OCTOBEK 1, 1898.] 



KNOWLEDGE. 



229 



■with the northern portion of the great spot, in an intricate 

 lacework of light on the two next days. By the 11th the 

 middle spots in the following stream had begun to disappear, 

 and by the 13th only one small dot remained in that part 

 of the group, the rearward spot being then separated 

 from its leader by a broad belt of photosphere. By this 

 day a very fine bright bridge, which was in process of 

 formation on the previous day, had forced its way across 

 the gi'eat umbra from north to south. The northern 



ii 



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/V/ 



Tracing of Vertical Force Pliotographic Kegister during tlie 

 Disturbance of 1898, September 9—10. 



portion of the great spot was still full of complicated detail. 

 On the following day the bright tongues which invaded 

 the spot lay mostly on the east. By September 15th the 

 great spot was seen only as a notch on the limb, and one 

 spot alone followed it. 



The accompanying plate shows the group at its fullest 

 presentation, namely on September 8th, before it had 

 reached the central meridian, and September 9th and 10th, 

 immediately after passing it. These were the days, too, 

 on which it attained its greatest area and extent ; the total 

 area of the group being then some two thousand seven 

 hundred millions of square miles, its greatest length nearly 

 one hundred and forty thousand miles, and its breadth 

 forty-four thousand miles. They are reproduced, by 

 the kind permission of the Astronomer Eoyal, from 

 photographs taken in the ordinary routine at the Eoyal 

 Observatory, Greenwich, with the photoheliograph pre- 

 sented by Sir Henry Thompson. This instrument has 

 an aperture of nine inches, stopped down to four inches 

 on the present occasion, and a focal length of eight 

 and a-half feet. The image in the primary focus is about 

 one inch in diameter, and is enlarged by a secondary 

 magnifier seven and a-quarter times. The resulting photo- 

 graph has been further enlarged some two and a-half 

 diameters, so that the present plate gives the spot on a 

 scale of eighteen inches to the sun's diameter. 



A special interest attaches to a great disturbance like 

 the present when it occurs at a normally quiet time, for 

 it brings out into clearer relief the peculiarities of the 



connection between these solar displays and the related 

 phenomena on this planet of magnetic storms and aurorae. 

 The accompanying trace, reproduced from the photo- 

 graphic sheet of the vertical force magnet at Greenwich, 

 shows that some fourteen hours after the great spot crossed 

 the central meridian of the sun, a sharp magnetic disturb- 

 ance set in, which was at its height from eight to eleven 

 o'clock on Friday evening, September 9th. 



During these three hours an aurora of a specially 

 brilliant and beautiful character was ob- 

 served generally throughout the British 

 Isles, the official report of the Greenwich 

 observer, j\Ir. Beadle, running as follows: — 

 "At 20h. 15m. a bright light was observed 

 in the northern sky from which issued 

 several white streamers. These became 

 especially distinct at 21h. (when they at- 

 tained an altitude of about 45 degrees), 

 and remained visible, more or less brightly, 

 till about 21^h. 



" By 22h. an arch had formed. This 

 was of bright yellow light and the ends 

 were separated by a distance of about 90 

 degrees ; it was most decided in form and 

 colour at about 23h. 15m. At this time 

 the summit of the arch was fifteen degrees 

 to twenty degrees above the horizon. By 

 23Jh. the phenomenon had quite disap- 

 peared." 



A fainter display was noticed also the 

 following night, and in more northern 

 latitudes, as in Norway, the aurorie were 

 most brilliant for several successive nights. 

 It will be noticed that we have here, 

 again, a striking case of the quick answer 

 of the earth to a really great solar dis- 

 turbance, of which I gave several instances 

 in my paper on " The great Sunspot and 

 its influence," in Knowledge for May, 

 1892, and that the terrestrial disturbance 

 was at its height about twenty or twenty-one hours after 

 the sunspot had reached the centre of the disc. My own 

 experience fully confirms that of Signor Ricco, the great 

 Italian solar observer, that this is the relationship 

 that most generally prevails. Dr. Yeeder, on the other 

 hand, considers that the influential position for a sunspot 

 is when it is on the east limb, a view in which I am not 

 able to coincide. A spot like the present occurring at a 

 comparatively quiet time is even more useful for settling 

 such a point than one at maximum. 



[The KditorB do not hold themselTes responsible for the opinioni or 

 statements of correspondents.] 



THE AIRORA BOREALIS. 

 To the Editors of Knowledge, 



Sirs, — As I was particularly well placed for watching 

 the very fine Aurora BoreaUs on the evening of the 9th 

 inst., I think you may possibly find some interest in com- 

 paring my observations with those of others. 



I went out in the garden just after 8 o'clock, and was 

 immediately struck by curious flecks of light in the south, 

 suggesting luminous clouds, and on going out on to 

 Bramshott Common, where there is an uninterrupted 

 view for many miles, I saw that in the north there was a 

 pale yellowish-white light, which gradually increased in 



