708 PKOGRESi^ OF ASTRONOMY FOR 18!tl AND 1892. 



tluiii 3,000 inillionths of the vSuiTs visible lieiiiispheie, did not eover 

 oue-fifteeutli of that area on ^larch 5, though it revived somewhat be- 

 fore it was hist seen at the west liinb on JVIarch 17, but did not survive 

 to make a sixth appearance at the east limb on March 31 or April 1. 



Accordinji' to Mr. Maund«'r tlic great spot, the largest on record at 

 Greenwich, was 02,000 miles long and 02,000 miles broad, while the 

 entire group of whieli it formed the j)rincipal ])art was 162,000 miles 

 long and 75,000 broad. The area of the spot on February 13, 1802 

 was 2,910 million sipiare miles, and the whole group 3,530 million 

 scjuare miles. This is about eighteen times the area of the earth, and 

 seventy globes as large as ours could have lain side by side in the im- 

 mense hollow, Mr, Maunder thinks that the effect upon the weather 

 of a spot even of such enormous size must be very slight, if apprecia- 

 ble. The magnetic needle, however, undergoes violent disturbance upon 

 their appearance. 



In an article in Knowledge for April and May, 1802, Mr. ]\Iaunder 

 brings forward some important evidence in regard to the connection 

 between sun spots and magnetic storms. The article concludes as fol- 

 lows : 



In a period of nearly nineteen years, therefore, we have three mag- 

 netic storms which stand out i)reeminently above all others during 

 that interval. In that same period we have three gieat sun sjiot dis- 

 plays — counting the two groups of April, 1882, together — which stand 

 out with e(pial distinctness far above all other similar displays. And 

 we find that the three magnetic storms were simultaneous with the 

 greatest development of the spots. Is there any escape from the con- 

 clusion that the two have a real and binding connection ? It may be 

 direct, it may be indir(M*t and secondary oidy, but it must be real and 

 effective, ' 



Consider that the period in question is practically some six tliousand 

 eight hundred days, A magnetic storm does not last many hours ; a 

 sun si>ot soon declines from its greatest development, or soon i)asses 

 away from the center of the apparent disk. Suppose we take an out- 

 side limit, and give a period of two days to a giant s[)ot to exercise its 

 intluence or a magnetic storm to expend its violence ; what are the prob- 

 abilities against 3 out of 3,400 of such periods of the one phenomenon 

 agreeing with 3 (uit of 3,400 of the other, if they are not related ? If 

 3,400 numbers were i)laced i]i one box and 3,400 more in a second, and 

 one from each box were drawn at a time, what is tlie chance that the 

 three highest numbers would be drawn from the one box simultane- 

 ously with the three highest from the other, each to each, if the matter 

 had not been prearranged ? Indeed, we might legitimately call the 

 coincidence of A]>ril, 1882, a double one, and ask the odds against the 

 four highest iuiml)ers from each box being so drawn. 



Between sun spots and storms of the second magnitude it is more 

 difficult to make a satisfactory comparison, because it is not so easy to 

 frame a satisfactory dehnition as to what constitutes a secondary dis- 

 turbance. Nevertheless, the following brief table of large sun spots 

 seen since the beginning of 1881, whicli were <;oinci<lent with consider- 

 able disturbances, may prove of interest. The spotted area is given in 

 millions of s(piare miles: 



