O'Eeilly — On the Dates of Volcanic Eruptions. 431 



sun-spot period, 11*1 years, of -wliich. a table was drawn up, and on 

 the other, a table of the actual intervals found to exist between suc- 

 cessive sun-spot maxima and minima, starting from the most recent 

 date available, and going bacli to 1610. The results have proved 

 most interesting, as may be judged by the details already given for 

 the different eruption dates prior to 1610, and also by the summaries 

 which follow. As regards the ascertained dates of maxima and minima 

 of "Wolf's list, the following table gives the number of concordances, 

 with the mean differences between the dates of eruptions and sun-spot 



dates : — 



Concordances. Mean diff. 



Hecla, ... 4 .. 1-03 



Katla, . . . 5 . . 0-94 



^tna, . . . 31 . . 0-56 



Vesuvius, . . 37 . . 0-76 



That is, in all 77 concordances, with a general mean difference of 

 0-706 year. 



As regards the eruption dates prior to 1610, for which estimates 

 were made as described, with a view to the determination of the sun- 

 spot year corresponding to each eruption, I have thought it best to set 

 out in a table (see next page) the intervals found between such 

 eruption years, and one or other of the sun-spot dates contained in 

 "Wolf's list, comparing the interval found with the multiples of the 

 sun-spot interval or period, 11"1. 



It has already been remarked with regard to the concordances 

 shown in the case of Etna, that the minima concordance dates pre- 

 dominate with an approximation of 1-09 relatively to 2*05 for the 

 maxima. Taking the whole of the concordance dates herein given, as 

 well as the estimated dates, the result comes out, that for the former 

 the maxima are 37 in number, with a mean difference of 1*22 between 

 the eruption date and the sun-spot date, while for the minima the total 

 is 40, with a mean difference of 0-89. As regards the estimates, the 

 maxima show a total of 18, with a mean difference of 1'26, while 

 for the minima the total being 15, the mean difference comes out 

 0"82. That is, the estimates come out pretty much as the actual 

 concordances, which so far proves in favour of their probable correct- 

 ness. 



