430 Proceedings of the Royal Irish Academy. 



The interval between 472 and 203 is 269 years, whicli differs 

 somewhat from the normal interval 266"4 (= ITl x 24), but which 

 compares fairly with the interval between -1889'6 and -1619 = 270-6. 

 The interval between this year, 203, and the sun-spot minimum year 

 -1610-8 is 1407-8, which differs from the normal interval, 1409-7 

 (= 11-1 X 127) by 1-9 only. If therefore the year 202 be taken as a 

 minimum sun-spot year, the approximation will be sufficient as regards- 

 203. 



The inteiwal between 203 and 79, or rather between 203 and 

 78-65 is 124-35, which exceeds the normal interval 122-2 by 2-15, but 

 if we seek the interval between 78-65 and the sun-spot minimum year, 

 - 1610-80, we get 1532-15, which differs by less than an unit from the 

 normal interval 1531-8 (= 11*1 x 138), so that this year may be taken 

 as a minimiim sun-spot year. 



The interval between 78*65 and 63 is 15-65, which exceeds the 

 normal interval 11-1 by 4-55. This may be read as representing the 

 interval from the probable minimum of 67-55 to the maximum 62, 

 which is here adopted. 



Erom the examination of the figures which result from the com- 

 parison of the eruption dates of the volcanoes hereinbefore detailed — 

 Hecla, Katla, Etna, and Yesuvius, with the sun-spot years of maximum 

 and minimum as given in Wolf's list, covering the period from 1610 to 

 the present — it is evident that the concordance is general, and even 

 greater than might be expected, considering the difficulties inherent, 

 on the one hand to the exact determination of the dates of the com- 

 mencements of the eruptions cited; and on the other, to the uncertainty 

 which necessarily arises as to the precise month of the year, or even of 

 the year in which the maximum or minimum occurs. At best these 

 sun-spot dates can only be determined to within a month, even at the 

 present time. 



The concordances that may be noted evidently justify the presump- 

 tion that did we possess the dates of the sun-spot maxima and minima^ 

 for the years anterior to 1610, the same concordance would be found 

 between them and the dates of the eruptions prior to this year. It is- 

 much to be regretted that Kluge's list of eruptions has not been pub- 

 lished, and it would still be desirable that such a list be made, and that 

 from many scientific points of view. 



The concordances noted justify the attempt to estimate the sun-spot 

 dates for the years prior to 1610 with reference to the eruptions of the 

 volcanoes considered which are dated before that year. This has been 

 carried out by utilizing, on the one hand, the multiples of the normal 



