THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Aet. XXXIX. — Vesuvius: Characteristics and Phenomena 

 of the present Repose-period ; by Frank A. Perret, K.I.C., 

 former Honorary Assistant at Royal Vesuvian Observatory. 

 (With Plate I.) 



The modern eruptive processes of this volcano show a 

 marked periodicity. Mercalli has published a list of twelve 

 eruptive periods* since 1700, each culminating in a paroxysm 

 followed by a distinct interval of complete inactivity, the 

 duration of which has varied from two to seven years, with 

 three and a half years as an average. During these periods 

 of repose the central conduit is obstructed, as a result of the 

 preceding paroxysm, and the volcano assumes the solfataric 

 condition marked externally by fumarolic emanations. 



It should be noted, however, that if- these repose-periods 

 mark the end of one era of activity, they also herald the dawn 

 of the next to come. The condition of repose is apparent and 

 external and represents a preparatory phase which forms a 

 part of the cycle of events. This should not be considered, 

 therefore, as an interval of time during which the volcano is 

 uninteresting, but rather as offering a precious opportunity for 

 investigating the methods by which the hidden forces develop 

 into a condition of external activity and for studying at close 

 range the chemical and other phenomena which may serve as 

 indices of future eruption. 



Owing to the exceptional duration of the last eruptive 

 period — 1875-1906 — and the violence of its culminating par- 

 oxysm, the present rest-period may be expected to be of greater 

 than the average length and of more than ordinary interest. 

 During the three years already elapsed the eruption of Strom- 



* G-. Mercalli, " I Vulcani attivi della Terra," Ulrico Hoepli, Milan. 



Am. Jour. Sci.— Fourth Series, Vol. XXVIII, No. 167.— November, 1909. 



28 



