September 1, 1894.] 



KNOWLEDGE 



199 



doubt given rise to the idea expressed in the quotation 

 which Dr. Brester makes from Prof. Young. 



But though this is typical of the behaviour of a large 

 number of groups, we must not associate the growth and 

 forward motion of the leader spot too closely together, and 

 it certainly is not motion of the nature of a " leap." It 

 will often happen that in a growing " train " the last spot 

 wiE increase even more rapidly than the leader, whilst the 

 leader will often continue its motion forward at a rapid but 

 equable pace some considerable time after it has reached 

 its greatest extension. Nor is the very opposite phenomenon 

 unknown, viz., that of a recession in longitude during the 

 development of a group. 



An actual example will, however, convey much more 

 information than mere general description, and for this we 

 will turn to the last volume of Greenwich Ohservtitions. 

 Here we find that a simple form of train took its rise on 

 October IHth, 1891 (No. 232.5), and, developing rapidly, was 

 observed till it reached the west limb on October 25th. On 

 the first day of observation only two small well-defined spots 

 were seen, their total area only nineteen of the Greenwich 

 units (millionths of the sun's visible hemisphere). By the 

 next day a number of spots were seen, and the total area 

 was one hundred and one units, the increase being 

 principally in the case of the preceding of the two spots of 

 the day before, which had increased from eight to seventy- 

 three units. The third day showed the group at its 

 maximum area of three hundred and twenty units. It 

 now consisted of two great spots. A few spots were seen 

 between the two, linking them as it were together, on the 

 succeeding days, but the first and last spots, the giants of 

 the group, were slowly diminishing the while. By October 

 23rd the last spot had broken up, and it dwindled with 

 great rapidity, and by October 2.5th the leader spot alone 

 remained, a solitary spot. 



But though the increase in size took place entirely 

 between October 16th and 18th, the drift apart of the two 

 principal spots continued right on to the time of the break- 

 up of the following spot on October 23rd. As the following 

 little table wiU show, the movement lay almost entirely 

 with the leader, the following spot remaining almost 

 stationary, for the apparent change in longitude near the 

 beginning and end of its career may at least be partly 

 attributable to its growth and decline taking place not 

 quite symmetrically : — 



Date. Heliographic Longitude. 



1891. Spoto. Spot J. a—b. 



October 16 B30-l° 327-3^ 2-8° 



„ 17 331-1 328-0 3-1 



„ 18 333-2 328-1 5-1 



„ 19 334-2 327-5 6-7 



„ 20 335-6 327-5 8-1 



„ 21 336-5 327-6 8-9 



,, 22 337-2 327-4 9-8 



,, 23 337-3 327-1 10-2 



„ 24 837-8 327-7 10-1 



In groups which have been longer under observation 

 than the above, it has sometimes been the case that the 

 disappearance of the following portion of the group has 

 been followed by a backward drift of the leader spot. In 

 the present case the group died out whilst in the unseen 

 hemisphere, but another group, which appeared in 1891, 

 shows this retrogression in a slight degree. The group 

 (No. 2255) was a fine train at its first appearance on 

 July 8th. The forward impulse was still active in the 

 leader, a well-defined circular spot, as the following 

 longitudes, taken on ifuly 9th and the succeeding days, 

 sufficiently show :— 177-0°, 178-8^ 179-7°, 180-3°, 181-6°, 

 181-8°, 182-5°. Here, however, its progress stopped. The 



retinue of small spots that had attended it hitherto had 

 all but disappeai-ed by the following day, and though the 

 leader itself continued still to grow until it reached the 

 west limb on .July 20th, the longitudes recorded for it show 

 no stable progress, but are accordant almost within the 

 limits of error of observation for so large a spot, the mean 

 place being 182-4^. A fortnight later the group was again 

 seen at the east limb as No. 2277 ; the retinue had entirely 

 disappeared, and the leader of the former apparition 

 appeared alone. The longitude had scarcely altered from 

 that it had on July 20th, being 183-1° as against 182-6° ; 

 the advance during an entire fortnight amounting only to 

 half a degree. It was now slowly diminishing in size, its 

 areas on succeeding days reading as 221, 221, 198, 190, 

 195, 185, 185, 177, 162, 140, 172, 107, and 104. In strict 

 correspondence with this diminution in area was a decline 

 in longitude, the readings for the same period being 183-1, 

 182-7^ 182-7, 182-5°, 182-4°, 182-2°, 181-9°, 181-7°, 181-6^ 

 181-2°, 181-3°, 181-5\ and 181-0°, the ephemeral recovery 

 in area on August 14th being accompanied by a feeble but 

 longer-hved return in longitude. 



A more complicated movement was witnessed in the 

 group numbered at its successive returns as 2256, 2278, 

 2294, and 2311. Nothing is more striking in the case of 

 a considerable disturbance than to see it repeat itself as it 

 were in another latitude, and an active train will often be 

 accompanied by a feebler copy of itself a few degrees north 

 or south. An outbreak of the first magnitude, indeed, will 

 reproduce itself in several directions, and will be both 

 preceded and followed by small groups on the same Une of 

 latitude, whilst other groups will form between it and the 

 equator, but on the same meridian. 



In the present instance, groups 2255 and 2256 both lay 

 on the same meridian and were of very equal dimensions ; 

 the more northern group, as of longer duration than its 

 companion, having perhaps the better right to be considered 

 the principal. As in the October group (No. 2325), the 

 train consisted principally of two great spots, and the 

 second of these remained almost stationary, its longitudes 

 reading as 168-5°, 168-2°, 168-2°, 168-0°, 168-1°, 168-3°, 

 168-4°, and 168-4° on July 11th and the succeeding days. 

 The leader during the same period had shown a slight ten- 

 dency to advance, its position varying as follows : — 174-4°, 

 174-8°, 174-9°, 175-6°, 176-3°, 175-7°, 175-7°, 176-3°. The 

 leader attained its maximum on July 12th, the rear-guard 

 on July 13th, but the latter had broken up by July 15th,* 

 and it dwindled rapidly. Its decay was accompanied by a 

 check to the advance of the leader, and by a sudden increase 

 in size, but after the disappearance of the rear spot the 

 leader again moved forward, its longitudes for July 18th, 

 19th, and 20th being 176-3°, 176-9°, and 177-8°, but its 

 area again steadily diminishing. Here, then, two periods 

 of contraction were associated with a steady advance, but 

 a sudden expansion with a halt and slight retrogression. 



At its return, as group 2278, the leader was foimd in 

 longitude 179-7°. It had considerably increased in size — 

 from one himdred and eight to three hundred and five 

 units — and was alone but for the presence of a faint area 

 apparently recently separated from its penumbra. During 

 this apparition the spot steadily moved backward in longi- 

 tude, and, with some slight irregularities, on the whole 

 decreased in area. The longitudes ran 179-7°, 179-0°, 

 178-3°, 178-1°, 177-4°, 177-2°, 177-0, 176-8°, 176-8° 

 176-5°, 176-1°, 175-9° and 175-3°. 



The group was seen during the two nest rotations ; from 

 August 31st to September 13th as No. 2294, and from 



* The letter b for the reir spot is retained after its diTision in the 

 Greenwich fiesults for its preceding portion. 



