374 



KNOWLEDGE & SCIENTIFIC NEWS. 



[March, 1906. 



TKe Aurora of 



November 15tK. 



By W. SiiACKi.KTOX, F.R.A.S. 



Ix \ io\\ of live wide-spread area over wliirli tlif aurora 

 ol Xoxeniber 15 was observed, together willi the simul- 

 taneous disturbance of the magnetic needle, it seems 

 desirable to add to llie few particulars given on page 

 293 in the issue of December last. Observations of the 

 aurora are recorded from Viirdo, Christi.inia, 

 Szczawnica in Galicia (Karpathian Mts.), Wilhelms- 

 haven (CJermany), many places in France, England, and 

 Ireland, and also from Nova -Scotia; thus practically 

 the whole of the northern parts of this hemisphere that 

 were in darkness at the time of the occurrence have re- 

 corded the phenomenon. Most observers agree that 

 the aurora was at its maximum about g p.m. (Green- 

 wich time), but Mr. Roberts, writing from Welshpool, 

 states that he ob.served a fine display, with a maximum 

 brilliancy about 6.10 p.m.; others also record this. In 

 Nova -Scotia displays are recorded as taking place about 

 6 p.m. and also about 9 p.m. (Halifax time). From 

 these observations it would appear that several displavs 

 occurred on the same date. The earlier display is 

 probably coincident with an easterly movement of the 

 declination needle, but the one coinciding with the 

 greatest disturbance is the display observed about 

 9 p.m., and curves from the magnetic observatories of 

 Wilhelmshaven, Kew, and Stonyhurst, all exhibit 

 precisely similar deflections. Through the kind per- 

 mission of the Director of the National Phvsical 

 Laboratory, Dr. Chree has supplied us with copies of 

 the Declination and Horizontal Force curves taken at 

 the Kew Observatory, and a similar declination curve 

 is exhibited from Stonyhurst College through the kind- 

 ne'is of Fathers .Sidgreaves and Cortie. 



DECLINATION loos 



Declination Curve, Kev 



It will be seen that aljout g p.m. a rapid easterlv 

 movement of the needle began, reducing the declination 

 some 33' in about 15 minutes, followed by an equally 

 r.ipid return to the normal. 



Declination Curve, Stonyhurst, Nov. isth. 



The similarity of the curves is obvious, and Dr. 

 Borgen, Director of the Magnetic Observatory, 

 Wilhelmshaven, states that the large disturbance 

 showed itself, in the same way, on the magnetic curves 

 taken there. 



The disturbanc<'s of the declination needle w<'re ac- 

 companied by a sympathetic' movement of the horizontal 

 force magnet; the most prominent displacement, how- 

 ever, coincides with the aurora observed in the early 

 evening, when the horizontal force increased by 1057 

 in about 10 minutes, and then fell 1507 in the next 20 

 minutes (17= ,,;•"' C.G.S. unit). 



HORIZONTAL FORCE i^JS 



' Two days' records are taken on eacli photographic sheet ; 

 the upper is the earlier. The base line is the time scale. 



Horizontal Force Curve, Kew, Nov. isth. 



.\fter the disappearance of the aurora the needles 

 continued to be disturbed, but not so \iolently, for 

 about 30 hours. 



Of the many theories put forward in explan:ition of 

 the origin of magnetic disturbances, one natiually 

 associates these particular ones with the co-existing 

 aurorae. 



The aurora of November 15 was particularly interest- 

 ing on account of its vivid crimson tint, a type of 

 aurora which is not common, the whitish yellow 

 aurorae being more frequent. Observations from 

 favourable localities like \'ardo, state that the aurora 

 was by far the most splendid seen for many years. 

 Strong aurorae of this kind are rare in England, and it 

 is a matter of regret that so few spectroscopic observa- 

 tions are forthcoming, especially as the origin of the 

 auroral lines is not definitely known, in spite of the 

 researches of Capron, Vogel, Huggins, Paulsen, and 

 others. Usually, however, it is inconvenient to put 

 aside a spectroscope suitable for this kind of work in 

 the hope of an aurora coming along, and this may ac- 

 count for the scarcity of observations; yet every ob- 

 server who looks for aurora? should be provided with a 

 spectroscope, for it is well known that the aurora line is 

 visible spectroscopically when the naked eye fails to 

 detect any display, and, again, many reputed aurorae 

 would be shown to be mere sky reflections if spectro- 

 scopic observations had been made. 



Both Professor Fowler and myself made independent 

 spectroscopic observations, but neither of us were able 

 to bring a spectroscope to bear upon the display whilst 

 the crimson rays lasted, and no trace of a red line was 

 seen, though special search was made. 



Ihi; common aurora line at X 5,572 was very strong, 

 .iiid remained visible till midnight (as long as observa- 

 tions were continued), whilst, in addition, three other 

 laint and more refrangible lines were seen. 



The .spectrum of the aurora is most probablv a vari- 

 able one, although the green line is persistent, but the 

 phenomenon of gases exhibiting different spectra ac- 

 cording to the nature of the electric discharge is 

 conuuon, and it seems probable that the various tints 

 of aurorae may be accounted for by the varying electric 

 discharges through some gas in the upper reaches of 

 our atmosphere. The nature of this gas has not been 



