200 



KNOWLEDGE 



[Septembee 1, 1894. 



September 28th to October 10th as No. 2311. As 

 No. 2294 it pursued an erratic course, moving irregularly 

 backwards and forwards over a small area, and was last 

 seen at the west limb of the -sun on nearly the same meri- 

 dian as that on which it had been first seen on the east 

 limb. But during the next fortnight, whilst m the unseen 

 hemisphere, it had drifted backwards at a very rapid pace, 

 a drift which still continued during its final apparition as 

 No. 2811, and the group first seen on .July 9th in longitude 

 175-1° was last seen on October 10th in longitude 168-1^. 



The accompanying diagram exhibits separately the 

 motion of the circular leader spot taken by itself, and of 

 the centre of gravity of the entire group. The minuter 

 irregularities are possibly due to errors of observation, a 

 discrepancy of one or two tenths of a degree being readily 

 admissible in the estimation of the centre of a spot some 



backwards along a course as even and straight as it had 

 previously been erratic. 



It would be tedious rather than instructive to multiply 

 examples of this forward and backward movement of spot 

 groups, which is extremely typical of the greater and more 

 long - Uved disturbances. The instance selected is an 

 ordinary one, and might be matched by the dozen. But 

 to my mind this extension of a group in longitude, tUl fi'om 

 being one or two degrees in length it becomes ten or twelve 

 degrees, the sympathy often manifested between the 

 extreme spots of a much extended group, and the tendency 

 of a group to return to its original meridian as it approaches 

 dissolution, are plain indications that the cause of the 

 outbreak is something very deeply seated in the sim, 

 whether the actual spots be deep or not. 



In other words, in spite of the obstacles which the solar 



CUETE SHOWIXS DkIFT OP THE LeADEE SpOT. 



168° 169= 170= 171= 172= 173= 174= 175= 176 177 



17S= 179= 180= 



26= 



25° 



27' 



26° 



24= 



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LOWCITBDE, 



Grouf iiu 



C.o-A ZJ-?* 



>V».j-.t 



Crou.fi Hqf 



LONCITtfDE. 



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Curve showi> g- Drift of the Cextre of G-eavity op Extiee Geocp. 



three degrees in diameter, and still more so in the com- 

 putation of the centre of gravity of an irregular group 

 which sometimes straggled over more than ten degrees. 

 But it would rather appear that they are real, for it wiU be 

 noted that, except during the first development of the 

 group, the cun-e for the euthe group is actually smoother 

 than for the leader alone. In other words, the waxing 

 and waning of the spots in the after part of the group, and 

 their osciUations of position, were so instantly responded to 

 by a partially compensating motion of the leader, that the 

 group, considered as a whole, kept a more even course 

 than the single spot. This was especially the case 

 during the third rotation, when the leader followed a 

 particularly zigzag path so long as it had a train of 

 attendants. When these had all disappeared it moved 



gravity and the rapid increase of temperature with depth 

 place upon vertical disturbance in the sun, vertical dis- 

 turbance and disturbance from a gi-eat depth is just the 

 very thing which the greater sunspots evidence. 



The groups we have taken as examples were only of the 

 third or fourth order of magnitude, yet they furnish us 

 with instances of spots with areas of two hundred or three 

 hundred milUons of square miles, moving at a speed of 

 from two hundred to four hundred miles an hour. There 

 can be no question of a calm atmosphere where move- 

 ments of this order are in frequent progress. Our wildest 

 tornadoes would appear but as zephyrs in comparison. 

 The argument, therefore, that the solar atmosphere must 

 be quiescent, because the external causes of storms and 

 cyclones at work upon the earth are absent there, is not 



