108 



KNOWLEDGE 



LMay 1, 1895. 



to nearly 7000 miles, or 800 miles an hour. It must, 

 however, be always borne in mind that even moving at 

 this tremendous speed it would take the spot four entire 

 days to cross its own diameter when at its greatest 

 extension. This kind of spot motion is, therefore, rather 

 the development of the disturbance forward than the 

 actual movement of the spot on the solar surface. The 

 spot has seemed to move, mainly because its growth has 

 been restricted on the following side, but has proceeded 

 freely to the west and north and south. 



Contrast with this growth and mobility that of the last 

 spot of the group. With an area of 27 millions of square 

 miles on March 27th, it extends to 31 millions on March 

 31st, and has shifted its position 0'1° in longitude in four 

 days. _ Then the bright faculous matter surrounding it 

 closes in upon it. It has shrunk to eight millions by the day 

 of our photograph, and has disappeared by the following day. 



This behaviour is most strictly typical of a large class 

 of spot-groups. The actual commencement of the gi-oup 

 was not witnessed in this case, as it took its rise in the 

 unseen hemisphere. But it was evidently not more than 

 two days old when first discerned near the east limit. 

 The gradual change of the compact irregular cluster into 

 the long straight stream by the forward rush of a single 

 small spot, which grew as it advanced, whilst the spots 

 in the rear remained almost motionless, is continually 

 being witnessed, and the rate of advance is practically 

 always the same— from 6000 to 8000 miles a day. Then, 

 as with the present group during April 3rd — 6th, there is 

 a gradual swing back of the leader spot as the train of 

 smaller spots which follow it die out. On April 5th, for 

 example, the leader has receded 1-4° from its place on 

 April 2nd, and is accompanied by but a single follower. 



The condition in which the group was now seen is apt 

 to be a stable one. In the present instance it was lost to 

 sight at the west hmb after April Gth, but a fortnight 

 later was seen again at the east limb, still a well-defined 

 circular spot, but little smaller in area to what it was on 

 April Gth, in practically the same longitude, but having 

 moved more than 2° towards the north in latitude. It 

 remained steadily in this position without materially 

 changing as to place, size or appearance. It reached the 

 west limb for the second time on May 3rd, and was not 

 seen again. 



The eastern group, like the one we have just noticed, 

 took its rise in the unseen hemisphere, but its regular 

 outline and stability of position point to its being already 

 an old spot when it first came into view on March 28th. 

 Besides, its period of growth was already over, and a slow 

 and steady shrinkage from an area of 225 millions of 

 square miles to one of 126, was practically the only change 

 it showed during this period of visibility. OccasionaUy 

 one or two little acolytes were seen near it ; there were 

 three on April 1st, but these soon disappeared again. The 

 group showed precisely the same features during its second 

 appearance, the shrinkage still continuing, until, by 

 May 4th, its place was entirely covered over. 



The third and largest of the three groups, the one 

 furthest to the north, showed by far the greatest activity, 

 and furnished during much of its course an example of 

 another class of spot stream from that afforded by the 

 western group. Its commencement was quiet enough — a 

 small solitary spot near the east limb on March 28th. Its 

 area was about 33 millions of square miles, it had obviously 

 not long formed, and it was surrounded by some bright 

 but not extensive faculse. Its development was very 

 striking. It covered an area of 224 millions of square 

 miles on March 29th, and three fresh spots had formed 

 behind it. These at first lay nearly at right angles to the 



direction of the solar rotation, but speedily fused together, 

 and tended to draw out in a line parallel to it. On March 

 30th the group consisted mainly of two fine composite 

 spots, each showmg several nuclei, and covered a total 

 area of nearly 1000 millions of square miles. A double 

 chain of very small spots united the two giants of the 

 group. The increase continues on the succeeding days 

 until April 3rd and 4th, when the total area reaches 1660 

 millions of square miles. After this the group begins to 

 decay, and it proves the most short-lived of the three. 

 Bright faculous matter begins to pour into the rear spot on 

 April 5th, and by April 7th it is completely broken up. 

 Simultaneously with this destruction of the following 

 portion of the group the great leader spot has moved 

 forward and northward with great rapidity, a movement it 

 evidently continues whilst in the further hemisphere, for 

 after its second appearance at the east limb on April 22nd 

 it is found to have travelled more than 10° in longitude 

 and 2- in latitude since April 5th. The leader, now 

 reduced to a small circular spot of about 80 millions of 

 square miles area, is all that remains of the great group, 

 and ten days later the last remnants even of this have 

 disappeared. 



The great difference between the western and the 

 northern groups lay in the fact that the growth of the 

 former was chiefly confined to its leader spot, but that 

 of the latter was divided nearly equally between the first 

 spot and the last. It is in strict accordance with this 

 circumstance that the elongation of the first group was due 

 to the rapid motion forward of its growing leader ; but 

 that of the other to the motion apart from each other of its 

 two end spots, the leader moving forward, the rear spot 

 moving backward, a motion which continued so long as 

 the growth of both was maintained. The break-up of the 

 rear spot was at once answered by a check to its motion, 

 and by an increase in the forward drift of the leader.'' 



Another striking feature of the northern group, and one 

 intimately connected with its chief development in two 

 directions, was that the disintegration of its two spots 

 proceeded on the inner side ; the front spot wasted on its 

 following side, the rear spot on its preceding side. The 

 outer edges of both spots were regular and well defined. 

 This alteration is partly shown in the present photograph, 

 but the process was better seen on the previous day, when 

 the group was in a condition of more active change. 



The three groups before us, therefore, though not of 

 extraordinary size, presented several points of interest. 

 There is yet another to which allusion should be made. 



As I have had occasion to point out before (" The Great 

 Sunspot and its Influence," Knowledge, May, 1892), it is 

 by no means easy to make a satisfactory comparison 

 between sunspots of what I have called the second order 

 of magnitude, and magnetic disturbances. They may or 

 may not be observed simultaneously. The present occasion 

 was, however, one in which a considerable display of sun- 

 spots was coincident with a marked but not an extra- 

 ordinary magnetic irregularity. As the accompanying trace 

 (Fig. 2) will show, the declination magnet at Greenwich 

 commenced to swing eastwards at about five p.m. on March 

 30th, and a set of oscillations commenced, of no great 

 rapidity but of considerable length, which, by the following 

 midnight, had reached a point nearly a degree removed 

 from the mean place. The oscillations which followed 

 tended to bring the magnet back to its normal position, 



* A rotation ])erio(i of 25'38 days was assumed in each cas3. 

 The use of either S()oerer's or Carrington's values for the special 

 latitudes concerned would alter the rates of motion given in the above 

 remarks, but would only account for a small portion of the change 

 of longitude. 



