July 1, 1892.] 



KNOWLEDGE 



129 



further south still, ^^z., in S. lat. 28-5°. It had shown, 

 therefore, a steady movement away from the equator from 

 November, when it lay iu S. lat. 19°, to March— the mouth 

 when it was last seen — when it was very nearly 10° 

 further south. 



Before Maximum. 



We have to bear in mind that this implies a drift of 

 more than 70,000 miles in less than four months — indeed, 

 in one month of the four, the drift amounted to 46,000 

 miles — and that it concerned an object which, whatever its 

 nature, showed on the average an area ot 600 millions of 

 square miles, and a maximum area of six times that 

 amount. 



.A.S the February group was the greatest in area of all 

 those observed during the period covered by the Green- 

 wich record, so it stands out as pre-eminent iu its drift. 

 But other groups also have shown a great and remarkable 

 di-ift ; indeed, a large and active group is almost sure to 

 display some motion of the kind, and in many instances 

 tlie evolutions performed by a lai'ge group are of the most 

 complicated character. 



If, however, we confine ourselves to the great groups 

 to which we have already drawn special attention in 

 earlier papers, viz., those of April and November, 1882, 



The thu-d great group of 1882, that of November, showed 

 little or no persistent drift, remaining almost stationary for 

 the greater part of the three months during which it was 

 under observation, but displaying a rather rapid move- 

 ment towards the equator dming the last four or five days 

 of its existence. 



A yet more curious feature of this drift in latitude, which 

 is frequently to be noted, is the tendency to retiurn to the 

 latitude from which the group first started. A single 

 example mav illustrate the point. 



The group of August 2nd, 1889, formed in S. lat. 21°, 

 and steadily moved towards the equator, being last seen in 

 its first rotation in S. lat. 20 . When it was seen at the 

 east limb again on August 27th, its latitude was 19^. It 

 steadily moved upwards dm-ing its appearance, and had 

 nearly regained its old position of latitude 20" before it 

 passed out of view at the west limb. During its third and 

 last appearance it oscillated between 21-5' and 20-5°, 

 finally disappearing when in the last-named latitude. 



Drift in latitude is therefore not at all an uncommon 

 feature of sunspots, though drift to so large an extent 

 as that shown by the great group ol February last, viz., 

 10° in four months, is most exceptional. Yet such 



At Maxiiuum. 



we find that the .\pril group showed a di-ift of Bi'' in 

 latitude during a single month ; slightly greater than the 

 maximum drift of the recent spot, but as the group of 

 April, 1882, was only observed diu'ing three rotations, it 

 did not show that long persistency in southward motion 

 which the later group has done. The second great group 

 of April, 1882, was onlv observed during one rotation. 



After Maximum. 



drift is not at all what we should have naturally ex- 

 pected. For there is a well - marked difl'erence in the 

 speed of rotation of the various zones of the solar surface. 

 Spots on the equator, for instance, not only have an actual 

 rate of speed higher than those north or south of them, 

 but a more rapid angular motion ; in other words, whilst 

 a spot on the equator takes 25 days to complete a rotation, 

 one iu latitude 36- takes 27. 



But curious as are these instances of motion in latitude 

 iu the case of specific groups, there is a far more curious 

 phenomenon, akin to it, shown by simspots as a whole — I 

 mean the change in the latitudes affected by spots at various 

 stages of the sunspot cycle. 



Let us suppose that we are watching the sun during 

 the period of decline after maximum ; during such a time 

 as that covered by the years 1886 and 1887, for example. 

 What should we notice ? In 1886 we should notice that 

 spots were seen over the entire belt from 20° N. to 20° S., 

 not a few lying quite close to the equator, their- mean 

 position in either hemisphere being about 10° of 

 latitude ; but that out of 128 separate groups only one, 

 and that a very small one which lasted but two days, was 

 .seen further from the equator than lat. 20°. In 1887 

 the tendency was to approach the equator still closer. 

 The mean latitude of the entire spotted area was 8-5°, and 

 no single group attained a distance from the equator of 



