AND OP TERRESTRIAL MAGNETISM. 



.'7.. 



Table XII appear on days n 28 and n + 28, and not on days n 27 and n-t-27 as in 

 the case of magnetic " character." It is also curious that the spot area on day n 28 

 should so largely exceed tint on day //+28. As this phenomenon, however, is not 

 shown in 1906 it may be "accidental." The sunspot uiv;i pulses, both primary and 



TAHLK XII. The 27-28-Day Period in Projected Sunspot Areas (n being the 



I!f|.i-.-.i'iii;iti\ r I i.iy !' Large Spot AI-.-.-I ). 



secondary, appear considerably rounder than those in magnetic " character," and this 

 is probably responsible for the greater variability in the position of the crest in the 

 subsidiary pulses of sunspot area than in those of magnetic " character." Thus in 

 190!) and 1910 the largest spot area in the subsidiary pulses appear on day n 30 ; 

 while the spot areas on day n 25 in 1909, and on days n + 27, + 28 and + 30 in 

 1910 are actually below the normal. 



21. The results obtained in S.M. and in the present paper put it Ixjyond a doubt 

 that there is in terrestrial magnetism a period of alxiut 27 days, in the sense that if 

 day TI is either decidedly more or decidedly less disturbed than the normal day, then 

 days n 27 show a distinct tendency to differ from the normal day in the same 

 direction as day n. The characteristic is just as clearly shown by quiet days as by 

 disturbed days. The phenomenon appears in disturbed years and in quiet years, in 

 years of many and in years of few sunspots. It was particularly prominent in 1911 

 when sunspots were few, and it was also well developed in 1910, a year in which only 

 one day \vas award.- 1 character " 2 " at Greenwich. 



Prof. SnnsTKii, as is well known, has adduced arguments which appear fatal to the 

 view that a magnetic sit. mi on the earth can l>e due to any limited jet of electrified' 

 particles emanating from the sun. It may thus seem a waste of time to consider 

 other difficulties, in the way of jet theories, suggested by the present enquiry. There 

 an-, however, physicists, with whom I to some extent sympathise, who have a feeling 

 that demonstrations of the impossibility of some physical hypothesis may prove in 

 the long run less conclusive than was at first supposed. Fresh physical discoveries 

 may remove what seemed at one time insuperable barriers. Thus it may not be 



2 N 2 



