Oct. 22, 1874 I 



NATURE 



501 



frequency, would be of no avail for enumerations ; the zone 

 selected mu5t be one of occasional auroras, arising only from 

 the southward spreading of the strongest disturbances of the 

 ever-beiming and sometimes forth-sallying illuminations of the 

 north. 



It is also for sucTi other obvious reasons,'"a3 that years of 

 arctic exploration tend to appear in general catalogues as 

 years of extraordinary auroral frequency, and that observations 

 in Asia, Western America, and in the whole of the southern 

 hemisphere have for the most part been made but recently or 

 at very irregular intervals, that the use of general auroral cata- 

 logues in questions of periodicity calls for much selection and 

 reduction of the miscellaneous mass of observations. A most 

 extensive general auroral catalogue appears to have been pub- 

 lished early last year, or at the end of the previous year,* by 

 Prof. Lovering, of the United States, of whicli Prof Loomis 

 has employed the materials, and of which he acknowledges the 

 completeness in terms of commendation. It extends from the 

 year 500 B.C. to the year 1S64, and includes with its supple- 

 ments upwards of 12,000 cases of observed auroras. For the 

 following years, from 1S64 to the end of 1S72, Prof. Loomis has 

 continued the catalogue for a restricted area suited to the question 

 of periodicity, partly from American sources, and partly (in 

 Earope) from the periodical journal published by Dr. Heis, 



IVochcnschrift fiir Astronomie und Uliteorologii. The selected 

 region of observation is limited on the north by an iso-auroral line 

 skirting the northern boundary of the State of Massachusetts 

 and crossing the Atlantic from near Boston to the north of Ire- 

 land, passing thence between England and Scotland, and through 

 the northern part of Jutland, a little south of Stockholm, to a 

 little north of St. Petersburg, where it continues its course in 

 Russia as far as long. 40° E from Greenwich. The meridian of 

 this longitude (nearly that of the eastern ends of the Black Sea 

 and Red Sea) limits the area on the east. It is similarly limited 

 westwards by the meridian of Sc W. from Greenwich, including 

 Washington, and the eastern, but none of the western States of 

 North America. A lengtliy general catalogue for this region 

 was extracted by Prof Loomis from Lovering's list, incluiing 

 all the auroras recorded in it in the years between the beginning 

 of the year 1776 and the end of the year 1S72, with their 

 month and dates. The whole of this long list, supplemented in 

 great measure by his own inquiries, is given at full length at the 

 end of the last paper {sup. cit.) by Prof Loomis. The number 

 of auroras in each year, or their annual frequency, is then 

 obtained and laid down in a curve for the whole interval of 

 ninety-six years of the observations. On the same plate is pro- 

 jected the mean daily range of magnetic declination, and th; 

 relative extent of black spots on the sun's disc for the sam; 



•\NUMBeR OF AURORAS OBSERVED- EACH YEAR 



jnlHi 



series of years of observation ; the latter from Wolff's numbers, 

 and the former from the average of magnetic observations made 

 at Prague from the year 1777 to the year 1S71 inclusive. Very 

 uncouth in appearance are all these curves ; and the curve of 

 annual auroral frequency is far the most shapeless in outline of 

 them all ; but the leading crests and troughs of the mling 

 eleven-year period of the sun-spot curve are conspicuously 

 reproduced in each of the other two curves, so that it is diflicult to 

 say whether tlie auroral curve or the curve of magnetic decli- 

 nation is the stricter in its adherence to the times of maxima 

 prescribed to it by the solar spots. In two cases, however, 

 the auroral maximum took place some three years, and in 

 another case about a year, too late (1840, 1S51, 1871). The 

 maximum of the magnetic disturbance curve also took place on 

 one of these occasions (1S3S) a year later than the sun-spot maxi- 

 mum, while in the year 1787 both the auroral frequency and 

 magnetic disturbance curves attained their maxima together 

 between one and two years earlier than the sun-spot curve. 



Prof Loomis concludes that the times of auroral minimum and 



maximum frequency happen on an average from half a year to a 



year later than the same critical times of magnetic disturbance 



«iid of the sun's relative obsciuation by black spots ; that they 



* " Memoirs of the American .'Vcademy," vol. x. 



are more nearly related to the same times for the magnetic daily 

 range than for the sun-spot curve, and that the time of greatest 

 auroral frequency lasts longer than that of the sun's obscuration 

 by spots or of the magnetic needle's greatest daily disturbances. 

 A period of very moderate activity in aU the curves is embraced 

 between the maxima of 17SS and 1830, which is particularly 

 noticeable in the scarcity of auroras and in the smallness of the 

 magnetic oscillations in that period. More than 4,000 auroras 

 are included even in the limited selection from Prof. Lovering's 

 catalogue used by Prof Loomis to establish these results, and 

 yet the interval of ninety-six years (during which the magnetic 

 declination had been continuously observed) to which it is con- 

 fined proves to be too short to detemiine certainly the long cycle 

 of activity and repose that seems to govern the times of greatest 

 auroral frequency for years together, in long recurring periods of 

 between half a century and a century. In a previous paper (the 

 second in the American Journal of Science above quoted), Prof. 

 Loomis had arrived at all the conclusions of the paper just 

 described from an auroral catalogue of his own construction, of 

 observations in not very northerly latitudes of Europe and in 

 the United States ; extending, however, only to the year 1850, 

 the closmg year of the magnetical observations at Prague then 

 accessible to him. A period ,of about sixty years, from the 



