouds 



