October 1, 1900.] 



KNOWLEDGE 



•^'>ri 



the aurorse of a rich year will enable the observer to 

 draw out a rough scale for himself, which will satisfy 

 the possibilities of the case. 



An important detail in auroral work is the fixing of 

 the position of some specially bright point from two 

 or three fairly distant stations with a view to the deter- 

 mination of its height. This can obviously be best done 

 by reference to the stars if many of these are visible 

 at the time. It would, however, be well to have at hand 

 some rough and ready means for obtaining the altitude 

 and azimuth of any given point, and for this it would 

 be casv to make a sort of rough wooden tiieodolite or 

 altazimuth witli a bar cai-rying a big easily seen pair 

 of sights upon it instead of the telescope. As the auroral 

 flashes come and go so quickly the time of any such 

 determination must be taken with jealous exactness. 



The value of having some means always at hand, how- 

 ever rough, for determining the position of an auroral 

 beam, together with the need for exactness in giving 

 the time of the observation, was well illustrated by the 

 remarkable auroral beam of 1882, November 17. A 

 great sunspot, tiic largest visible for eleven years, was 

 Hearing the central meridian of the solar disk. The 

 magnet, which had been uneasy from the time of the 

 first appearance of the spot at the east limb, began io 

 be seized with the most violent convulsions about two 

 hours before noon on the 17th, the disturbance lasting 

 till 6 o'clock the following morning. " Strong earth 

 currents were also observed at all the times of magnetic 

 disturbance, varying in magnitude with the intensity of 

 the magnetic changes, and the most violent electric storm 

 recorded for more than thirty years swept over Europe 

 iind America.'' In sympathy with these manifestations 

 a superb auroral display w;us witnessed on the evening 

 of the 17th, but bv far the most unique and striking 

 phenomenon occurred " at about 6 p.m., when a bright 

 beam of light i-ose from the eastern horizon and passed 

 majestically across the sky in much the same manner 

 as any ordinary celestial body might do, but with 

 several hundred times their rapidity." Some twenty- 

 six observations of the phenomenon were collected to- 

 gether by Mr. Rand Capron, but most of these were very 

 incomplete, and their discussion was therefore attended 

 with much difficulty, yet imperfect as the obsei-vations 

 were they seemed to show with considerable probability 

 that the height of the beam was 133 miles, and its speed 

 about 10 miles per second. The direction of its flight was 

 from east to west, magnetic not geographical. Had 

 three or four of the observers but possessed some simple 

 means for measuring the height of the beam at its 

 culmination and the azimuths of its rising and setting, 

 the precision of these conclusions would have been 

 greatly increased. 



The same charts that are useful for meteor observa- 

 tions may very conveniently be used for aurorse, the 

 positions of the streamers or of the auroral crown being 

 sketched in with reference to the stars. In all the work 

 the first thing to be aimed at is to make the record 

 as definite as possible. It is here that the difficulty of 

 auroral observation is most felt. They are beautiful 

 and impressive as spectacles, and the student will need 

 no instruction in the preparation of his general 

 descriptions. But to pick out the particular phenomena 

 to which the desirable amount of definiteness can be 

 ascribed will require practice. 



From time to time curious beams of light are seen 

 in the sky the exact nature of which it is difficult to 

 determine. Thus on March 4, 1896, a curious light was 



seen stretching up from the horizon towards the Pleiades 

 which some observers were inclined to regard as auroral, 

 some as the Zodiacal Light, and some actually regarded 

 it as being comctary. The fact that an unmistakable 

 aurora was seen the same evening pointed strongly in 

 favour of the auroral theory. On the other hand, as 

 its direction coincided nearly if not precisely with that 

 of the axis of the Zodiacal Light, and as similar beams 

 have been seen in the same position on other occiisions, 

 the question cannot be regarded as absolutely decided. 

 It would be a matter of the highest interest could it 

 be shown that certain definite regions of the heavens 

 were subject to recurrent flashes, and a careful collation 

 of observations made at widely separated stations would 

 soon settle as to whether we should regard them as 

 auroral or zodiacal, and could not fail to increase our 

 comprehension of one or the other plienoinenou. 



DARK MARKINGS IN THE SOLAR CORONA. 



By W. II. Wesley, f.h.a.s. 



Everyone who has examined a scries of photographs 

 of total solar eclipses is familiar with dark rifts or gaps 

 ill the corona. Most conspicuous at times of sun-spot 

 minimum are the polar rifts, which at such periods 

 open widely and occupy a considerable portion of the 

 sun's polar regions. Rifts, more or less dark, also occur in 

 other parts of the corona, sometimes sharply cutting into 

 the densest portions. There can be no doubt that these 

 rifts ai-e merely inter.spaces between coronal rays. They 

 show the extremely irregular manner in which the 

 corona is distributed over the sun's surface. In view 

 of the fact that the corona, whatever it may be, is not 

 flat, as it appears during an eclipse, but is an object 

 possessing three dimensions, it is obvious that a sharply 

 defined rift, cutting into a dense portion of the corona, 

 and traceable to the sun's limb, represents a gap of 

 most singular form. 



But striking as are these coronal rifts, there is a 

 still more interesting class of dark markings that in 

 many cases cannot be explained as mere interspacea 

 among the bright rays. Unlike the ordinary rifts 

 these dark markings are only occasionally seen. A close 

 examination of the original negatives is often necessary 

 to detect them, and as a rule they are lost in any 

 photographic reproduction. 



The first instance of their occurrence of which I know 

 was in 1871. On the eastern side of the corona, in the 



4////m: 



Fio. 1. — I)i!ii,'t;im 111' AFiii-kiDgs on tlio Coroiiii of 1S71. 



equatorial region, there appears on the photographs a 

 small dark spot about 9' from the sun's limb. It docs 



