250 



KNOWLEDGE 



[NovEMnER 2, 1896. 



and many others having expressed opinions more or less 

 in consonance with such an iiiea. (Dr. llrestor's name is, 

 by a ahp, no doubt, given as lirewster.) 



It would be entirely unfair to attempt to summarize in 

 a few words the theory here expounded. It may 

 sufVice to say that M. Deslandres linds a very close 

 analogy between solar and terrestrial meteorology ; 

 promineaces, for example, being, according to him, tho 

 solar representatives of our electric storms. 



A VERY EXTENDED STREAM OF SUNSPOTS. 



THE years 1893 and 18!)1 were markedly the epoch 

 of the sunspot maximum. 1895 showed a steep 

 descent of tho curve of spotted areas, and, indeed, 

 on four days in April and three days in May of the 

 current year the sun was entirely free from spots. 

 Drawings of the solar corona of August, 189G, made at 

 Bodi) and Nova Zembla, added strong testimony of the 

 near approach of the sunspot minimum. Nevertheless, on 

 September 9th, 189(i, a small solitary umbral spot was 

 observed close to the east limb, which on the following 

 day was seen to be the precursor of a group hardly sur- 

 passed in area even by the great group of February, 1892, 

 and equalled by none in length. September 11th and 12th 

 were cloudy ; but on September 13th the photograph 

 showed it well advanced on the disc, a sinuous length, 

 a narrow band of almost constant breadth, with central and 

 segmented nucleus and faintly continuous penumbra. To 

 the north of the final spot of the riband, itself curved, 

 there was a semi-elliptical ring of nebulosity with three or 

 four small nuclei situated on the inner side of the ring, 

 whose apse points north. Iq fact, the whole stream 

 formed a series of curves whose concavities lay to the 

 sun's equator. Little change was to be noted on 

 September 14th and 15th, and by September Kith the 

 great procession of spots was passing the central meridian, 

 and its fair proportions were spread unaltered by fore- 

 shortening before our eyes. The sharply nucleated pre- 

 ceding spots of the group showed a tendency to shed 

 indefinitely shaped nebulosities on their southern side. 



closely resembled the opening shells of a bivalve. The 

 herald spot had lessened its distance from the great group 

 very considerably, liy September 20th the spots had 

 aggregated to form three distinct groups, and as the stream 

 had now nearly approached to the west limb its form and 

 character could not be so easily recognized. On September 

 22ad only the iinal spots were to be seen on the sun's 

 edge. 



The accompanying plate will give, however, a better 

 idea of the form and changes of the group than much 

 verbal description. The photographs, which we are 

 enabled to reproduce by the kind permission of the 

 Astronomer Uoyal, were taken with the Dallmeyer photo- 

 heliograph of four inches (three inches effective) aperture, 

 and live feet focal length, of the Royal Observatory, 

 Greenwich, on September 13th, IGth, 19th, 21st, and 22nd 

 respectively. The primary image is enlarged fifteen times 

 in the telescope itself, and has been further enlarged for 

 reproduction twice more, so that the scale of the photo- 

 graphs here given is half an inch to a minute of arc, or 

 about sixteen inches to the solar diameter. 



The leading spot of the group on September lOth was 

 in hel. long. GC° and in liel. lat. IC N. ; the last spot 

 in hel. long. 12'' and hel. lat. 10^'= N. This gives a 

 total length to the group of more than 24° of solar longi- 

 tude, or fully one hundred and eighty thousand miles. 

 Its breadth, on the other hand, did not exceed one-fifth of 

 this at its broadest part. 



The tendency of long groups is t'centiiaUy to range 

 themselves parallel to the solar equator, but it is not at all 

 uncommon to find streams inclined at a very considerable 

 angle to it in their earlier days. The present group was 

 an illustration of the latter relation, being inclined at an 

 angle of about 15° to the equator, the preceding end of 

 the group being at the lower latitude. 



The total area of the group when fully on the central 

 meridian, which it took forty-four hours to cross, amounted 

 to two thousand four hundred millions of square miles. 



This stream was not of the type that is usually accom- 

 panied by magnetic disturbances. Nevertheless, as the 

 accompanying diagram, which represents the photographic 



1 i -f /tf.vV.) 



Copy of the Photographic Trace of the Declination Magnet (Royal Observatorj, Greenwich), 1896, Sept. 17d. 17h.— Sept. 19d. Oh., 



G. Civil Time. 



the centre of the stream was itself nebulous and ill-defined, 

 and the regular following spots had broken on their 

 northern edge into irregular penumbral patches. Thus 

 though the band had quite lost the waved appearance 

 which was its characteristic on September 13th, and the 

 nuclei of the spots all lay towards the median line of 

 the stream, a faintly spiral formation seemed indicated. 

 But the chief feature of the group was now the form 

 and method of segmentation of its separate spots. One type 

 recurred again and again in many stages, and its form 



trace of the declination magnet at the Royal Observatory, 

 Greenwich, will show, the quiescence of the magnets was 

 disturbed soon after sundown on September 17th by the 

 sharp, sudden twitch which is so typical of the commence- 

 ment of a cosmical magnetic storm. If this trace be 

 compared with the photographs given in Knowledge for 

 May, 1892, p. 92, it will be seen that the succeeding 

 oscillations were of a much milder type than those that 

 accompanied the giant spot of February, 1892. Yet they 

 were quite considerable, a wave of 13', which finished 



