O'Eeilly — On the Dates of Volcanic M'liptions. 399 



He cites (page 4) the extraordinary low barometer shown to have 

 existed over all Europe on the 25th December, 1821 [Mlin. JRIiiloso^jfi. 

 Journ., vol. vii., 1822, p. 155), and on page 6 he cites the eruption 

 of 22nd to 26th October, 1822 (= 1821-80), of Yesuvius, having given 

 rise to showers of ashes, the greatest known since the event of 

 Pompeii (a.d. 79). (It is worth noting that the interval between 

 this date = 1821-80 and 78-65, date of the destruction of Pompeii, 

 is 1742-15, which is nearly = 1742-7 = IM x 157, that is a multiple 

 of the sun-spot period.) He says — " "We have here to do with a mass 

 of volcanic phenomena, such as the history of vulcanism rarely 

 reproduces, and it must, therefore, be of importance to examine 

 whether similar eruption years repeat themselves after definite 

 periods, or whether similar phenomena stand in any determinate 

 relation with other periodic natural phenomena. A treatise by 

 Dr. Lament 'On the 10 year Period of the Daily Variation of the 

 Magnetic l^eedle and the Relation of Earth Magnetism with the 

 Sunspots' (Poggend. Ann. 4 Eeihe, Ed. 26, p. 607, 1862), in which 

 the year 1822 is taken as a minimum sun-spot year, allowed me an 

 opportunity of comparing the eruptions with the sun-spot periods. 

 At once the merest glance showed me that some of the minimum years 

 therein indicated, such as the years 1793, 1843, 1855, distinguish 

 themselves from neighbouring years by their numerous volcanic out- 

 bursts. Meanwhile appeared a work by Professor Rudolph Wolf of 

 Bern, on the same subject ('On the 11 year Period of Sunspots and 

 Earth Magnetic Yariations,' Poggend. Ann. Bd. 27, p. 502, 1862), 

 which being based on a greater amount of material, raised my opinion 

 on the matter to a certainty that ' years poor in sunspots, which are 

 also marked by a lesser vakie of magnetic variation, are eruption years, 

 and vice versa.' 



" In order that the reader may be in a position to decide for himself 

 m the matter, I herewith annex the full table as Wolf has published 

 it in. his remarkable work, and add thereto the number of eruptions 

 for each year. 



[Page 12.] " As to the existence of a connection between magnetic 

 variations, sunspots, and volcanic eruptions, the consideration of the 

 foregoing does not allow of a doubt." He calls attention to one point 

 in particular, as furnishing a support to the views he advances. It is 

 the periodic return of eruptions after a time interval of about one 

 hundred years (99 to 101), and he adds: — "The connection which 

 appears to exist between sunspots, magnetic variations, and volcanic 

 eruptions affords us, if we accept a 1 Hth years period for the first- 



