104 



KNOWLEDGE. 



[May, 1901. 



The table shows that in 1545 the standard alloy con- 

 tained only one-half of its weight of pure silver, and in 

 1550 it was still further debased, and contained only 

 one-fourth part of pure silver. The restoration of the 

 silver staiidaid, begun in the reign of King Edward VI., 

 was completed by Queen Elizabeth, and it has not been 

 since debased. (To be continued.) 



THE TYPES OF SUN-SPOT DISTURBANCES. 



By the Rev. A. L. Cortie, s.j.. f.r.a.s. 



During the last twenty years some 3800 drawings of 

 the solar surface, on a scale of lOi inches to the sun's 

 diameter, have been made at the St"ouyhurst College Ob- 

 servatory, during the period 1880 to 1890 by means of 

 the 8-inch equatorial, and since the latter year by means 

 of the Perry Memorial 15-inch ecjuatorial. The prin- 

 cipal spots in this long series of drawings have recently 

 been tabulated in a form wliich gives their complete life- 

 histories from their first appearance to their final ex- 

 tinction. Accompanying the tabulations, are a series 

 of charts which show at a glance the chief phases in the 

 development of a group of spots. The object of these 

 tables and charts was to compare individual spots on the 

 sun and magnetic storms on the earth, and the results 

 of the comparison were presented by Father Sidgreaves 

 to the Royal Astronomical Society last December. But 

 besides this primary object for which the tables and 

 charts were drawn up, they have served, and, it is hoped, 

 will serve for many other subsidiary studies with regard 

 to sun-spots. One such study led to a paper read at the 

 last meeting of the British Association at Bradford, of 

 which some account is presented in the following pages. 

 It is comparatively easy from the charts, not only to 

 study the changes in individual spots which take place 

 during their appearance on the sun, but likewise to 

 compare outburst with outburst. From this comparison 

 it was soon seen that there exists a great family likeness 

 between all outbursts of sun-spots, whether we deal with 

 the great storms when the solar activities reach their 

 maximum, or the feeble manifestations of the solar forces 

 at the epochs of minimum. Should an observer look at 

 the sun at a time of maximum activity, he will probably 

 see its surface covered with many spots seemingly in all 

 sorts of forms and shapes ; as scattered dots, or as trains 

 of spots, or m a form in which two larger spots pre- 

 dominate in a group, or again as a single deep black spot, 

 of round and regular outline. Yet all these varieties are 

 but phases of one well-marked and general type through 

 which, as a general rule, all groups of spots pass during 

 the course of their life-histories. In a normal outburst 

 the sequence of the phases is as follows. Wc must pre- 

 mise, however, that in short-lived spots all tiie various 

 stages or phases of the type may not be reached, but 

 only the earlier ones. 



A group of spots first appears in the form of a few 

 scattered dots or small spots. These spots grow with 

 great rapidity, and in three or four days at the most after 

 their birth, begin to aggregate, and reach a second 

 characteristic phase in the process of development. This 

 is marked by the predominance of two main spots in 

 the group, one in the forefront, and the other in the 

 very rearmost position of the group. In this phase the 

 preceding of the two spots is for the most part the more 

 compact and regular, its fellow being generally in-egular 

 and ragged in form, and made up of semi-detached 

 patches of umbra and penumbra. Many times, however, 

 it covers a greater total area than the leader of the group. 



This stage is reached by almost all groups of spots, even 

 very small ones, and any theory as to the origin and 

 development of sun-spots must needs explain why two 

 spots, one in the forefront and one at the roar of the 

 group should always predominate. The preceding of the 

 two main spots, as a rule, then, rushes forward with a 

 large proper motion in longitude, by this means in- 

 creasing its distance from the rearmost spot. At this 

 period in the life-history of the group, the space bet,veen 

 the two spots becomes filled by a train of irregular un- 

 formed spots, and penumbral patches of no veiy great 

 size, the whole train of spots thus formed arranging 

 itself into a stream more or less parallel to the solar 

 equator. This tyjDe is accordingly reckoned as the third 

 in the order of succession in a normal outburst. It 

 occurs ordinarily between five and seven days after the 

 first appearance of the group. At times a retinue of 

 companion spots follows both the chief members of the 

 group, and more rarely is confined to the rearmost spot. 

 This phase is not very stable or of long duration, and 

 the two main spots survive after its disappearance. 

 Hence there is a recurrence from the third type back 

 again to the second. Of the two surviving spots the 

 following one generally breaks up and disappears in a 

 few days, leaving the leader to form into a dense black 

 round spot of regular outline. This accordingly has 

 been reckoned as the fourth type or phase in the stages 

 of the life-history of a group of sjDots. 



Although it is the leader that generally survives to 

 form the single spot, yet instances are not wanting in 

 which the reverse has been the case, the leader dis- 

 appearing and the rearmost of the two spots remaining 

 (e.g., 1892, July 4 — October 5). A few cases are on 

 record when both the chief spots survived to form regular 

 spots (e.g., 1881, October 14 — December 17). Again, 

 the single surviving spot, accompanied or unaccompanied 

 as the case may be by small companions, may never 

 become a round regular spot, but may maintain an 

 irregular shattered appearance of a collection of umbr<e 

 and penumbrse, as for instance in the great spot of Feb- 

 ruary, 1892. The duration of the phase in which the 

 group has developed into a single round black spot is 

 generally for two solar rotations, and in one case 

 (1897, April 28 — August 27) was maintained for five 

 successive rotations. It may be remarked in passing that 

 the average life of a group of sun-spots, derived from a 

 study of 115 greater disturbances recorded at Stony- 

 hurst during the last twenty years, is 56 days, or a little 

 longer than two solar rotations. 



The last stage in the life-history of a group of sun- 

 spots is reached when the round black spot gradually 

 dwindles down to the dimensions of a mere dot, or dis- 

 appears as a group of small dots. This phase is so like 

 the opening phase of the series that it does not need 

 a new type number to describe it. It frequently 

 happens that when this final stage has been reached, 

 other smaller sjiots will appear in the neighbourhood of 

 the remnants of the storm that is passing away, and the 

 v.Iiole cycle of phases will be again repeated. A series 

 of outbursts linked together in this manner in approxi- 

 mately the same position on the solar disc was active 

 during a period extending from September 25, 1891, to 

 March 5, 1893, or during more than twenty solar rota- 

 tions. 



In addition to these four well-marked phases or types 

 described above, another type comprises irregular, 

 scattered groups, of short life-, which occasionally appear 

 and disappear without conforming to the normal se- 

 quence of changes. 



